Understanding Infrared Thermography Reading 3 SGuide-IRT

Infrared Thermal TestingReading III- SGuide-IRT My ASNT Level III Pre-Exam Preparatory Self Study Notes 29th April 2015

Charlie Chong/ Fion Zhang

Infrared Thermography






DEADLY French Military Mistral Anti Aircraft Missile System


■ https://www.youtube.com/embed/_3c0NpYapM0

https://www.youtube.com/watch?v=_3c0NpYapM0

See Through & Fun Thermal Camera Experiments


■ https://www.youtube.com/embed/pXAzZoWLzSo

https://www.youtube.com/watch?v=pXAzZoWLzSo

LEAKED Body Scan Images From The TSA!


■ https://www.youtube.com/embed/QRkWmRVs-nk

https://www.youtube.com/watch?v=QRkWmRVs-nk

How to see through clothing 2


■ https://www.youtube.com/embed/0wQlyCNPw8M

https://www.youtube.com/watch?v=0wQlyCNPw8M

Bf4 little bird ah-6j night vision infrared real combat footage helmet cam montage funker tactical. – 金头盔


■ https://www.youtube.com/embed/dRra63kOwWE

https://www.youtube.com/watch?v=XfXShaTzAhI&list=PL7D451B08CD9A119B

Apache IR Thermal Weaponry

Charlie Chong/ Fion Zhang https://www.youtube.com/watch?v=XfXShaTzAhI&list=PL7D451B08CD9A119B

■ https://www.youtube.com/embed/XfXShaTzAhI?list=PL7D451B08CD9A119B

Infrared Electrical Testing


■ https://www.youtube.com/embed/DgXsmvv7Q9o

https://www.youtube.com/watch?v=DgXsmvv7Q9o


Fion Zhang at Shanghai29th May 2015

http://meilishouxihu.blog.163.com/



Charlie Chong/ Fion Zhang http://greekhouseoffonts.com/



ASNT Certification GuideNDT Level III / PdM Level IIIIR - Thermal/Infrared Testing Length: 4 hours Questions: 135

1. Principles/Theory• Conduction• Convection• Radiation• The nature of heat and heat flow• Temperature measurement principles• Proper selection of Thermal/Infrared testing

2. Equipment/Materials• Temperature measurement equipment• Heat flux indicators• Performance parameters of non-contact devices


3. Techniques• Contact temperature indicators• Non-contact pyrometers• Infrared line scanners• Thermal/Infrared imaging• Heat flux indicators• Exothermic or endothermic investigations• Friction investigations• Fluid Flow investigations• Thermal resistance (steady state heat flow)• Thermal capacitance investigations

4. Interpretation/Evaluation• Exothermic or endothermic investigation• Friction investigations• Fluid flow investigations• Differences in thermal resistance• Thermal capacitance investigations


5. Procedures• Existing codes and standards• Job procedure development

6. Safety and health• Safety responsibility and authority• Safety for personnel• Safety for client and facilities• Safety for testing equipment

Reference Catalog NumberNDT Handbook, Third Edition: Volume 3, Infrared and Thermal Testing 143Fundamentals of Heat and Mass Transfer 952ASNT Level III Study Guide: Infrared and Thermal Testing 2265


IVONA TTS Capable.

http://www.naturalreaders.com/




Recalling: FIGURE 9-3.


The atmosphere is also fairly transparent, at least in two wavebands. In the rest of the thermal spectrum, water vapor and carbon dioxide absorb most thermal radiation. (6μm ~ 8μm)

(6μm ~ 8μm)

(2–6 μm) (8–14 (15) μm)


SGuide-IRTContentPart 1 of 2■ Chapter 1 - Introduction to Principles & Theory■ Chapter 2 - Materials and Their Properties■ Chapter 3 - Thermal Instrumentation

Part 2 of 2■ Chapter 4 – Operating Equipment and Understanding Results■ Chapter 5 – Applications■ Appendices A, B, C


Chapter 1Principles & Theory


1.1 Introduction to Principles & TheoryInfrared/thermal testing involves the use of (1) temperature and (2) heat flowmeasurement as a means to predict or diagnose failure.

This may involve the use of contacting or noncontacting devices, or a combination of both. A fundamental knowledge of heat flow and the thermal behavior of materials is necessary to understand the significance of temperature and temperature changes on a test sample.

Contacting devices include thermometers of various types, thermocouples, thermopiles and thermochromic coatings.

Noncontacting devices include convection (heat flux) devices, optical pyrometers, infrared radiation thermometers, infrared Line scanners and infrared thermal imaging (thermographic) equipment.

Infrared thermography is the nondestructive, non-intrusive. noncontact mapping of thermal patterns on the surface of objects. It is usually used to diagnosethermal behavior and, thereby, to assess the performance of equipment and the integrity of materials, products and processes.


Keywords:Principles: temperature and heat flow measurement as a means to predict or diagnose failure.

Techniques: contacting or noncontacting devices, or a combination of both.

Contacting devices include: thermometers of various types, thermocouples, thermopiles and thermochromic coatings.

Noncontacting devices include: convection (heat flux) devices, optical pyrometers, infrared radiation thermometers, infrared Line scanners and infrared thermal imaging (thermographic) equipment.


The infrared thermal imaging equipment used in infrared thermography is available in numerous configurations and with varying degrees of complexity.The thermal maps produced by infrared thermal imaging instruments are called thermograms. To understand and interpret thermograms, thethermograpber must be familiar with the fundamentals of temperature and heat transfer, infrared radiative heat flow and the performance of infrared thermal imaging instruments and other thermal instruments.

An understanding of the equipment, materials and processes being observed is also important to effectively assess the full significance of infrared/thermalmeasurements. A more detailed discussion of the performance parameters of infrared thermal imaging instruments is provided in Chapter 3.

Keywords:■ infrared thermography - The thermal maps produced by infrared thermalimaging instruments are called thermograms.


1.2 Fundamentals of Temperature and Heat Transfer

Heat is a transient form of energy in which thermal energy is transient. Whatis often referred to as a heat source (such as an oil furnace or an electricheater) is really one form or another of energy conversion – the energy storedin one object being converted to heat and nowing to another object.

Heat flow is thermal energy in transit and heat always flows from warmer objects to cooler objects. (transient)

Temperature is a measure of the thermal energy contained in an object - the degree of hotness or coldness of an object that is measurable by any of anumber of relative scales.

Comments:“HBNDEv C9 -Transfer of heat energy can be described as either steady-state or transient 暂态. In the steady-state condition, heat transfer is constant and in the same direction over time.” –However, In this PPT, both steady state and transient are both transient form of energy.


The three modes of heat transfer are: ■ conductive,■ convective and ■ radiative.

All heat is transferred by one of these three modes. In most situations, beat istransferred by a combination of two or all three modes. Of these three modes of heat transfer, infrared thermography is most closely associated with theradiative process, but it is essential to study all three to understand the meaning of thermograms and to pursue a successful program of thermography. As a result of heat transfer, objects tend to increase ordecrease their temperature until they come to thermal equilibrium with their surroundings. To maintain a steadystate heat flow condition, energy must becontinuously supplied by some means of energy conversion so that the temperature differential, and hence the heat flow remains constant.


The three modes of heat transfer are: ■ conductive,■ convective and ■ radiative.

http://www.chem.purdue.edu/gchelp/liquids/character.html


The three modes of heat transfer are: Water in 3 phases.

http://dli.taftcollege.edu/streams/Geography/Animations/WaterPhases.html

http://dli.taftcollege.edu/streams/Geography/Animations/WaterPhases.swf


Temperature and Temperature ScalesTemperature is expressed in either absolute or relative terms. There are two absolute scales called Rankine (English system) and Kelvin (metric system). There are two corresponding relative scales called Fahrenheit (English system) and Celsius or centigrade (metric system). Absolute zero is the temperature at which no molecular action takes place. This is expressed as zero Kelvin or zero degrees Rankin (0 K or 0° R). Relative temperature is expressed as degrees Celsius or degrees Fahrenheit (°Cor °F). The numerical relations among the four scales are as follows:

converting ºC to ºF, (9/5 x ºC +32) ºFconverting ºF to ºC, (5/9) x (ºF -32) ºC

T Rankine = T Fahrenheit+ 459.7T Kelvin = T Celsius + 273.16

Exercise: Temperature (not temperature interval)0 ºC = 32 ºFthus -273.16 ºC = (-273.16 x 9/5 + 32) ºF = 459.7 ºF


Temperature and Temperature Scales

■ http://www.mathsisfun.com/temperature-conversion.html


Temperature and Temperature Scales

REMEMBER

0ºC = 32ºF

converting ºC to ºF, (9/5 x ºC +32) ºF

for my ASNT exam


Boston Tea Party – New governances not the Old Fahrenheit & ⅝”.


Boston Tea Party – New governances not the Old Fahrenheit & ⅝”.


The Mighty Fahrenheit & ⅝”, English System.


The Mighty Fahrenheit & ⅝”, English System.


Absolute zero is equal to - 273.16 °Cand also equal to approximately - 459.7 °F. To conveIt, a change in temperature or delta T (∆T) between the English and metric systems, the simple 9/5 (1.8 to 1) relationship is used:

∆T Fahrenheit (or º Rankine) = 9/5 x ∆T Celsius (or Kelvin)

or simply;

∆T Fahrenheit (or º Rankine) = 1.8 x ∆T Celsius (or Kelvin)

Table 1.1 (pages 12 to 14) is a conversion table that will assist in the rapidconversion of temperature between fabrenheit and celsius values.Instructions for the use of the table are shown at the top of the table. (not in this PPT)


Conductive Heat TransferConductive beat transfer is probably the simplest form to understand. lt is thetransfer of beat in stationary media. It is the only mode of heat flow in solids,but it can also take place in liquids and gases.

Conductive heat transfer occurs as the result of atomic vibrations (in solids) and molecular collisions (in liquids) whereby energy is moved, one molecule at a time, from higher temperature sites to lower temperature sites. An example of conductive heat transfer is when one end of a section of metal pipe warms up after a flame is applied to the other end. There are physical laws that allow the amount of conductive heat flow to be calculated, and they are presented here to show the factors on which conductive heat flow depends.

Keywords:■ atomic vibrations■ molecular collisions (atomic collisions in inert gas)


The Fourier conduction Law expresses the conductive heat flow, Q per unit area A, through a slab of solid material of thickness L as illustrated in Figure 1.1. Thermal resistance Rt is defined as:

Thermal conductivity is defined as:

Heat flow per unit area is defined as:


Where:• Q/A = the rate of heat transfer through the slab per unit area (BTU/h∙ft2) or

(W/m2) perpendicular to the flow,• L = the thickness of the slab (ft or m),• T1 = (°F) or (ºC) is the higher temperature (at the left),• T2 = the lower temperature (at the right)• k = the thermal conductivity of the slab material (BTU/h∙ft∙ºF) or (W/m∙K)• Rt = the thermal resistance of the slab material (°F∙h∙ft2fBTU) or (m2∙K/W)


The Fourier conduction Law ( One dimension heat flow)The mathematical relationship that describes heat transfer as a function of the material that heat is conducting through is known as Fourier's law and is given below.

Fourier’s Law: q = k∙A∙(TH-TC)∙L-1

Where:q = heat transfer per unit time (W)A = heat transfer area (m2)k = thermal conductivity of material (W/m∙K)L = material thickness (m)


Thermal conductivity is highest for metals such as aluminum and lower forporous materials such as brick. It is inversely proportional to thermalresistance.

K= 1/Rt

Comment: k ∝ 1/R, R= thermal resistivity and the thermal resistance Rt = L∙R

Thermal conductivity is highest for metals such as aluminum and lower for porous materials such as brick. It is inversely proportional to thermal resistance. In real terms, the Fourier expression means that the rate of heat flow increases with increasing temperature difference. increases with increasing thermal conductivity and decreases with increasing slab thickness. Heat flow may be expressed in English units or metric units.


Convective Heat TransferConvective heat transfer takes place in a moving medium and is almostalways associated with heat transfer between a solid and a moving fluid (such as air). Forced convection takes place when an external driving force, such as a wind or an air pump, moves the fluid. Free convection takes place when there is no external driving force - the temperature differences necessary for heat transfer produce density changes in the fluid. The warmer fluid rises as a result of increased buoyancy. In convective heat flow, heat transfer takes effect by direct conduction through the fluid and the mixing motion of the fluid itself. Figure 1.2 illustrates convective heat transfer between a flat plate and a moving fluid.


Figure 1.2: Convective heat flow


Figure 1.2: Convective heat flow

fluid velocity

Distance from

boundary layer

Thermal Boundary layer

Tsurface

T∞


The presence of the plate causes the velocity of the fluid to decrease to zeroat the surface and influences its velocity throughout the thickness of aboundary layer. The thickness of the boundary layer depends on the free fluidvelocity V∞ - the higher the free fluid velocity, the thinner the boundary layer.It is greatest for free convection where V∞ = 0. The rate of heat flow depends,in turn, on the thickness of the boundary layer as well as the temperaturedifference between Ts and T∞ , Ts being the surface temperature and T∞being the free field fluid temperature outside the boundary layer.


Newton's cooling law defines the convective heat transfer coefficient as:

where: h = (BTU/b-ft2-°F) or (W/m2-K)

This is rearranged to obtain an expression for convective heat flow per unit area:

If Rc= 1/h is the resistance to convective heat flow, then:


Rc is easier to use than h when determining combined conductive andconvective heat transfer because then they are additive terms.

In real terms, this expression means that the rate of convective heat flow increases with increasing temperature difference, increases with higher convective heat flow coefficient and decreases with increasing convective thermal resistance.

Conductive and convective heat transfer are very similar. In both, the heattransfer is directly proportional to the temperature difference and the speed atwhich th is energy is transferred (rate of heat flow) depends on the transfercoefficient of the media or material through which the heat energy flows. Bycomparison, radiative heat transfer takes place in accordance with a differentset of rules.


Radiative Heat TransferRadiative heat transfer is unlike the other two modes because:1. it occurs by electromagnetic emission and absorption in a manner similar

to light;2. it propagates at the speed of light;3. like light, it requires a direct line of sight; 4. the heat energy transferred is proportional to the fourth power T4 of the

temperature of the objects; and5. it can take place across a vacuum – in fact, a vacuum is the most efficient

medium for radiative heat transfer.

The electromagnetic spectrum is illustrated in Figure 1.3 and shows that X-rays. radio waves. light waves (ultraviolet and visible) and infrared radiation are all related. Radioactive heat transfer takes place in the infrared portion of the spectrum, from 0.75μm to about 100μm, although most practical measurements can be calculated to about 20μm . The symbols μm (μm is preferred) stand for micrometers or microns. A micron is one-millionth of a meter and the measurement unit for radiant energy wavelength. Wavelength is inversely related to frequency (longer wavelengths have lower frequencies).


Figure 1.3: Infrared in the electromagnetic spectrum

Practical Infrared Thermography λ; 2μm to 6μm and 8μm to 14μm


Figure 1.4: Infrared radiation leaving a target surface (ρετσ)

ρε

τ


1.3 Fundamentals of Radiative Heat FlowRadiation Exchange at the Target SurfaceThe measurement of infrared/thermal radiation is the basis for non-contacttemperature measurement and infrared thermography. The surface to beevaluated is called the target surface. Thermal infrared radiation leaving asurface is called exitance or radiosity. It can be emitted from the surface,reflected by the surface, or transmitted through the surface. This is illustratedin Figure 1.4.

The total radiosity is equal to the sum of the emitted component (We), the reflected component (Wr) and the transmitted component (Wt ).

It is important to note that the surface temperature Te is related to the emittedcomponent We only.

Keywords:■ Exitance■ Radiosity


Thermal infrared radiation impinging on a surface can be absorbed, reflected, or transmitted as illustrated in Figure 1.5. Kirchhoff's law states that the sum of the three components is always equal to the total received radiation, Et The fractional sum of the three components equals unity or 100 percent:

Et = Eα + Eρ + Eτ , (for blackbody Eε = Eα )

where:Et = total energy

Likewise, the sum of the three material properties, transmissivity, reflectivity and emissivity, also always equals unity:

τ + ρ + ε =1


Figure 1.5: Infrared radiation impinging on a target surface

Kirchhoff's law


Reflections off Specular and Diffuse SurfacesA perfectly smooth surface will reflect incident energy at an anglecomplementary to the angle of incidence as shown in Figure 1.5. This iscalled a specular reflector. A completely rough or structured surface willscatter or disperse all of the incident radiation. This is called a diffuse reflector.No perfectly specular or perfectly diffuse surface can exist in nature, and allreal surfaces have some diffusivity and some specularity. These surfacecharacteristics will determine the type and direction of the reflectedcomponent of incident radiation. When making practical measurements, the specularity or diffusivity of a target surface are taken into account by compensating for the effective emissivity (ε*) of the surface. The thermographer's use of effective emissivity is reviewed as part of the detailed discussion of equipment operation in Chapter 5.

Keywords:■ Specular reflector■ Diffuse reflector


Reflections off Specular and Diffuse Surfaces


Reflections off Specular and Diffuse Surfaces


Transient Heat ExchangeThe previous discussions of the three types of heat transfer deal with steadystate heat exchange for reasons of simplicity and comprehension. Heattransfer is assumed to take place between two points, each of which is at afixed temperature. However, in many applications, temperatures are intransition so that the values shown for energy radiated from a target surfaceare the instantaneous values at the moment measurements are made. Inmany instances, existing transient thermal conditions are exploited to usethermography to reveal material or structural characteristics in test articles. Ininfrared nondestructive testing of materials, thermal injection or activethermography techniques are used to generate controlled thermal transientflow based on the fact that uniform structural continuity results in predictablethermal continuity. These techniques will be discussed in greater detail inChapter 5.


Radiant Energy Related to Target Surface TemperatureAll target surfaces warmer than absolute zero radiate energy in the infraredspectrum. Figure 1.6 shows the spectral distribution of energy radiating fromvarious idealized target surfaces as a function of surface temperature (T) andwavelength (A.). Very hot targets radiate in the visible as well, and our eyescan see this because they are sensitive to light. The sun, for example, is at atemperature of about 6000 K and appears to glow white bot. The heatingelement of an electric stove at 800 K glows a cherry red and, as it cools, itloses its visible glow but continues to radiate. This radiant energy can be feltwith a hand placed near the surface even though the glow is invisible. Theidealized curves shown in Figure 1.6 are for perfect radiators known asblackbodies. Blackbodies are defined and discussed in greater detail later inthis chapter. Figure 1.6 also shows two key physical laws regarding infraredenergy emitted from surfaces.


Radiant Energy Related to Target Surface TemperatureAll target surfaces warmer than absolute zero radiate energy in the infraredspectrum.


The Stefan-Boltzmann law: W= σεT4

Where:W = radiant flux emitted per unit area (W/m2)ε = emissivity (unity for a blackbody target)σ = Stefan-Boltzmann constant= 5.673 x I0-8 W/m-2∙K-4

T = absolute temperature of target (K)

(Comments: for blackbody ε=1, α=ε.)

illustrates that W, the total radiant flux emitted per unit area of surface, (the area under the curve) is proportional to the fourth power of the absolute surface temperature T4. It is also proportional to a numerical constant σ, and the emissivity of the surface, ε.


Figure 1.6: Typical blackbody distribution curves and basic radiation laws

Stefan-Boltzmann LawRadiant Flux per Unit Area In W/cm2

W= σεT4

ε = emissivity (unity for a blackbody target)σ = Stefan-Boltzmann constant

= 5.673 x I0-8 W/m-2∙K-4

T = absolute temperature of target (K)

Wien's Displacement Lawλmax = b/Twhere: λmax = peak wavelength (μm)b = Wien's displacement constant(2897 or 3000 approximately)

Charlie C

hong/ Fion Zhang

Figure 1.6: Typical blackbodydistribution curves and basic radiation

laws


Wien's displacement law:

λmax = b/T

Where:λmax wavelength of maximum radiation (μm)b Wien's displacement constant or 2897 (μm∙K)

illustrates that the peak wavelength, λmax (μm) at which a surface radiates, is easily determined by dividing a constant b (approximately 3000) by the absolute temperature T (Kelvin) of the surface.


1.4 Practical Infrared Measurementsln practical measurement applications, the radiant energy leaves a targetsurface, passes through some transmitting medium. usually an atmosphericpath, and reaches a measuring instrument.

Therefore, when making measurements or producing a thermogram, three sets of characteristics must be considered:

1. characteristics of the target surface,2. characteristics of the transmitting medium and3. characteristics of the measuring instrument.

This is illustrated in Figure 1.7.


Figure 1.7: Three sets of characteristics of the infrared measurement problem

εobjρambτ assumed = 0

εatmτatm


Characteristics of the Target SurfaceTarget surfaces are separated into three categories; blackbodies, graybodiesand nongraybodies (also called real bodies, selective radiators or spectralbodies).

The target surfaces shown in Figure 1.6 are all perfect radiators (orblackbodies). A blackbody radiator is defined as a theoretical surface havingunity emissivity at all wavelengths and absorbing all of the radiant energyimpinging upon it.

Emissivity, in turn, is defined as the ratio of the radiant energy emitted from a surface to the energy emitted from a blackbody surface at the sametemperature. Blackbody radiators are theoretical and do not exist in practice. The surface of most solids are graybodies, that is, surfaces with high emissivities that are fairly constant with wavelength. Figure 1.8 shows thecomparative spectral distribution of energy emitted by a blackbody, agraybody and a nongraybody, all at the same temperature (300 K).


Figure 1.8: Spectral distribution of a blackbody, graybody and nongraybody


Referring back to Figure 1.5, the total exitance available to the measuring instrument has three components:

• emitted energy (We),• reflected energy (Wr) from the environment and other reflecting sources,

and• for nonopaque targets, energy transmitted through the target (Wt) from

sources behind the target.

Because a theoretical blackbody has an emissivity ε of 1.00, it will reflect and transmit no energy ρ = 0, τ = 0.

Real targets, however, are not blackbodies. and figure 1.9 shows the threecomponents that comprise Wx, the total exitance that an instrument sees when aimed at a real target surface. Because only the emittedcomponent, We, is related to the temperature of the target surface, itbecomes apparent that a significant part of the measurement problem iseliminating or compensating for the other two components. This is discussedin greater detail in Chapter 4.


Figure 1.9: Components of energy reaching the measuring instrument


Characteristics of the Transmitting MediumBecause the infrared radiation from the target passes through sometransmitting medium on its way to the target, the transmission and emissioncharacteristics of the medium in the measurement path must be considered when making non contact thermal measurement. No loss of energy or self emission (εatm) is encountered when measuring through a vacuum. However. most measurements are made through air. For short path length (a few meters, for example), most gases (including the atmosphere) absorb and emit very little energy and can be ignored. However. when highly accurate temperature measurements are required, the effects of atmospheric absorption must be taken into account. (τatm, εatm).


As the path length increases to more than a few meters, or as the airbecomes heavy with water vapor, atmospheric absorption may become asignificant factor. Therefore, it is necessary to understand the infraredtransmission characteristics of the atmosphere. Figure 1.10 illustrates thespectral transmission characteristics of a 10 m (33 ft) path of ground levelatmosphere at a temperature of 25 °C(77 °F) and 50 percent humidity.

It is immediately apparent that the atmosphere is not as transparent in the infrared ponion of the spectrum as it is in the visible ponion. Two spectral intervals have very high transmission. These are known as the 3 to 5 μm andthe 8 to 14μm atmospheric windows, and almost all infrared sensing andimaging instruments are designed to operate in one of these two windows.

The absorption segments shown in Figure 1.10 were formed by carbondioxide and water vapor, which are two of the major constituents in air. Formeasurements through gaseous media other than atmosphere, it isnecessary to investigate the transmission spectra of the medium beforevalidating the measurements, which is explained in greater detail in Chapter 2.


Figure 1.10; Transmission of 10m (33ft) of ground level atmosphere at 50 percent humidity and 25 °C(77ºF)

Perc

enta

ge T

rans

mis

sion

Wave Length μm


When there is a solid material, such as a glass or quartz viewing port,between the target and the instrument, the spectral characteristics of the solidmedia must be known and considered. Figure 1.11 shows transmissioncurves for various samples of glass. Most significant is the fact that glassdoes not transmit infrared energy at 10μm where ambient (30 °C, 86 °F)surfaces radiate their peak energy. In practice, infrared thermalmeasurements of ambient targets can never be made through glass. Onepractical approach to this problem is to eliminate the glass, or at least aportion through which the instrument can be aimed at the target. If a windowmust be present for personal safety, vacuum, or product safety, a materialmight be substituted that transmits in the longer wavelengths. Figure 1.12shows the spectral transmission characteristics of several infraredtransmitting materials, many of which transmit energy past 10μm. In additionto being used as transmitting windows, these materials are often used aslenses and optical elements in infrared sensors and imagers. Of course, astargets become hotter, and the emitted energy shifts to the shorterwavelengths, glass and quartz windows pose less of a problem and are evenused as elements and lenses in high temperature sensing instruments.Characteristics of the measuring instrument are addressed in Chapter 4.


Figure 1.11: Transmission, absorption and reflectance characteristics of glass


Figure 1.12: Transmission curves of various infrared transmitting material


Figure 1.12: Transmission curves of various infrared transmitting material

More reading on measuring techniques: http://www.testo.in/knowledge-base/online-training/thermography/measurements-of-glass-and-metal-and-specular-reflection/index.jsp


Convective Heat TransferConvective heat transfer, often referred to simply as convection, is the transfer of heat from one place to another by the movement of fluids. Convection is usually the dominant form of heat transfer in liquids and gases. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes of conduction (heat diffusion) and advection (heat transfer by bulk fluid flow). The term convection can sometimes refer to transfer of heat with any fluid movement, but advection is the more precise term for the transfer due only to bulk fluid flow. The process of transfer of heat from a solid to a fluid, or the reverse, is not only transfer of heat by bulk motion of the fluid, but diffusion and conduction of heat through the still boundary layer next to the solid. Thus, this process without a moving fluid requires both diffusion and advection of heat, a process that is usually referred to as convection. Convection that occurs in the earth's mantle causes tectonic plates to move. Convection can be "forced" by movement of a fluid by means other than buoyancy forces (for example, a water pump in an automobile engine). Thermal expansion of fluids may also force convection. In other cases, natural buoyancy forces alone are entirely responsible for fluid motion when the fluid is heated, and this process is called "natural convection". An example is the draft in a chimney or around any fire. In natural convection, an increase in temperature produces a reduction in density, which in turn causes fluid motion due to pressures and forces when fluids of different densities are affected by gravity (or any g-force). For example, when water is heated on a stove, hot water from the bottom of the pan rises, displacing the colder denser liquid, which falls. After heating has stopped, mixing and conduction from this natural convection eventually result in a nearly homogeneous density, and even temperature. Without the presence of gravity (or conditions that cause a g-force of any type), natural convection does not occur, and only forced-convection modes operate. The convection heat transfer mode comprises one mechanism. In addition to energy transfer due to specific molecular motion (diffusion), energy is transferred by bulk, or macroscopic, motion of the fluid. This motion is associated with the fact that, at any instant, large numbers of molecules are moving collectively or as aggregates. Such motion, in the presence of a temperature gradient, contributes to heat transfer. Because the molecules in aggregate retain their random motion, the total heat transfer is then due to the superposition of energy transport by random motion of the molecules and by the bulk motion of the fluid. It is customary to use the term convection when referring to this cumulative transport and the term advection when referring to the transport due to bulk fluid motion.

http://en.wikipedia.org/wiki/Convective_heat_transfer


Chapter 1Review Questions 13. d

14. eI5. d16. e17. b18. d19. a20. d21. b22. e

1. b2. d3. c4. a5. c6. d7. b8. b9. d10. d11. a12. a

Q&A


Q1. At a temperature of absolute zero:a. hydrogen becomes a liquid.b. all molecular motion ceases.c. salt water is part solid and part liquid.d. fahrenheit and celsius readings are the same.

Q2. Conductive heat transfer cannot take place:a. within organic materials such as wood.b. between two solid materials in contact.c. between dissimilar metals.d. across a vacuum.

Q3. The only three modes of heat transfer are:a. resistive, capacitive and inductive.b. steady state, transient and reversible.c. conduction, convection and radiation.d. conduction. convection and absorption.


Q4. Heat can only flow in the direction from:a. hotter objects to colder objects.b. colder objects to houer objects.c. more dense objects to less dense objects.d. larger object to smaller objects.

Q5. Thermal resistance is:a. analogous to electrical current.b. proportional to the fourth power of emissivity.c. inversely proportional to the rate of heat flow by conduction.d. a measure of material stiffness.

Q6. The radiation of thermal infrared energy from a target surface:a. occurs most efficiently in a vacuum.b. is proportional to the fourth power of the absolute surface temperature.c. is directly proportional to surface emissivity.d. is all of the above.


Q7. The mode of heat transfer most closely associated with infrared thermography is:a. induction.b. radiation.c. convection.d. conduction.

Q8. To convert a fahrenheit reading to celsius:a. divide by 1.8.b. subtract 32 and divide by 1.8.c. multiply by 1.8 and add 32.d. add 273.

Q9. Thermal radiation reaching the surface of an object can be:a. absorbed only in the presence of atmosphere.b. reflection and absorbed only in a vacuum.c. transmitted only if the surface is organic.d. absorbed, reflected and transmitted.


Q10. The follow ing spectral band is included in the infrared spectrum:a. 0.1 to 5.5 μm.b. 0.3 to 10.6 μm.c. 0.4 to 20.0 μm.d. 0.75 to 100 μm.

Q11. Most instruments used in infrared thermography operate somewhere within the;a. 2 to 14 μm spectral region.b. 5 to 10 μm spectral region.c. 10 to 20 μm spectral region.d. 20 to 100 μm spectral region.

Q12. As a surface cools, the peak of its radiated infrared energy:a. shifts to longer wavelengths.b. shifts to shorter wavelengths.c. remains constant if emissivity remains constant.d. remains constant even if emissivity varies.


Q13. The peak emitting wavelength of a 300 °C(572 °F) blackbody is approximately:a. 1.5 μm.b. 3 μm.0. 10 μm.d. 5 μm.

Q14. An opaque surface with an emissivity of 0.04 would be:a. transparent to infrared radiation.b. a fairly good emitter.c. almost a perfect reflector. (τ=0, ε=0.04, ρ = 0.96)d. almost a perfect emitter.

Q15. If a surface has an emissivity of 0.35 and a reflectivity of 0.45. its transmissivity would be:a. impossible to detennine without additional information.b. 0.80.c. 0.10.d. 0.20. [1-(0.35+0.45)]

λmax = b/T( in K) = 2897/(300+273.15) μm


Q16. In forced convection, the boundary layer:a. increases as the fluid velocity increases.b. remains the same as the fluid velocity increases.c. decreases as the fluid velocity increases.d. increases in proportion to the fourth power of the fluid velocity.

Q17. When heating one end of a car key to thaw a frozen automobile door lock, heat transfer from the key to the lock is an example of:a. forced convection.b. conductive heat transfer.c. free convection.d. radiative heat transfer.

Q18. The infrared atmospheric window that transmits infrared radiation best is the:a. 2.0 to 3.0 μm region.b. 3.0 to 6.0 μm region.c. 6.0 to 9.0 μm region.d. 9.0 to 11.0 μm region.


Q19. The spectral band in which glass transmits infrared radiation best is the:a. 2.0 to 3.0 μm region.b. 3.0 to 6.0 μm region.c. 6.0 to 9.0 μm region.d. 9.0 to 11.0 μm region.

Q20. Reflectance of infrared radiation by a glass surface is greatest in the:a. 2.0 to 3.0 μm region.h. 3.0 to 6.0 μm region.c. 6.0 to 9.0 μm region.d. 9.0 to 11.0 μm region.

Q21. A diffuse reflecting surface is:a. a polished surface that reflects incoming energy at a complementary angle.b. a surface that scatters reflected energy in many directions.c. also called a specular reflecting surface.d. usually transparent to infrared radiation.


Q22. In the 8 to 14 μm spectral region:a. the atmosphere absorbs infrared radiant energy almost completely.b. the atmosphere reflects infrared radiant energy almost completely.c. the atmosphere transmits infrared energy very efficiently.d. infrared instruments do not operate very accurately.


Chapter 2Materials and Their Properties


2.1 Materials CharacteristicsA knowledge of the characteristics of materials is important to thethermographer for numerous reasons, but the two most important are theneed to know how a particular target surface emits. transmits and reflectsinfrared radiant energy. and the need to know how heat flows within aparticular material.

2.2 Surface Properties of MaterialsThe surface properties of materials include emissivity, reflectivity and transmissivity.


Emissivity εWhen using infrared thermography to measure surface temperature of a target. it is essential to know the effective emissivity (ε*) of the surface material. This is the value that must be set into the instrument's menu under the specific conditions of measurement for the instrument to display an accurate surface temperature value. When attempting to make temperature measurements on a target of unknown emissivity, an estimate of emissivity may be the only available alternative. There are numerous reference tables available that list generic values of emissivity for common materials and these can be used as guides. Table 2.2 is an example of a reference table. As previously noted. emissivity depends on the material and the surface texture. It may also vary with surface temperature and with the spectral interval over which the measurement is made. These variations, though usually small , cannot always be ignored.

Emissivity- factors affecting are; surface texture, surface temperature spectral interval over the measurement is made, angle of view.


For an emissivity reference table to be useful. conditions of target temperature and spectral interval (wavelength) must also be presented. If the temperature and wavelength listed do not correspond to the actual measurement conditions. the emissivity listed must be considered to be a rough estimate. Even if there is an exact match to the measurement conditions, the lookup method is not the best approach for accurate temperature measurement. Ideally. the way to determine effectiveemissivity is to measure it with one of the several established protocols. using a sample of the actual target surface material and the actual instrument to be used for the measurement mission. The protocols for measuring effective emissivity of material samples are discussed in Chapter 4.


Reflectivity ρReflectivity of a surface generally increases as emissivity decreases. Foropaque graybody surfaces τ= 0. the sum of emissivity and reflectivity is unity (1.0). Therefore. an opaque graybody surface with a low effective cmissivity will be highly reflective, which can result in erroneous temperature readings even if the correct emissivity is set into the instrument. These errors can be the result of either point source reflections, background reflections or both entering the instrument . There are two components of reflected energy the diffuse componenl and the specular component. If the surface is relatively specular (smooth). most of the reflected energy is specular, that is. it reflects off the surface at an angle complementary to the angle of incidenct. If the surface is relatively diffuse (textured) most of the renected energy is scattereduniformly (haphazardly?) in all directions regardless of the angle of incidence.

Keywords:Therefore. an opaque graybody surface with a low effective cmissivity will behighly reflective


Errors because of point source reflections are usually larger when the targetsurfaces are specular, and errors because of background reflections are notaffected by the specularity or diffusivity of the target surface.

Both types of reflective errors are more serious when the target surface is cool compared to the temperature of the point source or the background because the point source makes a greater contribution to the total radiant exitance than the target does. In practice, the thermographer can learn to recognize and avoid errors due to point source reflections. The thermographer also can learn to measure and compensate for errors due to background reflection. This is discussed in Chapter 4.

Keypoints:Errors because of point source reflections are usually larger when the targetsurfaces are specular


Transmissivity τWhen the target surface is a non-graybody, the target material may be partlytransparent to infrared radiation. This means the target material has atransmissivity greater than 0. Due to this transparency. radiant thermal energy may be transmitted through the target from sources behind the target. This energy may enter the instrument and cause temperature measurement errorseven if the correct emissivity is set into the instrument and reflective errors are eliminated. Although errors due to transmission are the least common in practice. errors due to energy transmiued through the target usually require the most sophisticated procedures to correct them. In most cases, spectral filtering (spectral adaptation) is the best solution. Methods for correcting theseerrors are discussed in Chapters 4 and 5.

Keywords:■ spectral filtering■ non-graybody (could be any others like black body, selective emitter, could be a body with τ > 0)


View AngleThe angle between the instrument's line of sight and the surface material willhave a minimal effect on the material properties described above, providingthis angle is kept as close as possible to normal (perpendicul ar) and nogreater than ±30 degrees from normal (for many nonmetallic surfaces thismay be increased to as large as ±60 degrees from normal. if unavoidable).

If it is not possible to view a target at an angle within this range, the effectiveemissivity may Change. particularly if it is low to begin with. This will mostlikely compromise the accuracy of temperature measurements. Note that the emissivities listed in Table 2.2 are normal emissivities and are not valid atacute viewing angles. On curved (nonflat) surfaces. view angle can be evenmore critical and measurements should be made cautiously.

Note:An acute angle is an angle whose degree measure is greater than 0 but less than 90.


2.3 Heat Conducting Properties of MaterialsThe use of infrared themlography for nondestructive material testing isgenerally based on the assumption that uniform structural continuity providesuniform thermal continuity. Both unstimulated and stimulated approaches tothermographic material testing depend on this assumption. as will bediscussed in greater detail in Chapters 4 and 5. It is necessary. therefore, thatthe thermographer have a clear basic understanding of the manner in whichheat flows within a material and the material properties that affect this flow.

Keywords:The use of infrared themlography for nondestructive material testing isgenerally based on the assumption that uniform structural continuity providesuniform thermal continuity.


Thermal ConductivityThermal conductivity k is the relative one dimensional capability of a material to transfer heat. It affects the speed (thus time, t) that a given quantity of heatapplied to one point in a slab of material will travel a given distance within thatmaterial to another point cooler than the first. Thermal conductivity is high formetals and low for porous materials. It is logical. therefore. that heat will beconducted more rapidly in metals than in more porous materials. Althoughthermal conductivity varies slightly with temperature in solids and liquids andwith temperature and pressure in gases, for practical purposes it can beconsidered a constant for a particular material. Table 2.1 is a list of thermalproperties for several common materials.


Heat Capacity (thermal capacitance)The heat capacity of a malerial or a structure describes its ability to store heat.It is the product of the specific thermal energy Cp and the density ρ of thematerial. When thermal energy is stored in a structure and then the structureis placed in a cooler environment, the sections of the structure that have lowheat capacity will change temperature more rapidly because less thermalenergy is stored in them. Consequently, these sections will reach thermalequilibrium with their surroundings sooner than those sections with higherheat capacity.

thermal capacitance (Volumetric heat capacity) J/(m³·K) = Cp x ρ

whereρ is density kg/m³Cp is specific heat capacity J/(kg·K)


The term thermal capacitance is used to describe heat capacity in terms of an electrical analog. where loss of heat is analogous to loss of charge on acapacitor. Structures with low thermal capacitance reach equilibrium sooner when placed in a cooler environmcnt than those with high thermalcapacitance. This phenomenon is exploited when performing unstimulatednondestructive testing of structures, specifically when locating water saturated sections on flat roofs. This is discussed in greater detail in Chapter 5,


Thermal DiffusivityAs in emissivity ε. the heat conducting properties of materials may vary from sample to sample. depending on variables in the fabrication process and other factors.Thermal diffusivity α is the 3D expansion of thermal conductivity in any given material sample. Diffusivily relates more to transient heat flow, whereas conductivity relates to steady state heat flow. It takes into account the thermal conductivity k of the sample, its specific heat Cp and its density ρ. Its equation is

α = k/(ρ∙Cp) cm2s-1.

whereα = Heat diffusivity m²/sρ is density kg/m³Cp is specific heat capacity J/(kg·K)k is thermal conductivity W/(m·K)

Together, can be considered the volumetric heat capacity J/(m³·K).https://en.wikipedia.org/wiki/Thermal_diffusivity


Because thermal diffusivity of a sample can be measured directly using infrared thermography, it is used extensively by the materials flaw evaluation community as an assessment of a test sample's ability to carry heat away, in all directions, from a heat injection site. Table 2.1 lists thermal diffusivities for several common materials in increasing order of thermal diffusivity. Several protocols for measuring the thermal diffusivity of a test sample are described by Maldague.


Thermal Diffusivity

Diffusivily relates more to transient heat flow, whereas conductivity relates to steady state heat flow.


Partial 2.1


Partial Table 2.1


Partial Table 2.2


Thermal DiffusivityAs in emissivity ε. the heat conducting properties of materials may vary from sample to sample. depending on variables in the fabrication process and other factors. Thermal diffusivity α is the 3D expansion of thermalconductivity in any given material sample. Diffusivily relates more to transient heat flow, whereas conductivityrelates to steady state heat flow. It takes into account the thermal conductivity k of the sample, its specific heat Cp, and its density ρ. Its equation is

α = k/(ρ ∙Cp) cm2s-1.

for my ASNT exam


Chapter 2Review Questions 1. c

2. b3. a4. d5. a6. b7. a8. b9. b10. b

Q&A


1. The best way to determine the effective emissivity of a target surface is:a. to look it up in a table.b. to calculate it.c. to measure the effective emissivity of the material itself or a similar

sample.d. all of the above.

2. For an opaque graybody target surface, emissivity equals:a. 1/refleclivity.b. 1- reflectivity.c. 1.0.d. reflectivity to the fourth power.

3. The effective emissivity of a surface is always affected by:a. the material, its surface texture and the viewing angle.b. the material, its thermal conductivity and humidity.c. the material, its surface texture and its thermal diffusivity.d. the material, its visible color and its thermal conductivity.


4. When measuring the temperature of a nongraybody target:a, the viewing angle is not critical.b. always assume an emissivity of 1.0.c. reflections off the near surface may be ignored.d. errors may be caused by hot sources behind the target.

5. The effective emissivity of a target surface:a, can vary at different wavelengths.b. is the same for all wavelengths if the viewing angle is kept constant.c. is always higher at longer wavelengths.d. is always lower at longer wavelengths.

6. Unfinished, unoxidized metal surfaces usually have:a. high and uniform emissivities.b. low and uniform emissivities.c. non-graybody characteristics.d. low specular reflectivity.


7. Thermal diffusivity is:a. high for metals and low for porous materials.b. the same for all metals.c, low for metals and high for porous materials.d. the same for all porous materials.

8. Thermal diffusivity is:a, the same as diffuse reflectivity.b. related more to transient heat flow than to steady State heat flow.c. related more to steady stale heat flow than to transient heat flow.d. the same as spectral transmittance.

9. Thermal capacitance:a. describes the heating of a condenser.b. expresses the heat capacity of a material in a form analogous to

electrical capacitance.c. is zero for a blackbody radiator.d. describes the maximum temperature rating of a capacitor.


10. A highly textured surface is said to be diffuse. A smooth surface is said to be:a. opaque.b. specular.c. convex.d. transparent.


Chapter 3Thermal Instrumentation


3.1 Thermal Instrumentation OverviewEquipment for temperature measurement and thermography includescontacting as well as noncontacting devices. Contacting devices fortemperature measurement include thermopiles. thermocouples, liquidthermometers, gas expansion devices (bourdon gas thermometers), liquidcrystals (cholesterol crystals ?), heat flux indicators and fiber optic sensors.

Aside from some specialized instruments, the vast majority of noncontactingtemperature measurement devices are infrared sensing instruments andsystems. Infrared sensing instruments and systems are divided into (1) pointsensors (radiation thermometers), (2) line scanners and (3) thermal imagers.

This chapter begins with a review of contacting thermal measurement instruments and a discussion of the basic configurations of infrared sensing and imaging instruments. This is followed by a discussion of performance parameters and, finally, descriptions of commercial thermal sensing and imaging equipment, thermographic image processing software and imagehard copy recording accessories.


What is ThermopileA thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Thermopiles do not respond to absolute temperature, but generate an output voltage proportional to a local temperature difference or temperature gradient.

Thermopiles are used to provide an output in response to temperature as part of a temperature measuring device, such as the infrared thermometers widely used by medical professionals to measure body temperature. They are also used widely in heat flux sensors (such as the Moll thermopile and Eppley pyrheliometer) and gas burner safety controls. The output of a thermopile is usually in the range of tens or hundreds of millivolts. As well as increasing the signal level, the device may be usedto provide spatial temperature averaging. Thermopiles are also used to generate electrical energy from, for instance, heat from electrical components, solar wind, radioactive materials, or combustion. The process is also an example of the PeltierEffect (electric current transferring heat energy) as the process transfers heat from the hot to the cold junctions.

http://en.wikipedia.org/wiki/Thermopile


Thermopile- Thermoelectric Seebeck module

http://en.wikipedia.org/wiki/Thermopile


The Working Principle: Thermopile, composed of multiple thermocouples in series. If both the right and left junctions are the same temperature, voltages cancel out to zero. However if one side is heated and other side cooled, resulting total output voltage is equal to the sum of junction voltage differentials.


Leopoldo Nobili (1784 - 1835) first used the thermoelectric effect for IR radiation measurement using a “pile” of Bismuth and Antimony contacts. The measure of this effect is called the thermoelectric- or Seebeck- coefficient. For most conducting materials this coefficient is rather low, only few semiconductors possess rather high coefficients. Since the voltage of a single thermoelectric cell is very low, lots of such cells arranged in a series connection achieve a larger signal, making a “pile” of thermo-elements.


What is a IR Thermopile? (non-contact)A thermopile is a serially-interconnected array of thermocouples, each ofwhich consists of two dissimilar materials with a large thermoelectric powerand opposite polarities. The thermocouples are placed across the hot andcold regions of a structure and the hot junctions are thermally isolated fromthe cold junctions. The cold junctions are typically placed on the siliconsubstrate to provide effective heat sink. In the hot regions, there is a blackbody for absorbing the infrared, which raises the temperature according to theintensity of the incident infrared. These thermopiles employ two differentthermoelectric materials which are placed on a thin diaphragm having a lowthermal conductance and capacitance.

http://www.ge-mcs.com/download/temperature/930-164A-LR.PDF


IR Thermopiles Sensor (non-contact)


IR Thermopile Quad Sensor (non-contact)


ThermocoupleGeneral description: Thomas Seebeck discovered in 1821 that when two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit. (Seebeck effect). The junctions can be exposed, grounded or ungrounded. The thermocouple is normally directly connected to a standard temperature controller. Thermocouples are among the easiest temperature sensors used in science and industry and very cost effective. (usually less than $50.00)

thermocouple embedded in Dalton cartridge heater

http://www.deltat.com/thermocouple.html


ThermocoupleA thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots, where a temperature differential is experienced by the different conductors (or semiconductors). It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples are a widely used type of temperature sensor for measurement and control, and can also convert a temperature gradient into electricity. Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (°C) can be difficult to achieve.

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements. Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes. Thermocouples are also used in homes, offices and businesses as the temperature sensors in thermostats, and also as flame sensors in safety devices for gas-powered major appliances.

http://en.wikipedia.org/wiki/Thermocouple


Liquid or Gas Expansion DevicesMany physical properties change with temperature, such as the volume of a liquid, the length of a metal rod, the electrical resistance of a wire, the pressure of a gas kept at constant volume, and the volume of a gas kept at constant pressure. Filled-system thermometers use the phenomenon of thermal expansion of matter to measure temperature change.

The filled thermal device consists of a primary element that takes the form of a reservoir or bulb, a flexible capillary tube, and a hollow Bourdon tube that actuates a signal-transmitting device and/or a local indicating temperature dial. A typical filled-system thermometer is shown in Figure 7-1. In this system, the filling fluid, either liquid or gas, expands as temperature increases. This causes the Bourdon tube to uncoil and indicate thetemperature on a calibrated dial.


Bourdon Gas Thermometers


Liquid Crystal ThermometerA liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change color to indicate different temperatures. Liquid crystals possess the mechanical properties of a liquid, but have the optical properties of a single crystal. Temperature changes can affect the color of a liquid crystal, which makes them useful for temperature measurement. The resolution of liquid crystal sensors is in the 0.1°Crange. Disposable liquid crystal thermometers have been developed for home and medical use. For example if the thermometer is black and it is put onto someone's forehead it will change colour depending on the temperature of the person.

There are two stages in the liquid crystals: 1. the hot nematic stage is the closest to the liquid phase where the molecules are freely moving around and only partly ordered. 2. the cold smectic stage is closest to a solid phase where the molecules align themselves into tightly wound chiral matrixes.

Liquid crystal thermometers portray temperatures as colors and can be used to follow temperature changes caused by heat flow. They can be used to observe that heat flows by conduction, convection, and radiation. In medical applications, liquid crystal thermometers may be used to read body temperature by placing against the forehead. These are safer than a mercury-in-glass thermometer, and may be advantageous in some patients, but do not always give an exact result, except the analytic liquid crystal thermometer which show the exact temperature between 35.5 to 40.5° Celsius.

http://en.wikipedia.org/wiki/Liquid_crystal_thermometer


Liquid Crystal ThermometerA liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change color to indicate different temperatures. Liquid crystals possess the mechanical properties of a liquid, but have the optical properties of a single crystal.


Thermocouple

http://www.omega.com/temperature/z/pdf/z021-032.pdf

Thermocouple grade wires

Stainless steel sheath

Wire junction

Adjustable nut

Flexible SS sheath


Bimetallic Thermometers

http://www.omega.com/temperature/z/pdf/z021-032.pdf


Resistance Thermometers - Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material, typically platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes and it is this predictable change that is used to determine temperature. They are slowly replacing the use of thermocouples in many industrial applications below 600 °C, due to higher accuracy and repeatability.

http://www.npl.co.uk/content/ConMediaFile/113

http://en.wikipedia.org/wiki/Resistance_thermometer


In RTD devices; Copper, Nickel and Platinum are widely used metals. These three metals are having different resistance variations with respective to the temperature variations. That is called resistance-temperature characteristics. Platinum has the temperature range of 650°C, and then the Copper and Nickel have 120°Cand 300°Crespectively. The figure-1 shows the resistance-temperature characteristics curve of the three different metals. For Platinum, its resistance changes by approximately 0.4 ohms per degree Celsius of temperature.

The purity of the platinum is checked by measuring R100 / R0. Because, whatever the materials actually we are using for making the RTD that should be pure. If it will not pure, it will deviate from the conventional resistance-temperature graph. So, α and β values will change depending upon the metals.

http://en.wikipedia.org/wiki/Resistance_thermometer


Platinum Resistance Thermometer

http://www.aoip.com/product/670-standard-platinum-resistance-thermometers/


Platinum Resistance Thermometer


Resistance Temperature Detector (RTD) - Principle of Operation, Materials, Configuration and Benefits by Innovative Sensor Technology

OverviewInnovative Sensor Technology is a world-class manufacturer of thin-film RTD temperature sensors, capacitive humidity sensors, and mass flow sensors at the component level. With our state-of-the-art manufacturing technology, Innovative Sensor Technology offers both standard and custom sensors to satisfy unique applications. Additionally, our highly qualified staff is now offering R&D consulting services for industrial applications. Our sensors have applications in the automotive, HVAC, appliance, controls, and test & measurement industries.

Resistance Temperature Detector (RTD) - Principle of OperationAn RTD (resistance temperature detector) is a temperature sensor that operates on the measurement principle that a material’s electrical resistance changes with temperature. The relationship between an RTD resistance and the surrounding temperature is highly predictable, allowing for accurate and consistent temperature measurement. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD resistance can be calculated, and the temperature can be determined.

http://www.azom.com/article.aspx?ArticleID=5573


RTD MaterialsDifferent materials used in the construction of RTD offer a different relationship between resistance and temperature. Temperature sensitive materials used in the construction of RTD include platinum, nickel, and copper; platinum being the most commonly used. Important characteristics of an RTD include the temperature coefficient of resistance (TCR), the nominal resistance at 0 degrees Celsius, and the tolerance classes. The TCR determines the relationship between the resistance and the temperature. There are no limits to the TCR that is achievable, but the most common industry standard is the platinum 3850 ppm/K. This means that the resistance of the sensor will increase 0.385 ohms per one degree Celsius increase in temperature. The nominal resistance of the sensor is the resistance that the sensor will have at 0 degrees Celsius. Although almost any value can be achieved for nominal resistance, the most common is the platinum 100 ohm (pt100). Finally, the tolerance class determines the accuracy of the sensor, usually specified at the nominal point of 0 degrees Celsius. There are different industry standards that have been set for accuracy including the ASTM and the European DIN. Using the values of TCR, nominal resistance, and tolerance, the functional characteristics of the sensor are defined.



RTD ConfigurationsIn addition to different materials, RTD are also offered in two major configurations: wire wound and thin film. Wire wound configurations feature either an inner coil RTD or an outer wound RTD. An inner coil construction consists of a resistive coil running through a hole in a ceramic insulator, whereas the outer wound construction involves the winding of the resistive material around a ceramic or glass cylinder, which is then insulated.

The thin film RTD construction features a thin layer of resistive material deposited onto a ceramic substrate through a process called deposition. A resistive meander is then etched onto the sensor, and laser trimming is used to achieve the appropriate nominal values of the sensor. The resistive material is then protected with a thin layer of glass, and lead wires are welded to pads on the sensor and covered with a glass dollop.

Thin film RTD have advantages over the wire wound configurations. The main advantages include that they are less expensive, are more rugged and vibration resistant, and have smaller dimensions that lead to better response times and packaging capabilities. For a long time wire wound sensors featured much better accuracy. Thanks to recent developments, however, there is now thin film technology capable of achieving the same level of accuracy.



Operations of RTD An RTD takes a measurement when a small DC current is supplied to the sensor. The current experiences the impedance of the resistor, and a voltage drop is experienced over the resistor. Depending on the nominal resistance of the RTD, different supply currents can be used. To reduce self-heating on the sensor the supply current should be kept low. In general, around 1mA or less of current is used. An RTD can be connected in a two, three, or four-wire configuration. The two-wire configuration is the simplest and also the most error prone. In this setup, the RTD is connected by two wires to a Wheatstone bridge circuit and the output voltage is measured. The disadvantage of this circuit is that the two connecting lead wire resistances add directly two the RTD resistance and an error is incurred.


2-Wire Configuration


The four-wire configuration consists of two current leads and two potential leads that measure the voltage drop across the RTD. The two potential leads are high resistance to negate the effect of the voltage drop due to current flowing during the measurement. This configuration is ideal for canceling the lead wire resistances in the circuit as well as eliminating the effects of different lead resistances, which was a possible problem with the three-wire configuration. The four-wire configuration is commonly used when a highly accurate measurement is required for the application.

4-Wire Configuration



Benefits of Thin Film RTD There are many options when considering contact temperature measurement, including thermocouples, thermistors, and RTD (wire wound and thin film). While thermocouples can handle very high temperatures and thermistors are inexpensive, there are many advantages of RTD. Some of these advantages include their accuracy, precision, long-term stability, and good hysteresis characteristics. Even beyond these, there are advantages of thin film RTD over wire wound, including smaller dimensions, better response times, vibration resistance, and relative inexpensiveness. New advancements has even made thin film technology just as accurate as wire wound at higher temperatures ranges.



ThermistorA thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiter, temperature sensors (NTC type typically), self-resetting overcurrent protectors, and self-regulating heating elements.

Thermistors differ from resistance temperature detectors (RTDs) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °Cto 130 °C

http://en.wikipedia.org/wiki/Thermistor


Thermistor

http://swordrock.wordpress.com/category/robotic-2/


Thermistor

http://en.wikipedia.org/wiki/Thermistor


3.2 Contacting Thermal Measuring DevicesThe most commonly used contacting devices include bimetallic thermometers, thermochromic liquid crystals, thermocouples, resistance thermometer,thermistors and heat flux indicators. These devices are discussed briefly here.For more detailed information, refer to ASNT Nondestructive TestingHandbook. third edition: Volume 3. Infrared and Thermal Testing.

■ Bimetallic ThermometersBimetallic thermometers are sensors constructed of dissimilar metallic strips bonded together. Typically. different iron nickel alloys are used. The strips differ in temperature coefficient of expansion such that temperature changes result in predictable bending of the assembly. Arranged in a spiral or helical configuration. one end of the bimetallic element is fixed and the other end is attached to a pointer. Properly calibrated, the angular position of the pointer can be made to indicate temperature on a scale.


■ Thermochromic Liquid CrystalsThermochromic liquid crystals (also called cholesterol crystals) change colorwith temperature. Coatings made of liquid crystals are commonly used astemperature threshold indicators. Depending on the mixture, a coatingapplied to a surface will change color predictably when the surface exceeds athreshold temperature. The color change may be reversible or irreversible.and the sensing range for most mixtures is limited to a narrow temperaturespan (here range and span are use interchangeably, actually range is the equipment detection range and span is the instrument adjustable interval of interest within the range) . Typically. a set of liquid crystal markers provides a selection of transition temperatures. This allows the user to select theappropriate marker for the desired temperature.

Keywords:Threshold temperature


■ ThermocoupleThermocouples are contact temperature sensors based on the thermoelectriceffect. or Seebeck effect. Thomas Seebeck discovered that, when two dissimilar metals are joined at both ends and these ends are at differenttemperatures, a predictable direct current will flow through the circuit. Thethermoelectric coefficient determines the relationship between this currentand the temperature difference between the two junctions. This coefficient isknown for each type of thermocouple. To configure a thermometer. the circuitis broken and the open-circuit voltage is measured by a volt meter. One of thetwo junctions is then held al a reference temperature. such as an ice bath,and the voltage is calibrated to indicate the temperature of the other junction. which then becomes the temperature sensing junction. Thermopiles are banks of thermocouples connected in parallel or in series to increase output gradient. The reference temperature is important because of the thermocouples' non linear response.

Keywords:■ thermoelectric coefficient■ nonlinear response


■ Resistance ThermometersResistance temperature detector (RTDs) are contact sensors thaI measuretemperature by a predictable change in resistance as a function oftemperature. Platinum is the most popular resistance temperature detectormaterial because of its excellent stability and its linear response totemperature change. Other materials used include nickel, copper, tungstenand iridium.

In operation, the resistance temperature detector may be placed in a bridge circuit such that the bridge output voltage is a measure of the resistance and hence the temperature at the resistance temperature detector. A more accurate measurement may be achieved by using a constant current source and a digital volt meter (DVM). such that the digital volt meter reading is proportional to the resistance temperature detector resistance and hence the temperature at the resistance temperature detector.

Keypoints:■ bridge circuit■ constant current source


■ ThermistorsThermistors are also sensors that measure temperature by a predictablechange in resistance as a function of temperature.

■ Thermosistors are made of semiconductor materials. Thermosistors are high impedance devices.

■ Whereas resistance temperature detectors are low impedance devices.

Thermistors typically are more sensitive to temperature changes than resistance temperature detectors but thermistors are not as stable.

Keywords:Thermistors typically are more sensitive to temperature changes than resistance temperature detectors


■ Heat Flux IndicatorsHeat flux indicators are heat flow meters and are used to measure rates inconduction, convection, radiation and phase change systems such asbuilding walls, boiler tubes and air conditioning ducts. A typical heat fluxindicator consists of a sensitive thermopile, composed of many fine gagethermocouples connected in series on opposite sides of a flat core with known and stable thermal resistance. The entire assembly is covered withprotective material.

The voltage generated across the thermopile is calibrated to be a measure of the steady state heat flux through the device. Transient heat flux can be related to the transient thermopile output and the geometry of the device.


A Heat Flux SensorIntroductionHeat flux sensors works to convert heat energy into an electrical signal. The intensity of the electrical signal is directly proportional to the heat rate being converted. During exposure of the heat flux sensor to heat energy, the sensor generates an electrical input that allows for the heat flux to be determined. To understand the working principle to a heat flux sensor, lets first look at this transducer in more detail.

How does it work?A working description of a basic heat flux sensor (also known as a heat flux gauge) is based on the attachment of thermocouple junctions aligned between an object making sure that these junctions are in contact with the hot and cold surface to this object (Figure 1). A standard measurement of heat flux density is presented as watts per square meter (W/m2).

http://www.azosensors.com/article.aspx?ArticleId=28


Figure 1. A basic heat flux sensor.

The principle to this thermopile concept is based on the transfer of heat through a thin film. This thin film is sandwiched between the two thermocouple junctions that both have contact with either the hot or cold surface to an object. As heat passes through the thermocouple junction and penetrates the thin film, the movement forces the thermocouple to initiate a voltage output that is proportional to the amount of heat penetrating the film. Many thermocouple junctions can be grouped together to help amplify the voltage output signal in response to heat energy.


Filling Materials


Heat Flux Sensor



Heat Flux Sensor



Heat Flux Sensor



3.3 Optical PyrometersOptical pyrometers include brightness pyrometers and infrared pyrometers.Infrared pyrometers are also called infrared radiation thermometers. Varioustypes are discussed in the next section. Brightness pyrometers are also calledmatching pyrometers. They incorporate a calibrated light source (lamp)powered by a calibrated current supply. Looking through a viewer, theoperator matches the brightness of the target to be measured with thebrightness of the calibrated lamp. The adjustment control is calibrated intemperature units, such that when the brightnesses are matched, the controlindicates the temperature of the target to be measured.


PyrometerA pyrometer is a device that is used for the temperature measurement of an object. The device actually tracks and measures the amount of heat that is radiated from an object. The thermal heat radiates from the object to the optical system present inside the pyrometer. The optical system makes the thermal radiation into a better focus and passes it to the detector. The output of the detector will be related to the input thermal radiation. The biggest advantage of this device is that, unlike a Resistance Temperature Detector (RTD) and Thermocouple, there is no direct contact between the pyrometer and the object whose temperature is to be found out.

Optical (brightness) PyrometerIn an optical pyrometer, a brightness comparison is made to measure the temperature. As a measure of the reference temperature, a color change with the growth in temperature is taken. The device compares the brightness produced by the radiation of the object whose temperature is to be measured, with that of a reference temperature. The reference temperature is produced by a lamp whose brightness can be adjusted till its intensity becomes equal to the brightness of the source object. For an object, its light intensity always depends on the temperature of the object, whatever may be its wavelength. After adjusting the temperature, the current passing through it is measured using a multimeter, as its value will be proportional to the temperature of the source when calibrated. The working of an optical pyrometer is shown in the figure below.

http://www.instrumentationtoday.com/optical-pyrometer/2011/08/


PyrometerA pyrometer is a type of remote sensing thermometer used to measure temperature. Various forms of pyrometers have historically existed. In the modern usage, it is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry and sometimes radiometry. The thermal radiation can be used to determine the temperature of an object's surface.

The word pyrometer comes from the Greek word for fire, "πυρ" (pyro), and meter, meaning to measure. The word pyrometer was originally coined to denote a device capable of measuring the temperature of an object by its incandescence, or the light that is emitted by the body as caused by its high temperature. Modern pyrometers are capable of interpreting temperatures of room temperature objects by measuring radiation flux in the infrared spectrum.

A modern pyrometer has an optical system and a detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (temperature T) is related to the thermal radiation or irradiance j* of the target object through the Stefan–Boltzmann law, the constant of proportionality σ, called the Stefan-Boltzmann constant and the emissivity ε of the object.

J* = εσT4

This output is used to infer the object's temperature. Thus, there is no need for direct contact between the pyrometer and the object, as there is with thermocouples and resistance temperature detectors (RTDs).

http://en.wikipedia.org/wiki/Pyrometer


Brightness Pyrometers



Brightness Pyrometers – Wien’s Law



The scientist Wilhelm Wien (1864–1928) has described the relation between a solid body’s temperature and its emitting peak wave length by following equation:

λmax =2898 / T

T = Temperature in K (Kelvin)λ = Wavelength in μm

Using this law we can calculate the specific peak emission wave length of any material or body: A human body, of a surface temperature of approx. 35°Cor 308 K calculates into a peak wavelength of 9,4 μm; a cat of 38°Ctemperature into 9,3 μm. According to Max Planck (1858 –1947) the intensity curve of all emitted wave lengths for a solid body is rather broad. For our example above this means we cannot distinguish human from the cat by their infrared spectrum.




3.4 Basic Configurations of Infrared RadiationSensing and Imaging Instruments

In terms of configuration and operation, most thermal imagers are considered to be extensions of radiation thermometers or radiation thermometers plus scanning optics. The performance parameters of thermal imagers are extensions of the performance parameters of radiation thermometers. To aid comprehension. the basic measurement problem is discussed in this chapter in terms of the measurement of a single point. It is then expanded to cover thermal scanning and imaging. Figure 3.1 illustrates the basic configuration of an infrared sensing instrument (infrared radiation thermometer), showing the components necessary to make measurements. Collecting optics (an infrared lens, for example) are necessary for gathering the energy emitted by the target spot and focusing this energy onto the sensitive surface of an infrared detector.


The processing electronics unit amplifies and conditions the signal from the infrared detector and introduces corrections for such factors as detector ambient temperature drift and target effective surface emissivity. Generally. a readout. such as a meter. indicates the target temperature and an analog output is provided. The output signal is used to record, display. alarm, control, correct or any combination of these.


Figure 3.1: Basic configuration of an infrared radiation thermometer


Infrared DetectorAn infrared detector is at the heart of every infrared sensing and imaging instrument. whatever its configuration. Infrared detectors can sense infrared radiant energy and produce useful electrical signals proportional to the temperature of target surfaces. Instruments using infrared detectors and optics to gather and focus energy from the targets onto these detectors are capable of measuring target surface temperatures with sensitivities better than 0.10 °C(0.18 ºF). and with response limes in the microsecond (μs) range. An instrument that measures the temperature of a spot on a target in this manner is called an infra red radiation thermometer. An instrument that combines this measurement capability with a means or mechanism for scanning the target surface is called an infrared thermal imager. It can produce thermal maps, or thermograms, where the brightness intensity or color hue of any spot on the map represents the apparent temperature of the surface at that point.


Figure 3.2 illustrates the spectral responses of various infrared radiation detectors. Radiant energy impinging on their sensitive surfaces causes all infrared detectors to respond with some kind of electrical change. This maybe an impedance change. a capacitance change, the generation of an electromotive force (emf) known as Voltage, or the release of photons, depending on the type of detector.

Infrared detectors are divided into (1) thermal detectors and (2) photon detectors. Thermal detectors have broad uniform spectral responses, somewhat lower sensitivities and slower response times (measured in millisecond): photon detectors (also called photo detectors) have limited spectral responses. higher peak sensitivities and faster response times (measured in microsecond). Thermal detectors usually operate at or near room temperature. whereas photon detectors are usually cooled to optimize performance.

Keywords:■ Thermal Detector- broad uniform spectral responses/ slower■ Photon Detector- limited spectral responses/ faster


Figure 3.2: Response Curves of Various Infrared Detectors


DiscussionSubject: Why (or How) there are 2 MCT; MCT(215K), MCT(77K)?


The mercury cadmium telluride (HgCdTe) detectors shown in Figure 3.2 arephoton detectors cooled to 77 K (-321°F) for operation from 8 to 12 μm and to 195 K (-109 °F) for operation from 3 to 5 μm. Because of their fast response, these detectors are used extensively in high speed scanning and imaging applications. In contrast to the mercury cadmium telluride detector, the radiation thermopile shown in Figure 3.2, is a broad band thermal detectoroperating uncooled. It is used extensively for spot measurements. Because itgenerates a direct current electromotive force proportional to the radiantenergy reaching its surface. it is ideal for use in portable, battery poweredinstruments. The lead sulfide (PbS) detector is typical of those used in radiation thermometers that measure and control the temperature of very hot targets. Its peak sensitivity at 3μm matches the peak energy emitted by a 1000K (727 °C= 1340 °F) graybody.

Because of the atmospheric absorption considerations previously discussed. most infrared thermal imagers operate in either the 3 to 5 μm or the 8 to 12 μm spectral region.

Note: 195K = [(-273+195) x 9/5] + 32 = -108 °F



Photon Detectors

Indium Antimony


Infrared Optics - Lenses, Mirrors and FiltersThere are two types of infrared optics; (1) refractive (lenses. filters, windows)and (3) reflective (mirrors). Refractive optics transmit infrared wavelengths ofinterest. When used for higher temperature applications. their throughputlosses can usually be ignored. When used in low temperature measurementinstruments and imagers, absorption is often substantial and must beconsidered when making accurate measurements.

Reflective optics. which are more efficient are not spectrally selective and somewhat complicate the optical path. Reflective optics are used more often for low temperature applications. where the energy levels cannot warrant throughput energy losses. When an infrared radiation thermometer is aimed at a target, energy is collected by the optics in the shape of a solid angle determined by the configuration of the optics and the detector.


The cross section of this collecting beam is called the field of view (FOV) of the instrument and it detennines the size of the area (spot size) on the target surface that is measured by the instrument at any given working distance. On scanning and imaging instruments this is called the instantaneous field of view (lFOV) and becomes one picture element on the thermogram. An infrared interference filter is often placed in front of the detector to limit the spectral range of the energy reaching the detector. The reasons for spectral selectivity will be discussed later in this chapter.

Processing ElectronicsThe processing electronics unit amplifies and conditions the signal from theinfrared detector and introduces corrections for factors such as detectorambient temperature drift and effective target surface emissivity.

In radiation thermometers, a meter is usually provided to indicate the target’sapparent temperature. An analog or digital output signal is provided to record,display, alarm, control, correct or any combination of these.


Field of View (FOV)A field of view (FOV) is a specification that defines the size of what is seen in the thermal image. The lens has the greatest influence on what the FOV will be, regardless of the size of the array. Large arrays, however, provide greater detail, regardless of the lens used, compared to narrow arrays. For some applications, such as work in outdoor substations or inside a building, a large FOV is useful. While smaller arrays may provide sufficient detail in a building, more detail is important in substation work. See Figure 4-7.


Figure 4-7. The field of view (FOV) is a specification that defines the area that is seen in the thermal image when using a specific lens.


What is IFOV?A measure of the spatial resolution of a remote sensing imaging system. Defined as the angle subtended by a single detector element on the axis of the optical system. IFOV has the following attributes:

■ Solid angle through which a detector is sensitive to radiation.■ The IFOV and the distance from the target determines the spatial

resolution.

A low altitude imaging instrument will have a higher spatial resolution than a higher altitude instrument with the same IFOV (?)

http://www.ssec.wisc.edu/sose/tutor/ifov/define.html


What is IFOV?IFOV (instantaneous field of view) – smallest object detectable. The IFOV (instantaneous field of view), also known as IFOVgeo (geometric resolution), is the measure of the ability of the detector to resolve detail in conjunction with the objective. Geometric resolution is represented by mradand defines the smallest object that can be represented in the image of the display, depending on the measuring distance. The thermography, the size of this object corresponds to a pixel. The value represented by mradcorresponds to the size of the visible point [mm] a pixel at a distance of 1 m.

http://www.academiatesto.com.ar/cms/?q=ifov


Instantaneous Field of View (IFOV)An instantaneous field of view (IFOV) is a specification used to describe the capability of a thermal imager to resolve spatial detail (spatial resolution). The IFOV is typically specified as an angle in milliradians (mRad). When projected from the detector through the lens, the IFOV gives the size of an object that can be seen at a given distance. An IFOV measurement is the measurement resolution of a thermal imager that describes the smallest size object that can be measured at a given distance. See Figure 4-8. It is specified as an angle (in mRad) but is typically larger by a factor of three than the IFOV. This is due to the fact that the imager requires more information about the radiation of a target to measure it than it does to detect it. It is vital to understand and work within the spatial and measurement resolution specific to each system. Failure to do so can lead to inaccurate data or overlooked findings.

H

D in meter

IFOV, θ in milli-radian

H in mm = D∙ θ


Figure 4-8. An IFOV measurement is the measurement resolution of a thermal imager that describes the smallest size object that can be measured at a given distance. IFOV is similar to seeing a sign in the distance while IFOV measurement is similar to reading the sign, either because it is closer or larger.

Instantaneous field of view (spatial resolution)/ IFOV measurement (measurement of resolution)


3.5 Scanning and ImagingWhen problems in temperature monitoring and control cannot be solved bythe measurement of one or several discrete points on a target surface. itbecomes necessary to spatially scan - that is to move the collecting beam orthe instrument's field of view relative to the target. This is usually done byinserting a movable optical element into the collecting beam as illustrated inFigure 3.3.


Figure 3.3: Adding the scanning element(s) for imaging


Line ScanningWhen the measurement of a single spot on a target surface is not sufficient.infrared line scanners can be used to assemble information concerning thedistribution of radiant energy along a single straight line. Quite often, this is allthat is necessary to locate a critical thermal anomaly. The instantaneousposition of the scanning element is usually controlled or sensed by anencoder or potentiometer so that the radiometric output signal can beaccompanied by a position signal output and be displayed on a recordingdevice and/or fed out to a computer based process control system. A typicalhigh speed commercial line scanner develops a high resolution thermal mapby scanning normal to the motion of a moving target such as a paper web or a strip steel process. The resulting output is a thermal strip map of the process as it moves normal to the scan line. The scanning configuration is illustrated in Figure 3.4. The output signal information is in a real time computer compatible format and can be used to monitor, control or predict the behavior of the target.


Figure 3.4: Line scanner scanning configuration


Two-dimensional Scanning - Thermal ImagingThe three common imaging configurations that produce infrared thermogramsare (1) optomechanical scanning, (2) electronic scanning and (3) focal plane array imaging.

Of the three, optomechanical scanning was the most common until the mid-1990s. Focal plane array imagers have replaced scanning imagers in most applications.


Optomechanical ScanningTo scan optomechanically in two dimensions generally requires two scanningelements. Although an almost infinite variety of scanning patterns can begenerated using two moving elements. the most common pattern is rectilinear.This scanning pattern is most often accomplished by two elements, eachscanning a line normal to the other. A representative rectilinear scanner isillustrated in the schematic of Figure 3.5. Its scanning mechanism comprisestwo oscillating mirrors behind the primary lens, a high speed horizontalscanning mirror and a slower speed vertical scanning mirror.


One performance limitation of single-detector optomechanical scanners is atrade off between speed of response and signal-to-noise ratio of the detector.These instruments require high speed cooled photodetectors that are pushedto their performance limits as the desired real time scanning rate is increased.Multidetector scanners reduce the constraints on detector performance byadding detector elements that share the temporal 瞬时频域；时间频率域,该属性的值会被自动赋值 spatial burden, allowing for faster frame rales with noreduction in signal-to-noise ratio or improving the signal-to-noise ratio with no decrease in frame rate.

Keypoints:■ One performance limitation of single-detector optomechanical scanners is

a trade off between speed of response and signal-to-noise ratio of the detector.

■ Multidetector scanners reduce the constraints on detector performance by

adding detector elements that share the temporal spatial burden.


Figure 3.5: Optomechanlcally scanned infrared imager


Electronic Scanning – Pyroelectric Vidicon Thermal ImagersElectronically scanned thermal imaging systems based on pyrovidicons andoperating primarily in the 8 to 14 μm (LWIR) atmospheric window arecommonly used. They provide qualitative thermal images and are classified as thermal viewers. A pyroelectric vidicon or pyrovidicon is configured the same as a conventional video camera tube except that it operates in the infrared (2 to 20 μm) region instead of the visible spectrum. Image scanning is accomplished electronically in the same manner as in a video camera tube.

Keywords:Operating primarily in the 8 to 14 μm (LWIR) atmospheric windowThey provide qualitative thermal images and are classified as thermal viewers.



Pyroelectric Vidicon Thermal Imagers


Focal Plane Array ImagingFirst introduced to the commercial market in 1987. cooled infrared focal planearray (IRFPA) imagers have evolved into compact, qualitative andquantitative thermal imagers without scanning optics. These devices havebeen replacing optomechanically scanned imagers for many applications.The first uncooled infrared focal plane array imagers have been used by themilitary for several years and became available to thermographers in 1997.Figure 3.6 is a schematic of a typical uncooled infrared focal plane arrayimager. Microbolometer arrays are also available.

Keypoints:FPA could be cooled or uncooled (photon detectors (QWIP) or microbolometers types)


Figure 3.6: Typical uncooled infrared focal plane array imager


FPA


IRFPA - Large IR mosaic prototype array with 35 H2RG arrays. The array has a total of nearly 147 million pixels. Each of the H2RG arrays has 2,048×2,048 pixels.

http://www.osa-opn.org/home/articles/volume_19/issue_6/features/high-performance_infrared_focal_plane_arrays_for_s/


IRFPA

http://ececavusoglu.girlshopes.com/cmoslineararraysirsensor/


Infrared sensors with 3D ROIC for cooled dual-band IR arrays

http://www.militaryaerospace.com/articles/2013/07/army-irfpa-roic.html


3.6 Performance Parameters of Infrared Sensing and Imaging Instruments

To select an appropriate instrument for an application, or to determinewhether an available instrument will perform adequately. it is necessary forthe thermographer to understand its performance parameters. Theperformance parameters for point sensing instruments (infrared radiationthermometers) are temperature range, absolute accuracy, repeatability,temperature sensitivity, speed of response, target spot size and working distance (field-of-view-spatial resolution), output requirements. sensorenvironment and spectral range.

For scanners and imagers the performance parameters include temperature range, absolute accuracy, repeatability, temperature sensitivity, total field of view (TFOV), instantaneous field of view (lFOV), measurement spatial resolution (IFOVmeas), frame repetition rate, minimum resolvable temperature (MRT), temperature sensitivity, image processing software,sensor environment and spectral range.


FOV-Field of viewThe field of view (FOV) of the thermal imager describes the area visible with the thermal imager (See Fig. 1.3). It is determined by the lens used (e.g. 32°wide-angle lens or 9° telephoto lens

http://www.testo.in/knowledge-base/online-training/thermography/measuring-spot-measuring-distance/index.jsp


IFOV- Instantaneous Field of View (Smallest measurable object) This defines the size of a pixel according to the distance. With a spatial resolution of the lens of 3.5 mrad and a measuring distance of 1 m, the IFOVgeometric has an edge length of 3.5 mm and is shown on the display as a pixel (See Fig. 1.4).

To obtain a precise measurement, the measuring object should be 2–3 times larger than the smallest identifiable object (IFOVgeo).

The following rule of thumb therefore applies to the smallest measurable object (IFOVmeasure):

IFOVmeas ≈ 3 x IFOVgeo



IFOV- Instantaneous Field of view

IFOVmeas ≈ 3 x IFOVgeo



More Reading: IFOVmeas: Measurement Spot Size

The one specification related to IFOV and Spatial resolution but perhaps of more practical importance to quantitative thermographers than the IFOV angle, and unfortunately continues to be missing from many imager data sheets, is “Measurement Spot Size“. Defined as the size of the area from which radiometric measurement data are derived, it is used to determine the minimum size of the measurement area where accurate measurements can be made for a given target / distance. As mentioned before, the measurement spot size is not a single IFOV footprint.

Measurement spot size is intrinsically dependent on the IFOV footprint size, but usually consists of several single IFOV footprint elements. Measurement Spot Size is not easily derived from IFOV because imager software algorithms typically rely on several pixels to derive the measurement value, even if ultimately only one pixel is used for the measurement. Without knowing how many pixels are used in the algorithm, or the effect of adjacent pixels on the one the data is taken from, it is impossible to use IFOV alone to calculate an accurate, real world spot size.

http://www.irinfo.org/11-01-2012-swirnow/


How many pixels (IFOV footprints) are needed for an accurate measurement and in what orientation is the manufacturer’s trade secret? Some manufacturers imply the measurement is made from a 3 x 3 array of pixels or 9 pixels total. Although this does not appear to be an absolute number, no other information is given. Some manufacturers have Field of View calculators on their websites; these will calculate the IFOV value for a given camera, lens and distance, but still give spot size as an IFOV value of a single footprint which then needs to be multiplied by some number of pixels. There is a term “MFOV” which stands for the “Measurement FOV”, also known as IFOVmeas, and it defines the resolution of the imager for measuring temperature. It is also expressed as an angle in mRad and because multiple IFOV elements are required to make a measurement, it is always larger than the IFOV value and is more representative of the actual spot size required for an accurate measurement.



There are a couple of ways to determine the measurement spot size. As previously stated, most manufacturers have a spot size measurement calculator which will allow you to approximate the measurement spot size. However, the best way to measure it is using your camera / lens combination with a procedure such as the one detailed in the standard for measuring distant / target size values for infrared imaging radiometers. This standard is available from Infraspection Institute.



Qualitative Versus Quantitative ThermographyFor scanners and imagers. one distinction based on instrument performancelimitations is that between qualitative and quantitative thermography.

A qualitative thermogram displays the distribution of infrared radiance over the target surface, uncorrected for target, instrument and media characteristics.

A quantitative thermogram displays the distribution of infrared radiosity overthe surface of the target. corrected for target, instrument and mediacharactcristics so as to approach a graphic representation of true surfacetemperature distribution.


Performance parameters of qualitative thermographic instruments. therefore, do not include temperature accuracy, temperature repeatability andmeasurement spatial resolution.

Generally, instruments that include the capability to produce quantitative thermograms are more costly than qualitative instruments and require periodic recalibration. Many applications can be solved without the time andexpense of quantitative thermography, but others require true temperature mapping. A discussion of the most appropriate applications for quantitative and qualitative thermal imagers is included in Chapter 5.

Keywords:Performance parameters of qualitative thermographic instruments, therefore, do not include temperature accuracy, temperature repeatability andmeasurement spatial resolution.


Performance parameters of qualitative thermographicinstruments, therefore, do not include temperature accuracy, temperature

repeatability and measurement spatial resolution.


Performance Characteristics of Point Sensing Instruments (Radiation Thermometers)The American Society for Testing and Materials defines infrared point sensinginstruments as infrared radiation thermometers even though they do notalways read out in temperature units. Some read out directly in apparentradiant power units such as W·m-2· s-1 (or BTU· ft -2∙ h-1), some provide aclosure or alarm signal at a selectable temperature and some others provideonly difference indications on a light emitting diode display.


Temperature RangeTemperature range is a statement of the high and low limits over which thetarget temperature can be measured by the instrument. A typical specificationwould be. for example. "temperature range 0 to 1000 °C(32 to 1832 ºF).“

Absolute AccuracyAbsolute accuracy, as defined by the National Lnstitute of Standards andTechnology (NIST) standard, entails the maximum error. over the full range,that the measurement will have when compared to this standard blackbody reference. A typical specification would be, for example. "absolute accuracy ±0.5 °C(±0.9 ºF) ± 1 percent of full scale.“


RepeatabilityRepeatability describes how faithfully a reading is repeated for the same target over the short and long term. A typical specification would be, for example, "repeatability (short and long term) of ±0.25 °C(±0.45ºF) “.Temperature range and absolute accuracy will always be interrelated; for example, the instrument might be expected to measure a range of temperatures from 0 to 200 °C(32 to 392 OF) with an absolute accuracy ±2 °C(±3.6ºF) over the entire range. This could alternately be specified as ±1 percent absolute accuracy over full scale. On the other hand, the best accuracy might be required at some specific temperature, say 100 °C(212 °F). In this case, the manufacturer should be informed and the instrument could be calibrated to exactly match the manufacturer's laboratory calibration standard at that temperature. Because absolute accuracy is based on traceability to the NIST standard. it is difficult for a manufacturer to comply with a tight specification for absolute accuracy. An absolute accuracy of ±0.5 °C(±0.9 °F) or ±1 percent of full scale is about as tight as can be reasonably specified. Repeatability, on the other hand, can be more easily ensured by the manufacturer and is usually more important to the user.


Temperature SensitivityTemperature sensitivity defines the smallest target temperature change the instrument will dctect. Temperature sensitivity is also called thermal resolution or noise equivalent temperature difference (NETD). It is the smallest temperature change at the target surface that can be clearly sensed at the output of the instrument. This is almost always closely associated with the cost of the instrument. so unnecessarily fine temperature sensitivity should not be specified. An important rule to remember is that. for any given instrument. target sensitivity will improve for hotter targets where there is more energy available for the instrument to measure. Temperature sensitivity should be specified, therefore, at a particular target temperature near the low end of the range of interest. A typical specification for temperature sensitivity would be, for example, “temperature sensitivity of 0.25 °C(0.45 ºF) at a target temperature of 25 °C(77 ºF)." In this case, the sensitivity of the instrument would improve for targets hotter than 2 °C(36 °F).

Keywords:Temperature sensitivity is also called thermal resolution or noise equivalent temperature difference (NETD).


Temperature sensitivity is also called: thermal resolution or

noise equivalent temperature difference (NETD).

for my ASNT exam


Speed of ResponseSpeed of response is how long it takes for an instrument to update ameasurement. It is defined as the time it takes the instrument output torespond to a step change in temperature at the target surface.

Figure 3.7 shows this graphically. The sensor time constant is defined by convention to be the time required for the output signal to reach 63 percent of a step change in temperature at the target surface. Instrument speed ofresponse is usually specified in terms of a large percentage of the full reading, such as 95 percent. As illustrated in Figure 3.7, this takes about five time constants, and is generally limited by the detector used (on the order of microseconds for photodctcetors and milliseconds for thermal detectors).


A typical speed of response specification would be, for example. "speed of response (to 95 percent) = 0.05 s.“ It should be understood that there is always a tradeoff between speed of response and temperature sensitivity.

As in all instrumentation systems, as the speed of response for a particular device becomes faster (instrumentation engineers call this a wider information bandwidth) the sensitivity becomes poorer (lower signal- to-noise ratio). If the speed of response is specified to be faster than is necessary for the application, the instrument may not have as good a temperature sensitivity as might be possible otherwise.


Figure 3.7: Instrument speed to response and time constant


Target Spot Size and Working DistanceTarget spot size D and working distance d define the spalial resolution of theinstrument. In a radiation thermometer, spot size is the projection of thesensitive area of the detector at the target plane. It may be specified directly,“1 cm at 1 m (0.4 in. at 3 ft)," for example, but it is usually expressed in more general terms such as a field of view solid angle ( 10 mrad, 1 degree, 2 degree) or a field-of-view ratio (ratio of spot size to working distance - for example, d/15, d/30, d/75.

A milliradian (mrad) is an angle with a tangent of 0.001. A d/15 ratio means that the instrument measures the emitted energy of a spot one-fifteenth the size of the working distance: 3 cm at 45 cm (1.2 in. at 18 in .) f

or example. Figure 3.8 illustrates these relationships and also shows how spot size can be approximated quickly based on working distance and field-of-view information furnished by the manufacturer. A typical specification for spot size would be. for example. "target spot size = 2 degrees from 1 m (39 in.) to ∞.“


Figure 3.8: Instrument field-of-view determination


This would take into account the shortest working distance at which theinstrument could be focused (1 m or 39 in.). For some instruments designedfor very close workiing distances, the simple d∙D-1 ratio does not always apply.

If closeup information is not clearly provided in the product literature, the instrument manufacturer should be consulted. For most applications and for middle and long working distance (greater than 1m or 3 ft), the following simple calculation (illustrated in Figure 3.8) will closely approximate targetspot size: where:

D ≡ αd

D = spot size (approximate),α = field-or-view plane angle in radians,d = distance to the target.

A 17.5 mrad (1 degree) field of view means a d∙D-1 ratio of 60 to1 and a 35 mrad (2 degree) field of view means a d∙D-1 ratio of 30 to 1.


D ≡ αdD = spot size (approximate),α = field-or-view plane angle in radians,d = distance to the target.

A 17.5 mrad (1 degree) field of view means a d∙D-1 ratio of 60 to1 and a 35 mrad (2 degree) field of view means a d∙D-1 ratio of 30 to 1. (?)

for D ≡ α∙d

given that α = 17.5mrad, D=17.5mm if d=1000mm, thusd/D = 1000/17.5 = 57.296 ≈ 60

This is to say the IFOV measurement ration = 1000 ∙ 1/α where α in mRad.


Calculating d/D ratiofor D ≡ α∙d1º = 0.0175rad, for D= d x radian thus; when d=1m, D= 0.0175md/D = 1/0.0175 = 57.3


EXAM score!

D=σ∙dIFOV ratio = d/D or 1/σ

(care on unit used!)

for my ASNT exam


Output RequirementsOutput requirements for radiation thermometers can vary widely - from asimple digital indicator and an analog signal to a broad selection of output functions, including digital output (binary coded decimal); high, low and proportional set points; signal peak or valley sensors; sample and hold circuits; and even closed loop controls for specific applications. On board microprocessors provide many of the above functions on even inexpensive standard portable models of radiation thermometers.


Sensor EnvironmentSensor environment includes the ambient extremes under which theinstrument will perform within specifications and the extremes under which itcan be stored without damage when not in operation. For a portable radiationthermometer. a typical specifi cation for sensor environment would be as followas.

1. Operating temperature is 0 to 37°C(32 to 100°F)2. Humidity is at 20 to 80 percent relative (not condensing).3. Atmospheric pressure is at -610 m to +2440 m (-2000 to +8000 ft) above

sea level.4. Storage temperature (nonoperating) ranges from -15 to +60°C(5 to140°F).

Frequently in process control applications, the sensor must be permanently installed in a somewhat more extreme environment involving smoke, soot. high temperature and even radioactivity. For these applications, manufacturers provide a wide range of enclosures that offer special protective featu res such as air cooling, water cooling, pressurization, purge gases and shielding.


Spectral RangeSpectral range denotes the portion of the infrared spectrum over which the instrument will operate. The operating spectral range of the instrument is often critical to its performance and, in many applications. can be exploited to solve difficult measurement problems. The spectral range is determined by the detector and the instrument optics. as shown in Figure 3.9. Here, the flatspectral response of a radiation thermopile detector is combined with that of a germanium lens and an 8 to 14 μm band pass filter. The instrument characterized is suitable for general purpose temperature measurement ofcool targets through atmosphere. The transmission spectrum of a 0.3 km (0. 19 mil) atmospheric ground level is also shown. An infrared interference filter is often placed in front of the detector to limit the spectral range of the energyreaching the detector.


Figure 3.9: Spectral response of an instrument determined by detector andoptics spectra


the following three classes of filters are common:

1. High pass filter pass (高通) energy only at wavelengths longer than adesignated wavelength.

2. Low pass filters (低通) pass energy only at wavelengths shorter than a designated wavelength.

3. Band pass filters similar to the one shown in Figure 3.9. pass radiation within a designated spectral band (8 to 14 μm. for example).

注意: 上述的高通,低通, 定义需要求证.作为学会书籍-上述定义为学会标准答案定义.


FiltersMany thermographic applications depend upon the use of specialized filters to obtain a useful image or measurement. Before using filters, it is importantto know the exact spectral response of the system as determined by thedetector and the lens material. Responses within the long or short wavebandscan vary from system to system. It is also important to understand how the selected filter interacts with the detector’s response.

There are three generic filter designations:1. High-pass filters, which allow only shorter wavelengths (SW) to pass2. Low-pass filters, which allow only longer wavelengths (LW) to pass3. Band-pass filters, which allow only a designated band of wavelengths to

pass

Comments: High-pass filters, which allow only shorter wavelengths (SW) to pass

(which only allow high frequency IR to pass)

Charlie Chong/ Fion Zhang Infrared Thermography Reading 1 HBNDEv-C9


Spectrally selective instrumems use band pass filters to allow only a veryspecific broad or narrow band of wavelengths to reach the detector. (Acombination of a spectrally selective detector and a filter can also be used.)This can make the instrument highl y selective to a specific material whosetemperature is to be measured in the presence of an intervening medium oran interfering background. Solving measurement problems by means ofspectrally selective instruments is discussed in greater detail in Chapter 4.

For general purpose use and for measuring cooler targets cooler than about500°C(932°F), most manufacturers of radiation thermometers offerinstruments operating in the 8 to 14μm atmospheric window. For dedicateduse on hotter targets. shorter operating wavelengths are selected. usuallyshorter than 3μm. One reason for choosing shorter wavelengths is that thisenables manufacturers to use commonly available and less expensive quartzand glass optics, which have the added benefit of being visibly transparent formore convenient aiming and sighting. Another reason is that estimatingemissivity incorrectly will result in smaller temperature errors whenmeasurements are made at shorter wavelengths.


Thermographers have learned that a good general rule to follow, particularly when dealing with targets of low or uncertain emissivities, is to work at theshortest wavelengths possible without compromising sensitivity or riskingsusceptibility to reflections from visible energy sources.

Keypoints:1. targets cooler than about 500°C(932°F), most manufacturers of radiation

thermometers offer instruments operating in the 8 to 14μm atmosphericwindow.

2. For dedicated use on hotter targets. shorter operating wavelengths are selected. usually shorter than 3μm. use commonly available and less expensive quartz and glass estimating emissivity incorrectly will result in smaller temperature

errors when measurements are made at shorter wavelengths.


MWIR OR LWIR?For general purpose use and for measuring cooler targets cooler than about500 °C(932 °F). most manufacturers of radiation thermometers offerinstruments operating in the 8 to 14 μm atmospheric window. For dedicateduse on hotter targets. shorter operating wavelengths are selected. usuallyshorter than 3 μm. One reason for choosing shorter wavelengths is that thisenables manufacturers to use commonly available and less expensive quartzand glass optics, which have the added benefit of being visibly transparent formore convenient aiming and sighting. Another reason is that estimatingemissivity incorrectly will result in smaller temperature errors whenmeasurements are made at shorter wavelengths.

Thermographers have learned that a good general rule to follow, particularly when dealing with targets of low or uncertain emissivities, is to work at theshortest wavelengths possible without compromising sensitivity or riskingsusceptibility to reflections from visible energy sources.


3.7 Performance Characteristics of Scanners and Imagers

Because an infrared thermogram consists of a matrix of discrete pointmeasurements, many of fhe performance parameters of infrared thermalimager are the same as those of radiation thermometers. The output of aninfrared line scanner can be considered as one line of discrete pointmeasurements. The parameters of temperature range, absolute accuracy. repeatability, sensor environment and spectral range are esscntially the samefor radiation thermometers, line scanners and imagers. Others are derivedfrom or are extensions of radiation thermometer performance parameters.Qualitative thermal imagers (also called thermal viewers) differ fromquantitative thermal imagers (also called imaging radiometers) in that thermal viewers do not provide temperature or thermal energy measurements. Forthermographers requiring qualitative rather than quantitative thermal images,therefore, some performance parameters are unimportant.


Total Field of View (FOVtotal)For scanners and imagers. total field of view denotes the image size in termsof total scanning angles for any given lens. An example of a typical total fieldof view specifi cation would be "TFOV = 20 degrees vertical x 30 degreeshorizontal" (with standard Ix lens) and would define the thermogram total target size by a simple trigonometric relationship:

d = working distance,H = total horizontal image size,V = total vertical image size,x = horizontal scanning angle,y = vertical scanning angle.

This is illustrated in Figure 3. 10.

θ = y or x

tan θ/2 = V/2∙d-1

V = 2 ∙ tan (y/2) ∙ d, for θ = y


The total field of view for a line scanner consists of one scan line as shown inFigure 3.4 and Figure 3.10. The horizontal image size H is equal to the scan sector. The vertical image size V is equal to the instantaneous field of view.All other parameters are the same as for an imager.

Figure 3.4: Line scanner scanning configuration


Figure 3.10: Total field of view (TFOV) determination for an infrared imager


Instantaneous Field of View IFOVInstantaneous field of view in an imager is very similar to that for a pointsensing instrument: it is the angular projection of the detector element at thetarget plane. (resolution?)

In an imager, however, it is also called imaging spatial resolution and represents the size of the smallest picture element that ean be imaged. An example of a typical instantaneous field of view specification would be "IFOV = 1.7 mRad at 0.35 MTF." The 0.35 MTF refers to 35 percent of the modulation transfer function test used to check imaging spatial resolution.

This is described in detail in Chapter 4. The simple expression. D = αd, can be used to estimate imaging spot size at the target plane from manufacturer's published data by substituting the published instantaneous field of view for α.

Keywords:IFOV, image spatial resolution, MTF-modulated transfer function


EXAM score!

IFOVis also called;

image spatial resolution

for my ASNT exam


Recalling!

Temperature sensitivity is also called: thermal resolution

or noise equivalent temperature

difference (NETD).

for my ASNT exam


Measurement Spatial ResolutionMeasurement spatial resolution (IFOVmeas) is the spatial resolution of theminimum target spot size on which an accurate measurement can be made inlens of its distance from the instrument. An example of a typical measurement spatial resolution specification would be "IFOVmeas = 3.5 mrad at 0.95 SRF.“ The 0.95 SRF refers to 95 percent slit response function test used to check measurement spatial resolution. This is described in detail in Chapter 4. The simple ex pression, D = αd, can again be used to estimate measurement spot size at the target plane from manufacturer's published data by substituting published measurement spatial resolution for α.

Keywords:SRF refers to 95 percent slit response function test used to check measurement spatial resolution.

Comments:IFOVmeas – IFOV measurement


The 0.35 MTF refers to:0.35 percent of the modulation transfer

function test used to check imaging spatial resolution.

for my ASNT exam

IFOV - MTF


95 SRF refers to:95 percent slit response function test used to check measurement spatial

resolution

for my ASNT exam

IFOVmeas - SRF


Fig. 2a. Slit Response Function. Camera sees slit lips of radiometric temperature T0 (back side radiometric temperature) and The body behind the slit of radiometric temperature T1 (“slit “ temperature). Slit width is d and D is the distance slit-camera (Figure is issue from reference 4)

http://qirt.gel.ulaval.ca/archives/qirt2006/papers/025.pdf


Frame Repetition RateFrame repetition rate replaces speed of response and is defined as thenumber of times every point on the target is scanned in one second. Thisshould not be confused with field rate. Some imagers are designed tointerlace consecutive fields. each consisting of alternate image lines. Thisresults in images less disconcerting 令人不安的 to the human eye. The frame rate in this case would be one half the field rate. An example of a typical frame repetition rate specification for an imager would be "frame repetition rate = 30 frames per second." For a line scanner. the term line scan rate is used and it is expressed in lines per second.

Comments:For interlace field rate scanning; The frame rate in this case would be one half the field rate.


Minimum Resolvable Temperature DifferenceMinimum resolvable temperature (MRT) or minimum resolvable temperaturedifference (MRTD) replaces temperature sensitivity and is defined as thesmallest blackbody equivalent larget lemperature difference that can beobserved out of system noise on a thermogram. As in radiation thermometry.this difference improves (becomes smaller) with increasing target temperatureand is expressed in those terms. An example of a typical minimum resolvabletemperature difference specification for a line scanner or an imager would be "MRTD = 0.05 °C at 25 °C target temperature (0.09°F at 77°F),“ Minimum resolvable temperature difference may also depend on the spatial frequency imposed by the test discipline. The test techniques for checking minimum resolvable temperature difference is described in Chapter 4,

Comments: Temperature sensitivity is also called: thermal resolution or noise equivalent temperature difference (NETD).


Thermal Imaging Display and Diagnostic Software OverviewThermography applications often require extensive thermal imaging displayand diagnostic software. Thermal imagers feature image processingcapabilities that may be divided into five categories. one or more of whichmay be used in the same application. These categories are quantitativcthermal measurements of targets; detailed processing and image diagnostics;image recording. storage and recovery; image comparison (differential ormulti spectral thermography); and database and documentation. Applicationsusing software capabilities, singly and in combination. will also be describedin Chapter 5.


EXAM score!


(when calculation IFOV ratio care on unit used!)

for my ASNT exam


FOV - Animation

http://www.imagerchina.com.cn/fov_calculator.html



Questions & AnswersSubject: Answer this web queries from: http://www.thesnellgroup.com/community/ir-talk/f/9/p/1402/5433.aspx

wonder if anyone can help me here. I am studying for my employer's Level 2 certification exam and I am using the ASNT supplement booklet to help. They ask a few question about IFOV and spot size calculation and I do not quite understand how they get the answers. basically it is not the answer I want but how they got to the answers.

Question #1: A camera has an IFOV of 1.9 mRad. What is it's theoretical minimum spot size at a distance of 100 cm? Answer is: 0.19 cm (What formula is used for this and are there any units conversion like mm to cm or mRad to something else?)

Question #2: The IFOV measurement of a radiometric system is 1.2 mRad. What is the maximum size object this system can accurately measure at a distance of 25 m? Answer is: 3 cm (now clearly there are unit conversions going on here from meters to cm. So how is it done?)

Question #3: You are looking at an electrical connection 20 m in the air. What IFOV measurement is required to accurately measure the temperature on the 2.54 cm (1 in.) head of a bolt? Answer is: 1.25 mRad (I know it's just a matter of transposing the formula, but again there is units changes and I do not know the formula to apply)

Last question: Using an IR system with an IFOV measurement ratio of 180:1. What is the smallest size object you can accurately measure at a distance of 3m (3.3 ft)? Answer is: 16.6 mm or (0.65 in).

NOW this one I kind of figured out using: 1/180 = 0.0055 & 3 m = 3000mm therefore 0.0055 x 3000 = 16.5Let me know if you all know how to do these problems. I think all I need is the formula and an understanding when and which units to convert.


Answer: D= σ•d, IFOV ration= 1/σ = d/DQuestion #1: A camera has an IFOV of 1.9 mRad. What is it's theoretical minimum spot size at a distance of 100 cm? Answer is: 0.19 cm (What formula is used for this and are there any units conversion like mm to cm or mRad to something else?)

Calculation: D= 1.9 x 1 = 1.9mm or 0.19cm, (100cm = 1m)

Question #2: The IFOV measurement of a radiometric system is 1.2 mRad. What is the maximum size object this system can accurately measure at a distance of 25 m? Answer is: 3 cm (now clearly there are unit conversions going on here from meters to cm. So how is it done?)

Calculation: D= 1.2 x 25m = 30mm = 3cm

Question #3: You are looking at an electrical connection 20 m in the air. What IFOV measurement is required to accurately measure the temperature on the 2.54 cm (1 in.) head of a bolt? Answer is: 1.25 mRad (I know it's just a matter of transposing the formula, but again there is units changes and I do not know the formula to apply)

Calculation: 25.4 = σ x 20, σ = 1.27mRad

Last question: Using an IR system with an IFOV measurement ratio of 180:1. What is the smallest size object you can accurately measure at a distance of 3m (3.3 ft)? Answer is: 16.6 mm or (0.65 in).

Calculation: 1/ σ = d/D = 180, σ = 1/180,D = σ∙d, D = 1/180 x 3 = 0.01667m = 16.7mm(when calculating IFOV ratio, good to use the same unit for all inputs)



Break Time– Kenya Coffee Picker

http://www.kickstartcafe.com/journal/kenyan-coffee#.VWuZY52S3IU


3.8 Descriptions of Thermal Sensing and Imaging Equipment

Point Sensors (Radiation Thermometers)Point sensors (radiation thermometers) can be further divided into temperature probes. portable hand held devices. online process control devices and specially configured devices.

■ Temperature ProbesTemperature probes are low priced, pocket portable, battery powered devices that usually feature a pencil shaped sensor connected to a small basic readout unit. Generally, they are optically pre-adjusted for minimum spot size at a short working distance. A 0.5cm (0.2 in.) spot al a 2 cm (0.8 in.) working distance is typical. Temperature usually ranges from about - 20 °Cto 300 °C (-4 °F to 570 °F) and a sensitivity of ±1°C (1.8°F) is achieved easily. Probes are designed for close-up measurements such as circuit board analysis. troubleshooting of electrical connections. inspect ion of plumbing systems and biological and medical studies.


Portable Handheld Devices


■ Portable Handheld DevicesPortable handheld radiation thermometers are designed for middle distancemeasurements and, with few exceptions, operate in the 8 to 14 μm spectralregion and are configured like a pistol for one-handcd operation and aiming.They are usually optically preadjusted for infinity focus.

A typical 2 degree field of view resolves a 7.5 cm (3 in.) spot at a 150 cm (60 in.) working distance and a 30 cm (1 ft) spot at a 9 m (30 ft) working distance.(9 x tan(2º) = 0.314m=31cm)

Most instruments in this group incorporate microcomputers with limited memory and some have data logging capabilities. An open or enclosed aiming sight is provided and in some models a projected laser beam is used to facilitate aiming of the instrument as shown in Figure 3. 11. Note that the laser beam docs not represent the field of view. A measurement readout is always provided and usually the temperature is shown on a digital liquidcrystal display. These instruments are powered with disposable batteries and have low power drain.Temperature ranges are typically from 0 to 1000 °C(30to 1800 ºF).


Temperature sensitivity and readability are usually 1 percent of scale 1°C(2 ºF) although sensitivities on the order of 0.1 °C(0.2 °F) are achievable. Response times are on the order of fractions of a second, usually limited by the response of the readout.

Hand held radiation thermometers are used extensively in applications where spot checking of target temperatures is sufficient and continuous monitoring is not required. Handheld radiation thermometers have become an important part of many plant energy conservation programs. Process applications include monitoring mixing temperatures of food products. cosmetics and industrial solvents. Microcomputers enable handheld instruments to incorporate special features such as the ability to store sixty readings for future retrievals and printout.


Figure 3.11: Hand held infrared radiation thermometer with laser aiming


Hand Held Infrared Module


Note that the laser beam docs not represent the field of view.Figure 1. Use the Fluke 66 within 5 m (15 ft.) of the intended target.At greater distances, the measured area will be larger (approximatelythe distance divided by 30). Field of view θ= tan-1 (1/30) = 1.91º

http://www.fluke.com/fluke/m3en/products/thermometers


Note that the laser beam docs not represent the field of view.Figure 2. Use the Fluke 68 within 8 m (25 ft.) of the intended target.At greater distances, the measured area will be larger (approximatelythe distance divided by 50). Filed of view θ= tan-1 (1/50) = 1.14º

http://www.fluke.com/fluke/m3en/products/thermometers


■ Online Process Monitoring and Control DevicesOnline monitoring and control sensors are for dedicated use on a product or a process. Permanently installed where it can measure the temperature of one specific target. this type of instrument remains there for the life of the instrument or the process. With few exceptions. these instruments operate on line power. The measurement value can be observed on a meter. but it is more often used to trigger a switch or relay or to feed a simple or sophisticated process control loop. Most of the online monitoring and control sensors send signals to universal indicator control units that accept inputs from various types of industrial sensors. Because this instrument group is selected to perform a specific task, a shopping list format is provided to the customer by the manufacturer so that all required features can be purchased. including environmental features such as water cooled housings. air purge fittings and air curtain devices.


Emissivity set controls, located in a prominent place on a general purpose instrument are more likely to be located behind a bezel 嵌槽/柜 on the sensor on these dedicated units. where they are set once and locked. The spectral interval over which the sensing head operates is selected to optimize the signal from the target, to reduce or eliminate the effect of an interfering energy source or to enable the instrument to measure the surfacetemperature of thin films of material that are largely transparent to infrared radiation. The capability for spectral selectivity has made these instruments important in the manufacture of glass and thin film plastics. Applications in these atres are discussed in Chapters 4 and 5.


IR Sensor Module


IR Sensor Module


IR Sensor Module


IR Sensor Module


■ Devices with Special ConfigurationsSpecial configurations of infrared radiation thermometers include ratio pyrometers (also called two color pyrometers), infrared radiometric microscopes, laser reflection pyrometers and fiber-optic coupled pyrometers.

1. Two-color pyrometers or ratio pyrometers, are a special case of the online instrument. Ratio pyrometers are particularly useful in high temperature applications above 300°C(572°F) and in measuring small targets of unknown emissivity, provided the background is cool, constant and uniform. The emissivity of the target need not be known if it is constant and reflections are controlled. The target does not need to fill the field of view. provided the background is cool, constant and uniform. The measurement is based on the ratio of energy in two spectral bands. so impurities in the optical path resulting in broad band absorption do not affect the measurement. Ratio pyrometers are usually, not applicable to measurements below 300 °C(572 °F).


Two-color Pyrometers or Ratio Pyrometers





https://www.eutech-scientific.de/products-services/power-generation/euflame.html



https://www.eutech-scientific.de/products-services/power-generation/euflame.html


Two-color Pyrometers or Ratio PyrometersRatio Radiation - Also called two-color radiation thermometers, these devices measure the radiated energy of an object between two narrow wavelength bands, and calculates the ratio of the two energies, which is a function of the temperature of the object. Originally, these were called two color pyrometers, because the two wavelengths corresponded to different colors in the visible spectrum (for example, red and green). Many people still use the term two-color pyrometers today, broadening the term to include wavelengths in the infrared.

The temperature measurement is dependent only on the ratio of the two energies measured, and not their absolute values as shown in Figure 3-4.

Any parameter, such as target size, which affects the amount of energy in each band by an equal percentage, has no effect on the temperature indication. This makes a ratio thermometer inherently more accurate. (However, some accuracy is lost when you're measuring small differences in large signals). The ratio technique may eliminate, or reduce, errors in temperature measurement caused by changes in emissivity, surface finish, and energy absorbing materials, such as water vapor, between the thermometer and the target. These dynamic changes must be seen identically by the detector at the two wavelengths being used.

Emissivity of all materials does not change equally at different wavelengths. Materials for which emissivity does change equally at different wavelengths are called gray bodies. Materials for which this is not true are called non-gray bodies. In addition, not all forms of sight path obstruction attenuate the ratio wavelengths equally. For example, if there are particles in the sight path that have the same size as one of the wavelengths, the ratio can become unbalanced.

http://www.omega.com/literature/transactions/volume1/thermometers2.html


Figure 3-4: The “Two-Color” IR Thermometer


T1

T2

E1

E1

E2

E2


Phenomena which are non-dynamic in nature, such as the non-gray bodinessof materials, can be dealt with by biasing the ratio of the wavelengths accordingly. This adjustment is called slope. The appropriate slope setting must be determined experimentally. Figure 3-5 shows a schematic diagram of a simple ratio radiation thermometer. Figure 3-6 shows a ratio thermometer where the wavelengths are alternately selected by a rotating filter wheel.

Figure 3-5: Beam Splitting in the Ratio IR Thermometer



Figure 3-6: Radio Pyometry Via a Filter wheel

Figure 3-7: Schematic of a Multispectral IR Thermometer.



Some ratio thermometers use more than two wavelengths. A multi-wavelength device is schematically represented in Figure 3-7.

These devices employ a detailed analysis of the target's surface characteristics regarding emissivity with regard to wavelength, temperature, and surface chemistry. With such data, a computer can use complex algorithms to relate and compensate for emissivity changes at various conditions. The system described in Figure 3-7 makes parallel measurement possible in four spectral channels in the range from 1 to 25 microns. The detector in this device consists of an optical system with a beam splitter, and interference filters for the spectral dispersion of the incident radiation. This uncooled thermometer was developed for gas analysis. Another experimental system, using seven different wavelengths demonstrated a resolution of +/-1°Cmeasuring a blackbody source in the range from 600 to 900°C. The same system demonstrated a resolution of +/- 4°Cmeasuring an object with varying emittance over the temperature range from 500 to 950°C

Two color or multi-wavelength thermometers should be seriously considered for applications where accuracy, and not just repeatability, is critical, or if the target object is undergoing a physical or chemical change. Ratio thermometers cover wide temperature ranges. Typical commercially available ranges are 1652 to 5432°F (900 to 3000°C) and 120 to 6692°F (50 to 3700°C). Typical accuracy is 0.5% of reading on narrow spans, to 2% of full scale.



2. Infrared radiometric microscopes are configured like a conventional microscope and by using reflective microscope objectives and beam splitters, the operator can simultaneously view and measure targets down to 10 μm in diameter with accuracy and resolution of about 0.5°C (1°F).

3. Laser reflection pyrometers use the reflected energy of an active laser to measure target reflectance. A built-in microcomputer calculates target effective emissivity and uses this figure to provide a corrected true temperature reading. This instrument. though expensive, is useful for measurement of high temperature specular target surfaces in adverse environments.

4. Fiberoptic coupled pyrometers make possible the measurement of normally inaccessible targets by replacing the optic with a flexible or rigid fiberoptic bundle. This limits the spectral performance and hence the temperature range to the higher values, but has allowed temperature measurements to be made when previously none were possible.


Infrared Radiometric Microscopes


Fiberoptic Coupled Pyrometers

http://www.omega.com/temperature/pdf/4121_ir.pdf


Line ScannersLine scanners are divided into online process control devices and special purpose scanners.

■ Online Process Control Devices Online (monitoring and control) line scanners are high speed online commercial line scanners that develop high resolution thermal maps by scanning normal to the motion of a moving target such as paper web or a strip steel process. The vast majority of commercial infrared line scanners are in this configuration. The output signal information is in a real time computer compatible format and can be used to monitor, control or predict the behavior of the target. Like the online point sensor, these line scanners are usually permanently installed where they monitor the temperature profile at one site of the process, remaining there for the life of the instrument or the process. Likewise they are usually fitted with environmental housings and preset emissivity compensation sets. The best applications for this scanner are in online, real time process monitoring and control applications where they are integrated with the process host computer system.


It is not unusual to find line scanners at multiple locations in a process with all of them linked to the host computer. In the 1990s, infrared line scanners based on a linear focal plane array came into use. This type of instrument frequently uses an un-cooled array of thermal detectors radiation thermopiles.

This scanner has no moving parts. The linear array is oriented perpendicular to a process or a target moving at a uniform rate. The scanner output may be used to develop a thermograms or the data for each pixel can be fed directly to a host computer and used to monitor and control the process. Instruments of this type have been used to monitor moving railroad cars for overheated wheels and brake assemblies.


Special Purpose DevicesSpecial purpose configurations of line scanners include one type of portable line scanner and a number of aerial mappers that scan a line normal to themotion of the aircraft and develop a thermal strip map. Many of thesemappers have been replaced by low cost forward looking infrared scanners(FLIRs) based on staring focal plane arrays.


FLIR- Forward Looking Infrared


FLIR- Forward Looking Infrared


Imagers (Thermographic Instruments)Imagers (thermographic instruments) consist of both qualitative and quantitative imagers.

■ Qualitative Thermal ImagersQualitative thermal imagers are also called thermal viewers. They include mechanically scanned, electronically scanned (pyrovidicon) and staring focal plane array FPA imagers.

● Mechanically Scanned Thermal ViewersMechanically scanned thermal viewers are moderately priced battery powered scanning instruments that produce a qualitative image of the radiosity over the surface of a targct. The battery packs are rechargeable and usually provide 2 to 3 h of continuous operation. These one-piece, lightweight instruments, designed to be simple to operate, feature thermoelectric detector,cooling provided by a battery powered cooler. Although not designed for absolute temperature measurements, they can demonstrably sense temperature differences of tenths of degrees and can be used for targets from below 0°C up to 1500°C (32 of up to 2372 °F).


Typically, the total field of view is from 6 to 8 degrees high and from 12 to 18 degrees wide, with spatial resolution of 2 mRad 10 mm at 2.0 m (0.4 in. at 7 ft). Images are video recorded by means of a conventional video tape recorder output jack and video recorder accessories. The broad applications for thermal viewers are generally limited only to those in which the temperature measurements are not critical and recording quality does not need to be optimum. The combination of a thermal viewer (to locate thermal anomalies) and a hand held thermometer (to quantify them) can be a powerful and cost effective ombination.


● Electronically Scanned Viewers (Pyrovidcon Imagers)Pyrovidicon imagers are electronically scanned video cameras. The cameratube is sensitive to target radiation in the infrared rather than the visiblespectrum. Aside from the tube and germanium lens, which are expensive, these systems use television recording accessories, in comparison with other infrared imaging systems, the picture quality and resolution are good, approaching conventional television format.

The thermal image can be viewed or videotaped with equal convenience and no cooling is required. Pyrovidicon systems do not intrinsically offer quantitative measurement capability, but some manufacturers offer models in which an integrated radiation thermometer is bore sighted with the scanner and its measurement is superimposed on the video display along with a defining reticle in the center of the display thermal resolution of flicker freepyrovidicon instruments is between 0.2 and 0.4°C (0.4 and 0.7°F).


Pyroelectric devices have no direct current response, and a basic pyrovidicon imager 's display will fade when the device is aimed at an unchanging thermal scene. Early pyrovidicon imagers needed to be panned to retain image definition.

To enable fixed monitoring, crude, flag type choppers were devised to interrupt the image at adjustable chop rates. However, this resulted in a blinking image that was disconcerting to the eye. These choppers have been replaced by synchronous choppers that chop the image in synchronism with the electronic scan rate and produce flicker free images on the display. Pyrovidicon viewers operate well in the 8 to 14 μm atmospheric transmission window. Operating costs are very low because no cooler or coolant is required.


● Staring Infrared Focal Plane Array Thermal ViewersStaring infrared focal plane array (lRFPA) thermal viewers are directadaplations of devices developed for military and aerospace night vision andmissile tracking applications. For these applications, performance emphasis is on picture quality rather than measurement capability. Instruments usingcooled platinum silicide (PtSi) staring arrays with as many as 512 x 512elements are available. Instrument using cooled indium antimonide (InSb)focal plane arrays are available in models designed to compete with top-of-the-line commercial thermal imagers. Some instruments in this category havethe size and weight of a commercial video camera that fits in the palm of thehand, as illustrated in Figure 3.12.


Figure 3.12: Infrared focal plane array imager for qualitative thermography


Infrared focal plane array imager




Qualitative IrFPA




■ Quantitative Thermal ImagersQuantitative thermal imagers include (1) mechanicatly scanned thermal imagers (imaging radiometers) and (2) focal plane array radiometers.

● Mechanically Scanned Thermal ImagersMechanically scanned thermal imagers (imaging radiometers) provide a means for measuring apparent target surface temperature with high resolution image quality and sometimes with extensive on-board diagnostic software. Mosl commercially available imaging radiometers use a single detector. but some manufacturers offer dual detector or multidctcctor (lineararray) instruments. Most require detector cooling. Imaging radiometers userefractive reflective or hybrid scanning systems and operate in either the 3 to5μm or the 8 to 14μm atmospheric window. They generally offer instantaneous fields of view on the order of 1 to 2 mrad with standard optics and minimum resolvable temperature differences of 0.05 to 0.10°C (0.09 to 0.18°F).


On-board capabilities include isotherm graphics features, spectral filtering.interchangeable optics for different total field of views. color or monochrome(black and white) displays, flexible video recording capabilities and computercompatibility. Most feature compact, field portable, battery operable sensingheads and control/display units. A complete system including battery andvideo recorder can be handled by one person by mounting the componentson a cart or by assembling them on a harness.


● Focal Plane Array RadiometersFocal plane array radiometers are adaptations of military and aerospace forward looking infrared scanners. but are designed to measure the apparent temperature at the target surface and to produce quantitative thermograms. The capabilities of early infrared focal plane array imagers were slow in developing. The quality of measurement capabilities has improved since 1990. Infrared focal plane array cameras offer minimum resolvable temperature differences comparable to imaging radiometers (0.1 to 0.2°C; 0.18 to 0.36°F) and instantaneous field of views considerably better than imaging radiometers (1 mRad or better with standard optics).

Commercially available quantitative infrared focal plane array cameras use detector arrays made of platinum silicide or indium antimonide, either of which requires cooling. Quantitative thermal imagers based on uncooled focal plane arrays (using bolometrie and ferroelectric detectors) have also been developed. With inherently faster response, no moving parts and superior spatial resolution infrared focal plane array cameras have been replacing infrared imaging radiometers for most applications.


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Platinum Silicide IrFPA

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Quantitative IR Image


Quantitative IR Image


3.9 Thermal Imaging Display and Diagnostic Software

When the personal computer was introduced as part of thermal imagingsystems, the typical imager produced raw radiometric data. whereas all of thediagnostic software was contained in an ancillary. separately packagedcomputer that performed all of the diagnostics back on the bench. Withimproved packaging technology in both computers and thermal imaging equipment, there has been a gradual trend toward providing more and more on board software so that more diagnostics can be performed on site.Depending on manufacturer and model, some software is incorporated into instruments and some is available only on computer driven software packages. Although thermographic diagnostic software packages are usually proprietary to a particular manufacturer, there is a trend toward universality in image storage. Common formats for storing electronic images include taggedimage file format (TIFF) and other bitmapped formats. Retrieving images from these format is fast and easy.


Quantitative Thermal MeasurementsSome qualitative thermograms can be converted to quantitative thermograms.The raw image produced by a quantitative imager may be converted to aquantitative thermogram; the raw image produced by a viewer may not.

Quantitative thermal measurements provide the user with the true radiance orapparent temperature value of any or all points on the target surface. Topresent the thermogram in true radiance measurements, the systemthroughput attenuation must be considered as well as losses through themeasurement medium (atmosphere, in most cases). To present thethermogram in true temperature values. the target effective emissivity mustalso be considered. When this capability is provided, a menu instructs theuser to enter system calibration constants on initial setup and a system ofprompts assures the operator that changes in aperture settings, targetdistance, inter-changeable lenses. etc., will be fed into the keyboard each time a change in operating conditions occurs.


Changes in the corrections setting for target effective emissivity are also monitored. In addition. digital cameras are available to save visible images in computer compatible format for arehiving with corresponding thermograms.For most systems. the displayed temperature readings are based on the assumption that the entire target surface has the same effective emissivity. Some systems. however. allow the assignment of several differentemissivities to different areas of the target selected by the operator with the resulting temperature correction. A color scale or gray scale is provided along one edge of the display with temperature shown corresponding to each color or gray level in the selected range. The operator can place one or more spotsor crosshairs on the image and the apparent temperature value of that pixelwill appear in an appropriate location on the display. The isotherm featureallows the operator to select a temperature band or interval and all areas on the target within that band then appear enhanced in a predetennined gray shade or color hue. Detailed processing and image diagnostics relies onsoftware that allows manipulation and analysis of each pixel in the thermogram prescnting information in a wide variety of qualitative and quantitative forms for the convenience of the user. Some of these capabilities are described in this chapler.


In addition to the spot measurement capability discussed previously. lineprofiles may be selected. The analog trace. in X, Y. or both. of the lines on theimage intersecting at the selected spot will then appear at the edge of thedisplay. Some systems allow the operator to display as many as seven setsof profiles simultaneously. Profiles of skew lines can also be displayed onsome systems. Selected areas on the thermogram in the form of circles,rectangles or point-to-point free forms, can be shifted, expanded. shrunk orrotated or used to blank out or analyze portions of the image.

Detailed analysis of the entire image or the pixels within the area can includemaximum, minimum and verage values. number of pixels or even afrequency histogram of the values within the area. Color scales can becreated from 256 colors stored in the computer. Electronic zoom featuresallow the operator to expand a small area on the display for closerexamination. or to expand the colors for a small measurement range.Autoscale features provide the optimum display settings for any image ifselected. Three-dimensional features provide an isometric thermal contourmap or thermal profile map of the target for enhanced recognition of thermal anomalies.


Image Recording, Storage and RecoveryImages and data can be stored in and retrieved from memory, hard disk, floppy diskette, video tape, optical disks (writable compact disks and digitalvideo disks) and Personal Computer Memory/Computer IndustryAssociation (PCMCIA) cards.

Commercial thermal imaging systems incorporate some means, such as afloppy disk drive or a PCMCIA card to store images in the field. Usually. about forty images. with all accompanying data, can be stored on a 3.5 in diskette.

Some analysis usually can be done with on-board software; more extensive diagnostics usually require a separate computer. Options include IEEE or RS232 ports for access to additional storage and a video recorder option so that an entire measurement program can be recorded on video tape. Video tapes can be played back into the system and images can be saved to disk. Images can be stored from a frozen frame thermogram of a live target on operator command. or the operator can set up an automatie sequence and a preset number of images will be stored at preset time intervals.


Stored images can be retrieved, displayed and further analyzed. Image comparison (differential thermography) allows the automatic comparison of thermograms taken at different times. This includes time based comparison ofimages taken of the same target as well as the comparison of images taken of different but similar targets.

A special software program allows the operator to display two images side-by- ide or in sequence; and to subtract one image from another or one area from another; and to display a pixel-by-pixel difference thermogram. Comparison (subtraction) of images can be accomplished between two images retrieved from disk, between a live image and an image retrieved from disk and between a live image and an image stored in a computers random access memory, in this way, standard thermal images of acceptable components, assemblies and mechanisms can be arehived and used as models for comparison to subsequently inspected items. It is also possible to subtract a live image from a previous baseline image for subsequent time based thermal transient measurements.


Database and DocumentationRecords, files, data and documents can be saved in an orderly fashion. Thiscapability provides thc thermographers with a filing system so that records ofall measurement missions can be maintained on magnetic media, includingactual thermograms, time, date, location, equipment, equipment settings, measurement conditions and other related observations.

Most manufacturers of thermal imaging equipment have developed comprehensive report preparation software to facilitate timely and comprehensive reporting of the findings of infrared surveys and othermeasurement missions. These packages provide templates that allow thc thermographer to prepare reports in standard word processor formats into which tagged image file format (TIFF) images. imported from various imaging radiometers. can be directly incorporated. Additional diagnostic software is customarily provided in these packages so that analysis and trending can beadded to reports.


Calibration AccessoriesInfrared radiation reference sources are used by manufacturers to calibrateinfrared sensing and imaging instruments in the laboratory before they are shipped. These same reference sources are used later at periodic intervalsthereafter to ensure calibration stability. A radiation reference source isdesigned to simulate a blackbody radiator: that is. a target surface with astable, adjustable known temperature and a uniform emissivity approaching1.0 at all appropriate wavelengths. In addition to laboratory reference sources.there are field portable models suitable for periodic calibration checks offielded thermographic equipment and for other tasks. The setup anddeployment of radiation reference sources is discussed in Chapter 4.


3.10 Photorecording Accessories for Hard CopiesSince the advent of the personal computer and its integration with thermalimagers, magnetic storage and arehiving of data (labels. dates. conditions ofmeasurement. instrument settings. etc.) as well as thermograms havebecome routine. Soft copies can be made of real time images, processedimages enhanced images and combined images on floppy disks, analog and digital magnetic tape, recordable optical disks and Personal ComputerMemory/Computer Industry Association (PCMCIA) cards.

Report preparation software allows images to be inserted into word processing documents and printed by conventional laser or inkjet printers. Making a hard copy directly from a stored or displayed image is done in a variety of ways. A number of devices were introduced before magnetic media were available for directly photographing the display between with conventional or instant film. Using them generally required considerable skill because the ambient lighting and the screen curvature had to be considered. For this reason. it was difficult to achieve repeatable results. online printers and plotters provide reliable, good quality copies when speed is not a consideration.


Online printers and plotters are relatively slow and may tie up the computer and related software during operation. For real time or high speed photo-recording, portable video printers are usually selected. The video printer connects to the system's video output. It presents the current image on a remote display where it is frame grabbed and reproduced in real time under optimized conditions. Most video printers produce output on integral recorder paper. Available accessories allow a choice of direct instant hardcopies, negatives or slide transparencies. Although video printers are costly. they provide consistent quality in a reasonable time and do not require the use of the thermal imager or the computer during production time.


Chapter 3Review Questions 14. b

15. a16. b17. d18. b19. e20. a21. d22. a23. a24. d25. b

1. b2. d3. a4. b5. d6. a7. c8. c9. d10. d11. b12. a13. b

Q&A


Q1. The thermal resolution of an instrument is the same as:a. the temperature accuracy.b. minimum resolvable temperature difference.c. temperature repeatability.d. the minimum spot size.

Q2. The speed of response of an instrument is:a. the time constant of the detector.b. one half the time constant of the detector.c. the same as the field repetition rate.d. the time it takes to respond to a step change at the target surface.

Q3. The instantaneous spot size of an instrument is related to the:a. instantaneous field of view and the working distance.b. thermal resolution.c. spectral bandwidth and the working distance.d. speed of response and the working distance.


Q4. The performance parameters that are important for qualitative thermography are:a. absolute accuracy, repeatability and resolution.b. spatial resolution and thermal resolution.c. spatial resolution and absolute accuracy.d. measurement spatial resolution and thermal resolution.

Q5. Thermal viewers do not provide:a. high resolution thermograms.b. recording capabilities.c. real time scan rates.d. quantitative thermograms.

Q6. The thermal resolution of an instrument tends to:a. improve as target temperature increases.b. degrade as target temperature increases.c. remain constant regardless of target temperature.d. improve with increasing working distance.


Q7. The 3 to 5μm spectral region is ideally suited for operation of instruments:a. measuring subzero temperature targets.b. measuring targets at extremely long working distances.c. measuring targets warmer than 200 °C(392 °F).d. operating at elevated ambient temperature.

Q8. The total field of view of an imaging instrument determines the:a. imaging spatial resolution (lFOV) of the instrument.b. measurement spatial resolution (IFOVmeas) of the instrument.c. image size at the target plane for any given working distance.d. operating spectral range of the instrument.

Q9. The frame repetition rate of an imager is defined as the:a. number of imaging pixels in a thermogram.b. number of frames selected for image averaging.c. electronic image rate of the display screen.d. number of times every point on the target is scanned in one second.


Q10. The purpose of adding an infrared spectral filter to an instrument may be to limit the spectral band:a. to only wavelengths longer than a specified wavelength.b. to only wavelengths shorter than a specified wavelength.c. to only wavelengths between two specified wavelengths.d. any of the above.

Q11. To quickly calculate target spot size, a useful approximation is:a. π =3.1416.b. an instantaneous field of view of 1 degree represents a 60: 1 ratio between working distance and spot size.c. there are 2π radians in 360 degrees.d. a 1°F temperature change is equivalent to a 1.8 °Ctemperature change.

Q12. For online process control instruments, important features are:a. environmental housings and long term stability.b. ready access to emissivity compensation setting.c. portability and battery life.d. precision sighting.


Q13. A line scanner can be used to produce a thermogram of a sheet process only when:a. emissivity is known.b. the sheet process is moving at a uniform rate.c. the process material is a non graybody.d. the sheet process is hotter than 200 °C(392 °F).

Q14. Most quantitative infrared thermal imagers:a. are heavier than quantitative imagers and usually require line power.b. can store thermograms on floppy disks in the field.c. require frequent infusions of detector coolant in the field.d. use detectors that operate at room temperature.

Q15. Infrared focal plane array imagers:a. have no scanning optics.b. cannot be used for quantitative thermography.c. cannot be used for very cool targets.d. cannot operate on rechargeable batteries.


Q16. Most infrared focal plane array imagers:a. use more costly optics than scanning radiometers.b. offer better spatial resolution than scanning radiometers.c. offer better thermal resolution than scanning radiometers.d. offer more diagnostics features than scanning radiometers.

Q17. The number of detector elements in an infrared focal plane array imager:a. affects the measurement accuracy of the imager.b. affects the thermal resolution of the imager.c. affects the spectral band of the imager.d. affects the spatial resolution of the imager.

Q18. The fact that all elements in a focal plane array imager are always looking at the target make this kind of imager better suited than scanning imagersfor observing:a. distant low temperature targets.b. targets with rapidly changing temperatures.c. targets with low emissivities.d. targets with high emissivities.


Q19. For which of the following applications are quantitative thermograms most critical?a. Search and rescue.b. Nondestructive material testing.c. Process monitoring and control.d. Security and surveillance.

Q20. Infrared thermal detectors:a. have a broad, flat spectral response.b. usually require cooling to operate properly.c. have much faster response times than photon detectors.d. have much greater sensitivity than photon detectors.

Q21. The characteristics of infrared photodetectors(photon detectors) include:a. faster response times than thermal detectors.b. a requirement for cooling to operate properly.c. selective spectral response based on operating temperature.d. all of the above.


Q22. Filters, lenses and transmitting windows:a. are all examples of refractive optical elements.b. have negligible transmission loss in the infrared.c. are all examples of reflective optical elements.d. are not spectrally selective.

Q23. Resistance temperature detectors and thermistors operate on the same principle. that is:a. a predictable change in resistance as a function of temperature.b. the inverse square law.c. the known expansion of dissimilar materials.d. the comparison of target brightness with a calibrated reference.

Q24. Infrared radiation thermometers are used to measure temperature:a. without contacting the target.b. very rapidly.c. without causing a temperature change at the target.d. all of the above.


Q25. Two-color (ratio) pyrometers measure the temperature of a target by:a. taking into account the size and distance to the target.b. comparing the radiant energy from the target in two narrow spectral bands.c. incorporating tables of known emissivity.d. calibrating and correcting for the infrared absorption in the measurement path.


SGuide-IRTContentPart 1 of 2■ Chapter 1 - Introduction to Principles & Theory■ Chapter 2 - Materials and Their Properties■ Chapter 3 – Thermal Instrumentation

Part 2 of 2■ Chapter 4 – Operating Equipment and Understanding Results■ Chapter 5 – Applications■ Appendices A, B, C


Chapter 4Operating Equipment and Understanding Results


4.1 Temperature ChangesDistinguishing real temperature changes from apparent temperature changesis one of the biggest challenges facing thermographcrs. Thermal imaginginstruments register temperature changes in response to changes in radiosityat the target surface when in many cases, there is no change in real surface temperature. To complicate matters further, external mechanisms canexaggerate these misleading readings. To combat this situation, thermographers should understand the basic causes of apparent temperature change - some of which are only apparent and some of which are the result of real temperature changes at the target surface.

Causes of Apparent Temperature ChangesApparent temperature changes can be caused by difrerences in emisivity ε,reflectivity ρ, transmissivity τ and target geometry G.


■ Emissivity Differences ∆τEmissivity differences at the target surface can change the target radiosity.even on an isothermal target. and may give the appearance of temperaturevariations on the thermogram. Frequently, these can be seen on painted metal surfaces where scratches expose bare metal that has a differentemissivity than the paint.

■ Reflectivity Differences ∆ρReflectivity djfferences may become apparent when heat sources external to the target surface reflect off low emissivity target (low emissivity ≡ high reflectivity) surfaces into the instrument. These can be point sources or extended sources and they can add to or subtract from the apparenttemperature reading as will be discussed later.


■ Transmissivity Differences ∆τTransmissivity differences can be caused by heat sources behind the target ifthe target is partly transparent in the infrared range. These will only be seen if the target transmissivity is high enough and the heat source is different enough in temperature from the target to contribute significantly to the total target radiosity.

■ Target Geometry Differences ∆DTarget geometry differences are caused by multiple reflections within recesses or concavities on the target surface. They are actually variations in effective emissivity caused by changes in surface configurations. An example of this is the apparent temperature gradient in the far corner of an enclosure that is at a uniform temperature. Geometric differences diminish as target surface emissivity approaches unity. (blackbody does not affected by G)


Causes of Real Temperature ChangesReal temperature changes may be caused by differcnces in mass transport(fluid flow), phase change (physical state). thermal capacitance, inducedheating, energy conversion (friction. exothermic reactions and endothermic reactions), direct heat transfer by conduction, convection and radiation (thermal resistance) or a combination of two or more of these causes.

■ Mass Transport Differences (Fluid Flow)Mass transport differences are real temperature changes at the target surfacecaused by various forms of fluid flow. Free and forced convection are twoexamples of mass transport differences. Cool air exiting an air conditioningregister will cause the register to become cooler. Hot water flowing within apipe will cause the inside surface of the pipe to become warmer. (This willresult in the outside of the pipe also becoming warmer.)


Mass Transport Differences (Fluid Flow)


Mass Transport Differences (Fluid Flow)


■ Phase Change Differences (Physical State)Phase change differenccs occur when materials change physical stale. An example of this is water evaporating off the surface of a building. As the waterevaporates, it has a cooling effect on the entire surface. Thermal imaging equipment aimed at the building will register this cooling effect.

■ Thermal Capacitance Differences ∆CpThermal capacitance differences cause temperature changes in transient conditions when one part of a target has a greater capacity to store heat than another. In the thermogram of a water tank. as shown in Figure 4.1. the water level inside the tank is apparent because of the contrast in temperature, which is caused by the difference in thermal capacitance between water and air. This real temperature change is also evident in roof surveys as illustrated in Chapter 5.


Figure 4.1: An indication of water level In a storage tank


Thermal Capacitance Differences ∆Cp

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http://garoofingandrepair.com/infrared-thermal-roof-scan-atlanta-ga/




■ Induced Heating Differences (by electromagnetic induction)Induced heating differences occur when ferrous metals are within a magnetic field. Depending on the orientation of the parts and the strength of the magnetic field, induced currents within the ferrous parts can cause substantial heating. An example of this is when an aluminum bolt in a structure is mistakenly replaced with a ferrous bolt. If the structure is within a magnetic field, the bolt may become hot. This induction effect is exploited in the thermographic location of steel reinforcing bars embedded in concrete structures. Here a magnetic field is introduced to the structure and the resultant warm spot on the thermogram indicate the presence of the reinforcing bars.


Induced Heating Differences

http://processmodeling.org/








■ Energy Conversion DifferencesEnergy conversion differences occur when energy is converted from one form to another. Friction (mechanical energy converted to thermal energy) is acommonly observed example of temperature changes because of energyconversion. Another is electrical energy converted to thermal energy, asillustrated in Figure 4.2, where the current carrying wire of a twisted pairgenerates heat revealing insulation discontinuities. Exothermic orendothermic reactions (chemical energy converted to thermal encrgy) arefurther examples. typified by the heating that accompanies the curing ofpolymers.

■ Direct Heat Transfer DifferencesDirect heal transfer differences are also commonly observed in thermographicsurvey programs. An example of this is shown in the direct transfer of thermal energy through the wall of a catalytic cracker reformer vessel as illuslrated in Figure 4.3. The differences in heat flow illustrate the differences in thermal resistance between good refractory material and degraded material.


Figure 4.2: The current carrying wire of a twisted pair generates heat thatreveals insulation defects


Figure 4.3: Catalytic reformer vessel with insulation defects


Energy Conversion Differences


Infrared ThermogramEnergy Conversion Differences


Infrared Thermogram Energy Conversion Differences


Infrared Thermogram Energy Conversion Differences


Direct Heat Transfer Differences


■ Combination of Heat Transfer MechanismsThermal images of operating equipment and systems will often exhibit heat flow by a combination of mechanisms working simultaneously. Figure 4.4depicts the investigation into the thermal design of a new motorcycle engine.The thermal signature is a combination of fluid flow (in the cooling fins),exothermic reactions (within the cylinders) friction (at the piston rings andwithin the bearing) and thermal resistance (in the exhaust system).


Figure 4.3: Thermogram of a new motorcycle engine heat flow by a combination of mechanisms working simultaneously


Infrared Thermogram of a running motor


Image InterpretationA clearer understanding of the pitfalls possible in image interpretation helpsthe thermographer to perform the required tasks competently. As in the threemodes of heat transfer (conduction, convection & radiation), these mechanisms frequently occur in combinations. Although the ability of the thermographer to clearly identify the causes of temperature change in aparticular target environment may be unnecessary when making measurements, it is absolutely essential for the correct and responsible interpretation of results. In situations where the thermographer is unfamiliarwith the measurement environment, a knowledgeable facility representative should accompany the thermographer during the measurements or be available for consultation. By providing expert information concerning the processes taking place and the likely sources of temperature differences, the thermographer will be able to anticipate thermal behavior and betterunderstand and interpret the thermographic results.


■ Spectral Considerations in Product and Process ApplicationsMany products, both simple and complex have complex spectral characteristics in the infrared region. Spectral filtering of the measuring instrument can exploit these complex spectral characteristics to measure and control product temperature without contact. For example, if it is necessary to measure the temperature of objects from 200 to 1000 °C (392 to 1832 °F) inside a heating chamber with a glass port , or inside a thin walled glass bell jar, an instrument operating in the 2 to 3 μm band will see through the glass and make the measurement easily. On the other hand, an instrument operating at wavelengths longer than 4.8 μm will measure the surface temperature of the glass.


Infrared Thermogram of Glass of Water - Spectral Considerations

2 to 3 μm band will see through the glass and make the measurement easily.

4.8 μm will measure the surface temperature of the glass.


Infrared Thermogram


Infrared Thermogram


Infrared Thermogram


Spectral characteristics are exploited in the monitoring of incandescent lamptemperatures during production as illustrated in Figures 4.5, 4.6 and 4.7.Figure 4.5 shows the spectral characteristics of the imaging radiometer aswell as the transmission spectra of glass envelopes of various thicknesses.Using a 2.35 μm band pass filter with the instrument allows the instrument tosee through the glass and monitor the temperature of critical internal lampcomponents. Substituting a 4.8 μm high pass filter allows the instrument tomonitor the glass envelope temperature.Figures 4.6 and 4.7 are thermograms of the glass envelope and the internal lamp components respectively, recorded in immediate sequence. An important generic example of the need for spectral selectivity is in the measurement of plastics being formed into films and other configurations. Thin films of many plastics are virtually transparent to most infraredwavelengths, but they do emit at certain wavelengths. Polyethylene, polypropylene and other related materials have a very strong, though narrow, absorption band at 3.45 μm. Polyethylene film is formed at about 200 °C (392 °F) in the presence of heaters that radiate at a temperature near 700 °C ( 1292 °F).


Figure 4.5: Spectral selectivity for measuring the surface and internal temperatures of incandescent lamps


Figure 4.6: Surface temperature thermogram of an incandescent lamp


Figure 4.7: temperature thermogram of an incandescent lamp


Incandescent Lamp

2.35 μm band pass filter - Incandescent filament temperature

4.8 μm high pass filter - bulb surface (envelope)temperature


Figure 4.8 shows the transmission spectra of 40 μm ( 1.5 x 10-3 in.) thickpolyethylene film and the narrow absorption band at 3.45 μm. The instrument selected for measuring the surface of the film has a broad band thermal detector and a 3.45 μm spike band pass filter. The filter makes the instrument blind to all energy outside of 3.45 μm and enables it to measure the temperature of the surface of the plastic film without being influenced by the hot process environment. Figure 4.9 shows a similar solution for 13 μm (5 x 104 in.) thick polyester (polyethylene terephthalate 聚对苯二甲酸乙二醇酯) film under about the same temperature conditions. Here the strong polyester absorption band from 7.7 to 8.2 μm dictates the placement of a 7.9 μm spike filter placed in front of the same broad band detector as that used in the polyethylene application.


Figure 4.8: Measuring temperature of polyethylene


Figure 4.9: Measuring temperature of polyester

7.7 to 8.2 μm


IR Filter


IR Filter


Using Line Scanners for Monitoring Continuous ProcessesContinuous processes are most often processes in constant and uniform motion. When this happens. an imaging system may not be required to cover the full process image. To monitor and control processes in motion, an infrared line scanner can be used, scanning normal to the process flow, to generate a thermal strip map of the product as it passes the measurement site line as illustrated in Figure 4.10. If more than one measurement site line is required. additional line scanners may be deployed.


Figure 4.10: Line scanner for continuous process monitoring


4.2 Infrared Thermographic Equipment OperationBecause of product performance advances and meticulous (小心翼翼的)human engineering on the part of manufacturers, infrared thermographic equipment is far easier to operate in the twenty-first century than it was in the 1990s. It is relatively simple for the novice (新手) thermographer to turn on the equipment. aim at a target and acquire an image. Consequently, it is also easier than ever to misinterpret findings.


Preparation of Equipment for OperationEven when using point sensing instruments. preparation for makingmeasurements requires an instrument operation check. a battery status checkand a simple calibration check. This preparation follows a simple checklist, which is a critical element in the successful field operation of thermal imagingequ ipment. Equipment preparation is crucial in field measurements becauseof time consumption, measurement scheduling and the availability of on-silepersonnel.

A seemingly small oversight in equipment preparation can wasteconsiderable time and money. Calibration against a known temperaturereferencc is required for all infrared measuring instrumcnts and is normallyaccomplished through radiation reference sources also known as blackbodysimulators. These temperature controlled cavities or high emissivity surfacesthat are designed to simulate a blackbody target at a specific temperature orover a specific temperature range with traceability to the National Institute forStandards and Technology (NIST). Factory calibration and traceability isprovided by the manufacturer.


Bccause most quantitative thermographic instruments measure radiantenergy values converted to temperature readings by a computer, calibrationinformation is usually stored in the computer software and is identified with aspecific instrument serial number. If a specific instrument calibration is notavailable in the software, the computer will usually default to a generic calibration for that class of instrument. In addition to a blackbody calibration, the software is usually provided with correction functions for ambient effects such as atmospheric attenuation as a function of working distance and for emissivity correction.

Default settings for these values are normally in effect unless the operator chooses to alter them. Checking calibration of a thermal imaging system in detail requires placing a blackbody reference source in front of the instrument so that ilt subtends a substantial area in the center of the displayed image (much greater than the instantaneous field of view). The correct measurement conditions must be set into the computer where applicable [example. working distance = 10 m (33 ft), ambient temperature = 25 °C (77 ° F), emissivity = 1, etc.] and the temperature reading compared to the reference source setting. The spot measurement software diagnostic should be used if available. The detailed calibration should include the widest range of temperatures possible.


If the instrument is out of cal ibration. it may be possible to recalibrate it underceltain conditions. (Refer to the operator's handbook.) Otherwise, it may benecessary to return it to the factory for recalibration.

A detailed calibration check should be made at least every six months.Periodic calibration spot checks should also be performed. Ideally, calibration checks should be done before and after each field measurement mission and can be accomplished by means of a high quality radiation thermometer and high emissivity sample targets. To perform a spot check. place the target in front of the instrument. Set emissivity the same for both instruments and measure the apparent temperature simultaneously with the imager and the radialion thermometer. Spot checks should be run at a few temperatures covering the range of temperatures anticipated for the specific measurement mission. Because the fields of view and spectral ranges of the twoinstruments may not match, exact correlation may not be possible.The errors should be repeatable from day to day, however, and the procedure will provide a high degree of confidence in the results of the measurement mission.


Transfer caljbration using a radiation reference source in the field is effectivewhere extremely accurate measurements are required within a narrow rangeof temperatures. Typically, instrument calibrations are performed over a broadrange of temperatures. with certain maximum allowable errors occurring attemperatures within this broad range. The transfer calibration can optimizeaccuracy over a limited range. The procedure requires introducing a radiation reference source into the total field of view along with the target of interest with the reference set very close to the temperature range of interest. Using the diagnostic software to measure the apparent temperature differences between the reference and various points of the target of interest should provide improved accuracy. The equipment checklist used in preparation for a day of field measurements helps ensure that there will be no surprises on site. A standard checklist should be prepared to include all items in the thermographic equipment inventory. These should include instrument spare lenses, tripods, harnesses, transport cases, carts, batteries, chargers, liquid or gaseous cryogenic coolant, safety gear, special accessories, film, diskettes, spare fuses, tool kits, data sheets, operator manuals, calibration data, radiation reference sources, interconnecting cables, accessory cables and special fixtures.


The batteries mentioned on the mission checklist should be fully chargedbatteries. It is the thermographer's responsi bility to ensure that there is acomfortable surplus of battery power available for each field measurementsession. The fact that batteries become discharged more rapidly in coldweather also must be considered when preparing for field measurements.


Procedures for Checking Critical Instrument Performauce ParametersThere are established procedures for checking the critical performanceparameters di scussed in Chapter 3. The parameters that are most importantto most measurement programs are:

1. Thermal resolution or minimum resolvable temperature difference (MRTD).2. Imaging spatial resolution or instantaneous field of view (lFOV). and3. Measurement spat ialresolution (IFOVmeas).

Comments on item 1■ Thermal resolution or ■ minimum resolvable temperature difference (MRTD) or ■ noise equivalent temperature difference (NETD).the above describe the same phenomenon.


Comments:■ Thermal resolution or ■ minimum resolvable temperature difference (MRTD) or ■ noise equivalent temperature difference (NETD).


Recalling!

Temperature sensitivity is also called: thermal resolution

or minimum resolvable temperature difference (MRTD) or

noise equivalent temperature difference (NETD).

for my ASNT exam


Figure 4.11 : Test configuration for minimum resolvable temperaturedifference measurement


■ Thermal Resolutionthermal resolution can be measured using a procedure developed for military evaluation of night vision systems. This procedure uses standard resolution targets as illustrated in Figure 4.11 and is described as follows:

1. Set up the test pattern such that ΔT exceeds the manufacturer's specification for minimum resolvable temperature difference.

2. Determine the spatial frequency It of the target in cycles per milli-radian as follows:

a. the number of radians equals the bar width W divided by the distance d to the target (example: 2 mm at 1 m = 2 mRad); and

b. the spatial frequency, It = 1 cycle / (1 bar + 1 space) = 1 / (W + S).(If W = 2 mRad and S = 2 mRad, then It = 1/(2 + 2) = 0.25 cycles per milli-radian).


3. Reduce the ΔT until the image is just lost (note ΔTH) . Raise ΔT until the image is just reacquired (note ΔTC) then:

ΔT = ABS(ΔTH) + ABS(ΔTC)2

4. Then change distances or use different size bar targets to plot minimum resolvable temperature difference for other spatial frequencies.


Figure 4.12: Test configuration formodulation transfer functionmeasurement


■ Imaging Spatial ResolutionImaging spatial resolution of scanning imagers can be ensured using another procedure that stems from military night vision evaluation protocol and uses the same standard bar target. The procedure measures the modulation transfer function (MTF), a measure of imaging spatial resolution. Modulation is a measure of radiance contrast and is expressed:

Modulation = Vmax – VminVmax + Vmin

where:V = the voltage analogue of the instantaneous radiance measured.


Modulation transfer is the ratio of the modulation in the observed image to that in the actual object. For any system the modulation transfer function will vary with scan angle and background and will almost always be different when measured along the high speed scanning direction than it is when measured normal to it. For this reason. a methodology was established and accepted by manufacturers and users alike to measure the modulation transfer function of a scanning imager and, thereby to verify the spatial resolution for imaging (night vision) purposes.


A sample setup is illustrated in Figure 4.12 for a system where the instantaneous field of view is specified at 2.0 mRad using the same setup as illustrated in Figure 4.10. The procedure is as follows:

1. Set ΔT (where ΔT = T2 – T1) to at least 10x the manufacturer's specified minimum resolvable temperature difference (MRTD).

2. Select distance to simulate the manufacturer's specified imaging spatial resolution. The bar width W represents one resolution element. For example. instantaneous field of view can be calculated where bar width W= 2 mm and distance d = 1 m.

IFOV = W/d = 2mm/1m

where: d= distances to target, W=bar width


3. Display imager's horizontal line scan through the center of the bar target.4. Calculate the modulation transfer function:

Modulation = Vmax – VminVmax + Vmin

where:MTF = modulation transfer function (a ratio).Vmax = maximum measured voltage V.Vmin = minimum measured voltage V.


5. If the modulation transfer function (MTF) = 0.35* or greater. the imagermeets the imaging spatial resolution specification. (If the signal representingthe horizontal scan line is not accessible, consult the manufacturer for analternate means by which modulation transfer function can be verified. In adigital image, the gray level may replace the Voltage value. Note: There aredisagreements among users and manufacturers regarding the acceptableminimum value of modulation transfer function to verify imaging spatial resolution with values varying between 0.35 and 0.5, depending on the manufacturer and the purpose of the instrument.)


■ Measurement Spatial ResolutionMeasurement spatial resolution (IFOVmeas) can be measured using a procedure that measures the slit response function (SRF) of the imaging system. This procedure was developed by instrument manufacturers and is generally accepted throughout the industry. In this technique, a single variable slit is placed in front of a blackbody source and the slit width is varied until the resultant signal approaches the signal of the blackbody reference.Because there are other errors in the optics and the 100 percent level of slit response function is approached rather slowly, the slit width at which the slit response function reaches 0.9 is usually accepted as the measurement spatial resolution. Again, there are some disagreements as to whether 0.9 or 0.95 should be considered acceptable. The test can establish whether the imager meets the manufacturer's specifications for measurement spatial resolution. The test configuration for slit response function determination is illustrated in Figure 4.13.


Figure 4.13: Test configuration for slit response function measurement


1. Set ΔT (where ΔT = T2 – T1) to at least 10x the manufacturer's specified minimum resolvable temperature difference (MRTD).

2. Select distance and slit width to simulate the manufacturer's specified measurement spatial resolution. The bar width W (mm) represents one resolution element. For example, for a 3 mRad measurement spatial resolution, if d= 1m, W = (1.0 x 0.003) = 3 mm.

3. Display imager's horizontal line scan through the center of the bar target.


4. Open slit until Vmeas = Vmax.

5. Close slit until Vmeas = 90% of Vmax and measure slit width (W).

6. Compute: IFOVmeas = W·d-1. This should be equal to or smaller than the manufacturer 's imaging spatial resolution specification.

Again, if the signal representing the horizontal scan line is not accessible, consult the manufacturer for an alternate means by which measurement spatial resolution can be verified.


Common Mistakes in Instrument OperationRemembering a few key cautions regarding proper equipment application canhelp the thermographer to avoid some common mistakes. The followingguidelines should be observed.

1. Select an instrument appropriate to the measurement application inaccordance with the guidelines reviewed in Chapter 3.

2. Leam and memorize the startup procedure.3. Leam and memorize the default values.4. Set or use the correct emissivi ty and be particularly cautious with

emissivity settings below 0.5.5. Make sure the target to be measured is larger than the measurement

spatial resolution of the instrument. (FOV? or IFOV?)6. Aim the instrument as close to normal (perpendicular) with the target

surface as possible.7. Check for reflections off the target surface ρ and either avoid or

compensate for them.8. Keep sensors or sensing heads as far away as possible from very hot

targets.


■ Learning the Startup ProcedureLearning the startup procedure thoroughly is essential, particularly for thermographers who operate several different models of thermographic and thermal sensing equipment. Efficient startup lets the data gathering process begin with no unnecessary delays; it saves valuable on-site time and inspires confidence of facility personnel. A quick review of the operator's manual and a short dry run before leaving home base is usually all that is required.


■ Memorizing the Default ValuesMemorizing the default values provided in the operator's manual is another important contribution to time efficiency and cost effectiveness. These include default values for several important variables in the measurement such as emissivity, ambient (background) temperature, distance from sensor to target, temperature scale (degrees Fahrenheit or Celsius), lens selection and relative humidity. It is important to remember that the instrument's data processing software automatically uses these values to compute target temperature unless the thermographer changes these values to match the actual measurement conditions. Typical default values are 1 m (3 ft) distance to target, emissivity of 1.0 and background temperature of 25 °C (77 °F). Failure to correct for these can result in substantially erroneous results, if, for example the target is known to be 10 m (33 ft) away is known to have an effective emissivity of approximately 0.7 and is reflecting an ambient background temperature of 10 °C (50 °F). By memorizing the default values, the thermographer will know when it is necessary to change them and when time can be saved by using them unchanged without referring to a menu.


■ Setting the Correct Effective EmissivitySetting the correct effective emissivity is critical in making temperature measurements. Table 2.2 can be used as a guide when obtaining absolute temperature values is not critical. When measurement accuracy is important. it is always better to directly determine the effective emissivity of the surface to be measured using the actual instrument to be used in the measurement and under similar operating conditions. This is because emissivity may vary with temperature, surface characteristics and measuremenl spectral band and may even vary among samples of the same material. There are several methods that may be used to quickly estimate target effective emissivity. One known as the reference emitter technique can be used to detennine the emissivity setting needed for a particular target material. The determination uses the same instrument that will be used for the actual measurement. The procedure is illustrated in Figure 4.14 and is described as follows:


Table 2.2: Normal spectral emissivities of common materials


Figure 4.14: Test configuration for the determination of effective emissivityusing the reference emitter method.

#2 This area was painted, taped or conditioned with material of known emissivity.

#4; Set instrument emissivity using the known emissivity value and observed the material temperature

#5: Adjust emissivity to obtained the sample temperature obtained in #4

Non conditionedSurface


1. Prepare a sample of the material large enough to contain several spot sizes or instantaneous fields of view of the instrument. A 100 mm x 100 mm (4.0 in. x 4.0 in.) sample may be big enough.

2. Spray half of the target sample with flat black (light absorbing) paint, cover it with black masking tape or use some other substance of known high emissivity.

3. Heat the sample to a uniform temperature as close as possible to the temperature at which actual measurements will be made.

4. Make certain that the value for background temperature has been properly entered. Then set the instrument emissivity control to the known emissivity of the coating and measure the temperature of the coated area with theinstrument. Record the reading.

5. Immediately point to the uncoated area and adjust the emissivity set until the reading obtained in step 4 is repeated. This is the emissivity value that should be selected in measuring the temperature of this material with this instrument.


■ Measuring and Reporting Temperature Accurately - Filling theInstantaneous Field of View IFOVmeas

If true temperature measurement of a spot on a target is required, the spot must completely fill the instrument’s measurement spatial resolution (IFOVmeas). lf it does not. some useful information about the target can still be learned. but an accurate reading of target temperature cannot be obtained. The simple expression. D= α∙d, can be used to compute measurement spot size D at the target plane from a working distance “d” where α is taken to be the manufacturer's published value for measurement spatial resolution.

For example. if the target spot to be measured is 5 cm (2 in.) and the calculated spot size D is 10 cm (4 in .), move the instrument closer to the target or use a higher magnification lens if either is possible. If not, expect the reading to be affected by the temperature of the scene behind the target. Also. be sure to allow for aiming errors and instrument imperfections. An extra 30 percent should be enough.


■ Aiming Normal to the TargetAiming normal (perpendicular) to the target surface or as close as possible to normal is important because the effective emissivity of a target surface is partially dependent on the surface texture. It stands to reason, then, that if the surface is viewed at a skimming angle, the apparent texture will change, the effective emissivity will change greatly and the measurement will be affected by misleading reflections. These can result in cold errors as well as hot errors.

A safe rule is to view the target at an angle within 30 degrees of normal (perpendicular). If the target emissivity is very high this can be increased to as high as a 60 degree angle if necessary.

Note:where: θ angle from normal θ = 30ºθ = 60º where ε is high


■ Recognizing and Avoiding Reflections from External SourcesRecognizing and avoiding reflections from external sources is an important acquired skill for the thermographer. If there is a concentrated source of radiant energy (point source) in a position to reflect off the target surface and into the instrument, steps should be taken to avoid misleading results. There is the greatest likelihood of errors due to point source reflections when the (1) target emissivity is low, (2) the target is cooler than its surroundings or (3) the target surface is curved or irregularly shaped.

It should be noted that, although most errors due to reflections are from sources hotter than the target, reflective errors from cold sources can also occur and should not be discounted. A common source of reflective error is the reflection of the cold sky off glass or other reflective surfaces. If a temperature anomaly is caused by a point source reflection, it can be identified by moving the instrument and pointing it at the target from several different directions. If the anomaly appears to move (changes/ varies) with the instrument, it is a point source reflection. Once identified. the effect can be eliminated by changing the viewing angle, by blocking the line of sight to the source or by doing both.


Errors due to the reflection of an extended source, however, cannot be eliminated in this manner. The ambient instrument background (what the instrument sees reflected off the target surface) is the most commonly encountered example of an extended source reflection.

Errors due to extended source reflections are more likely when the target emissivity is low or when the target is cooler than its surroundings. Most instrument menus include a provision for entering the ambient background temperature if it is different from the default setting. The system will automatically correct the temperature reading. This will also work if the ambient background is an extended source such as a large boiler. In this situation, substituting the boiler's surface temperature for the background ambient setting will correct the temperature reading.

Keywords:Reflection due to Point sourceReflection due to Extended source


■ Measuring the Appropriate Background Temperature Using the Instrument

A technique commonly used by thermographers to determine an appropriate setting for "ambient background temperature" requires a piece of aluminum foil large enough to fill the total field of view of the instrument. First. crush the foil into a ball and then flatten it so that it simulates a diffuse reflecting surface. Next, place the foil so that it fills the instrument's total field of view and reflects the ambient background into the instrument. Allow the foil to come to thermal equilibrium. With the instrument's emissivity ε set to 1.00, measure the apparent temperature of the foil. Use this apparent temperature reading as the ambient background temperature setting.


■ Avoiding Radiant Heat Damage to the InstrumentAvoiding radiant heat damage to the instrument is always important. Unless an infrared sensing or imaging instrument is specifically selected or equipped for continuous operation in close proximity to a very hot target. it may be damaged by extensive thermal radiation from the target. A good rule for the thermographer to follow is "don't leave the instrument sensing head in a location where you could not keep your hand without suffering discomfort.“ Accessories such as heat shields and environmental enclosures are available from manufacturers for use when exposure to direct radiant heat is unavoidable. These accessories should be used to protect the instrument when appropriate.


■ Temperature Differences Between Similar MaterialsParticularly in electrical applications, it is critical to measure and report temperature differences between similar components with similar surface materials. such as the fuses on different phases of the same supply. Strict observance of the procedures regarding the use of the correct (1) effective emissivity value, (2) filling the measurement spatial resolution, (3) using the correct background temperature, (4) setting and using the correct viewing angles θ will ensure that these differences are measured and reported correctly


4.3 Safety and HealthSafety and health considerations are critical to successful thermographyprograms as well as to the welfare of the thermographer and client personnel.Strict adherence to applicable codes is the responsibility of thethermographer. It is essenlial that the basics of these regulations beunderstood.


■ Liquid and Compressed GasesSome instruments in the field use liquid or compressed gases for detector

cooling. The handling of these materials can be hazardous and it is thethermographer's responsibility to learn safe practices and to adhere to them. In general, these procedures are included in the safety regulations for each facility. They can also be found in the operator's manuals for these instruments. Some instruments use liquid nitrogen LN as a detectorcoolant Liquid nitrogen is not very hazardous but some safety precautions should be observed. The following four guidelincs for using and storing liquid nitrogen are taken from the AGEMA Model 782 Operator'sHandbook:

1. Never store the liquid in sealed containers. Liquid nitrogen and similar cryogenic liquids are always stored in Dewar flasks or the equivalent insulated containers, with loosely fitting covers that allow the gas to vent without building up dangerous pressures,

2. Never come into direct contact with liquid nitrogen. Serious frost bite injury (similar to a bum) can result if the liquid is allowed to splash in to the eyes or onto the skin.


3. Always re place the filler cap after filling to avoid the risk of spillage and condensation.

4. It is advisable to transfer some of the liquid from the storage Dewar to a smaller vessel (that is a vacuum jug) to effect more convenient filling and minimize spillage. Slowly pour a small amount into the instrument's liquid nitrogen chamber and wait until boiling ceases. This ensures that the chamber is at the same temperature as the liquid and minimizing splashing and spillage. Fill the chamber completely and replace the filler cap.“


Dewar Flask for LN Loosely fitting covers that allow the gas to vent without building up dangerous pressures


■ BatteriesProcedures for the handling of batteries and their safe disposal must also be followed by the thermographer. In general these procedures are included in the safety regulations for each facility. They can also often be found in the instrument operator's manuals. Generally, instructions for the safe disposal of batteries are provided in the literature accompanying the batteries. In the absence of such instructions, exhausted batteries should be considered as hazardous waste and handled accordingly.


■ Electrical SafetyFailure to recognize and observe electrical safety regulations can result in electrical shock and irrepairable damage to the human body. Electrical current flowing through the heart, even as small as a few milli-ampere can disrupt normal heart functions and cause severe trauma and some times death. In addition. body tissue can be severely and permanently damaged. Shock hazards are proportional to equipment operating voltage levels and distance from the hazard. Voltage levels as low as 60 V causing current to flow through the chest area with low skin resistance can be lethal. Examples of electric shock current thresholds and typical electrical contact resistances are given in Table 4.1 .

Safety practices are important as well. One good safety rule to follow is to never touch electrical contacts unless qualified to do so. areing can also be lethal and even low voltage equipment may produce killing areas. It is important that only trained personnel wearing are protective gear be permitted to approach energized equipment. Spectators should not approach at all.


Safety codes have been developed that specify the minimum distances to be maintained from live equipment and, in addition, protective clothing and devices (face shield, protective clothing and insulated gloves) are required in all facilities. Although the codes may vary from facility to facility, they all spell out the safety rules to which thermographers are expected to adhere. Examples of National Electrical Safety (NES) codes currently being observed in facilities in the United States and Canada that specify the minimum clearance zone from operating high voltage equipment in terms of voltage and distance are described in Table 4.2. thermographers must be aware of the safety regulations in force and know the recommended protective clothing. It is recommended that the applicable safety guidelines set forth in the following documents be reviewed:

1. National Fire Protection Association NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplace, 1995, and

2. National Fire Protection Association NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, 1994.


Table 4.1: Electric shock current thresholds and skin contact resistances


Table 4.2: Examples of specified clearance distances from high voltage equipment


4.4 Record KeepingKeeping thorough and detailed records is very imponant to the thermographer, particularly when performing a comprehensive program of thermographic facility surveys. As discussed in Chapter 3, most equipment manufacturers sell software that provides a filing system to maintain records of all images and accompanying data and comprehensive report preparation software for timely and comprehensive reporting of the findings of infrared surveys and other measurement missions. Although recording the actual findings is the basic reason for record keeping, support records are also important. These records include equipment status history as well as personnel qualification documentation.


Records of surveys should be documented to include:

1. day, date, location, identification of test site and equipment or components inspected;

2. thermographer's identification and qualifications; 3. equipment used and calibration history (when last calibrated. when last

spot check was made, etc.); 4. what was inspected, what was not inspected and why; 5. visual test reports of cracking, etc. with photographs if appropriate; 6. other observations noted by the inspector. such as noise and aroma; 7. backup video tapes of the entire measurement survey; and 8. specific mention of any critical findings.

All images should be maintained as files for future reference and trending. Reports may be tailored to include only those items considered significantbut records should be maintained for all measurements. Maintenance and repair records of all equipment and accessories should also be kept. Easily accessible and easily understood notes and records are a measure of the competence and professionalism of the thermographer and lead to credibility in the eyes of management, whatever the industry or discipline.


Easily accessible and easily understood notes and records are a measure of the competence and professionalism of the thermographer and lead to credibility in the eyes of management, whatever the industry or discipline.



2. d3. b4. a5. b6. a7. c8. a9. d10. d

Q&A


Q1. Apparent but not real temperature changes recorded by an infrared instrument can be due to:a. emissivity, reflectivity and mass transport differences.b. emissivity, reflectivity and geometric differences.c. thermal capacitance, reflectivity and geometric differences.d. thermal capacitance, mass transport and emittance differences.

Q2. Apparent temperature changes recorded by an infrared instrument that are, in fact real temperature changes can be due to:a. emissivity, reflectivity and mass transport differences.b. emissivity. reflectivity and geometric differences.c. thermal capacitance, reflectivity and geometric differences.d. thermal capacitance, mass transport and energy convertion.


Q3. Sun glints 闪耀 cause false indications of temperature changes. In this respect, they are similar to:a. solar heating.b. emissivity artifacts.c. resistive heating.d. mass transport.

Q4. The lower the temperature of a target to be measured, the more important it is to:a. correct for ambient reflections.b. fill the instrument 's measurement spatial resolution with the target.c. use a cooled detector.d. keep batteries fully charged.


Q5. The higher the temperature of a target to be measured, the less important it is to:a. fill the instrument's measurement spatial resolulion with the target.b. correct for ambient reflections.c. correct for atmospheric absorption in the measurement path.d. keep batteries fully charged.

Q6. Placing a blackbody reference source next to a distant target will usually help correct for:a. the effect of atmospheric absorption in the measurement path .b. ambient reflections off the target surface.c. target surface emissivity artifacts.d. point source reflections.


Q7. To make an effective infrared temperature measurement, the anglebetween the target surface and the instrument's line of sight should be:a. always 90 degrees (perpendicular).b. any angle providing the target always fills the measurement spatial

resolution of the instrument.c. as close at possible to 90 degrees but not less than 60 degrees.d. anywhere between 30 degrees and 45 degrees.

Q8. If a target does not fill the measurement spatial resolution of the measuring instrument at a convenient measurement distance, it may benecessary to:a. use a higher magnification lens or move in closer,b. place a blackbody reference next to the target.c. use the instrument 's electronic zoom feature,d. use more than one isotherm to make the measurement.


Q9. Differential thermography can be very useful because it:a. tends to minimize the effects of surface emissivity artifacts.b. tends to emphasize only those areas where temperature changes occur.c. helps record changes for thermal trending purposes.d. is all of the above.

Q10. When planning a measurement mission, it is important to remember thatbatteries:a. may never reach fu ll charge.b. are about the least reliable element at a thermographer's disposal.c. lose their charge more rapidly in cold weather.d. are all of the above.


Chapter 5Applications


5.1 Overview of ApplicationsBecause temperature is, by far, the most measured and recorded parameter in industry, it is no surprise that applications for temperature measurement and thermography are found in virtually every aspect of every industry. Because of the wide spread applicability of thermal sensing and thermography, attempting to classify applications into formal categories meets with considerable overlap. Because of this ambiguity, and because the thermal principles of investigation involved should be well known by the qualified thermographer, applications are presented in this chapter by thermal principles of investigation categories, as set forth in the Infrared Thermal Testing Method, Level III Topical Outline contained in Recommended Practice No. SNT-TC-JA (1996) as follows:

1. exothermic and endothermic investigations,2. friction investigations,3. fluid flow investigations,4. thermal resistance investigations and5. thermal capacitance investigations.


5.2 Exothermic and Endothermic InvestigationsAn exothermic process is one that releases heat and exhibits a temperatureincrease. An endothermic process is one that absorbs heat and exhibits atemperature decrease. The link between these methods is that theinvestigator does not need to apply any thermal stimulation. The relevantthermal pattern exists in the subject because of another process performed on (or within) the subject. Applications for thermograpgy in this area arecommonly found in electrical and electronic diagnostics, chemical processessuch as the application of foam-in-place insulation, fire detection, night visionand surveillance, animal studies, heating and cooling systems and otherareas where thermal energy is released or absorbed.

Keywords:the investigator does not need to apply any thermal stimulation.


Electrical ApplicationsElectrical findings represent the primary use of infrared thermography in facilities and utilities. They also represent the most straightforward application of the equipment. The most common electrical findings are caused by high electrical resistance, short circuits, open circuits, inductive currents and energized grounds. Much of the routine scanning is done qualitatively, but quantitative thermography is required in many instances to estimate true temperature rises. Specifically in electrical applications, the flow of current through a conductor generates heat in direct proportion to the power dissipated. This is directly proportional to the electrical resistance and to the square of the current (P = I2R) and is commonly called I2R loss. A poor connection or, in some cases, a defective component, will have an increased resistance, resulting in a temperature increase and, consequently, a temperature rise in the area of the discontinuity.


Alas!

Electrical Applications is NOT

“thermal resistance investigations”

for my ASNT exam


High electrical resistance is the most common cause of thermal hot spots in electrical equipment and power lines. When the line current is relatively constant and resistance is higher than it should be, additional power is dissipated and a thermal anomaly occurs. This is frequently dangerous and always costly in terms of valuable waits lost, unwanted heat and accelerated aging of equipment, which results in premature replacement of equipment.

Typical examples of resistive heating include loose connections, corrodedconnections, missing or broken conductor strands and undersized conductors.Figure 5.1 is an example of excessive heating caused by high resistance at aconnection (upper center) because of deterioration of the connection. Theconnection appears to be more than 5°C (9°F) warmer than the adjacentconnections. In power lines and switch yards, hot connections caused bydeterioration are the most common findings that are associated with potentialfailures. Short circuits are another cause of electrical failure. When they occurin a power line, they usually are extremely brief in duration and haveimmediate and disastrous results. Within an operating component, however. the shorted section will cause excessive current to flow with resultant heating, and this frequently can be detected and diagnosed using thermographic equipment.


Figure 5.1: Excessive heating due to a defective electrical connection

The connection appears to be more than 5 °C (9 °F) warmer than the adjacentconnections.


Excessive heating due to a defective electrical connection


One example of this would be shorted sections of a current transformer winding causing the transformer to appear hotter than normal and/or hotter than other similar devices. Similar problems can occur within power supplies and within rotating equipment such as motors and generators. Open circuits do not generally show up as hot spots and are often overlooked by inexperienced thermographers as indications of potential problems. An operating element running cooler than normal may indicate that the element is open and inoperative. A common problem with inverters, for example, is blown (open) capacitors that appear cool. Power supplies, resistor or integrated circuit chips that are open and inoperative will usually be cooler than normal, although the malfunction may cause excessive heating elsewhere in the operating clement.


Inductive currents flowing with in ferrous components or elements that arewithin the magnetic field of large equipment (i.e., the main generator in apower plant) can cause excessive heating. Warm areas can appear in motorframes and structural elements and several examples have been documentedwhere steel bolts have been inappropriately used to replace nonferrous boltsin framework supporting large rotating machinery. Heat caused by inductive heating does not always lead to failure, but should be documented by the conscientious thermographer. Energized grounds occur in plants and facilities, sometimes as the result of partial insulation breakdown in an operating element. These findings are, in many cases, considered life safety situations. Because an energized ground connection is usually extremely hot , there is seldom difficulty identifying it thermographically. The problem is tracing the cause, which may be elusive 难找的. The ground connection may also be carrying induced currents because of a breakdown of an element in close proximity. Most often the diagnosis requires considerable input from knowledgeable facilities personnel.


When starting new thermography programs, it is necessary to establishguidelines to determine how much temperature deviation from normalconstitutes an electrical problem. There is no simple standard because thereare so many factors, including ambient variations, that can influencetemperature. With this caution in mind, it is reasonable to set forth guidelinesto assess the severity of findings based on common sense and experience aswell as on temperature readings. Most facilities have rule-of-thumb systemswhereby they classify the potential severity of a finding based on temperaturerise and known load conditions.

Assessing Severity:Common sensePat experiencesTemperature reading ∆TRule of thumbLoad condition


Moisture in AirframesThe detection of moisture in airframe sections can be accomplishedthermographically because of the endothermic process that takes place whenthere is moisture ingress in an airbomc structure and this water freezes.When thermal images are taken immediately after the aircraft lands. the skinabove the sections where moisture has entered show up as cool spots on thethermogram, as seen in Figure 5.2.

In chemical processes, an example of an exothermic reaction is the installation of foam-in-place polyurethane insulation. As the liquid chemicals are released into the cavity, they solidify into a foam and release heat. This heat is conducted into the walls of the cavity wherever the foam is produced. As a result. the cavity walls are uniformly heated by a successful blow. A thermographic investigation can evaluate the effectiveness of this process by mapping the uniformity of the temperature distribution on the outside walls cool spots would indicate sections where the foam had not migrated.


Figure 5.2: Water ingress in an aircraft section


Process Control and Product MonitoringFor many years, infrared sensors have been used for quantitative surfacetemperature monitoring of products and processes. When measurement ofone point in the process, or even a number of points, is not consideredadequate to characterize the process for successful monitoring or control,infrared thermography can be used, The most significant aspect of thisapproach is that it is unique and unprecedented. Infrared point sensors areused, when appropriate, in place of conventional temperature sensors.Infrared scanners and imagers, however, are the only practical means toacquire a high resolution thermal map of an entire surface in real time (at ornear television display rates). Full surface thermal process control was not aviable option until the integration of computers and image processingsoftware with thermal scanners.


Line Scanners or Imagers for Mapping of Continuous ProcessesFull image process control can be defined as using an infrared thermal imageas a model against which to compare, and thereby control, part or all of thethermal surface characteristic of a product or process. If the process ismoving at a uniform, predictable rate, a thermal image can be produccd by aline scanner scanning normal to the motion of the process as illustrated inChapter 4. Figure 4.10. The control method is similar to thaI used in pointsensing applications, although far broader in scope. The scanner or imager isfirst used to characterize the thermal map of the product under idealconditions to produce, digitize and store a criterion image - what the idealthermal distribution would be if the process resulted in perfectly acceptableproducts as designed. During the actual process, the thermal map, or anycritical portion of the map, is constantly compared to the stored criterionimage model by means of image subtraction and/or statistical analysistechniques.


The differences produced by this comparison are used to adjust or correct the settings of the process mechanisms that govern the heat applied, or to alarm and automatically reset the process. Figure 5.3 shows the evaluation of a web process used on the outside of drywall construction material.

The thermogram clearly shows excessive heat on the right edge of the material, a condition that can cause the paper to become brittle. The information derived from. The thermogram is used to correct the temperaturedistribution, thus resulting in a more acceptable product. Although the image is not of an automatically controlled process. it would be possible to close the loop to maintain ideal temperature distribution automatically.


Figure 5.3 Thermogram of paper process


Spectral Considerations in Product and Process ApplicationsMany products. both simple and complex, have complex spectralcharacteristics in the infrared region. Spectral filtering (adaptations) of the measuring instrument can exploit these complex spectral characteristics to measure and control product temperature without contact. A good example of the exploitation of spectral characteristics in the monitoring of incandescent lamp temperatures during production. An important generic example of the need for spectral selectivity is in the measurement of plastics being formed into films and other configurations. Several examples of this exploitation are illustrated in the detailed discussion of spectral considerations in Chapter 4.


Night Vision, Search, Surveillance, Security and Fire DetectionThe level of heat given off by the human body makes it readily detectable tothermographic instruments. Similarly. exothermic actions of engines andmoving vehicles make them good targets for infrared surveillance applications.Night vision, search, surveillance and security applications are, with very fewexceptions, qualitative applications of infrared thermography. They providethe user with the capability to see through an atmospheric path in totaldarkness. The clarity of the image is of critical importance and temperaturemeasurement is not required. Ideally, in these applications. the objective is todisplay (and sometimes to record) an image that has the very best spatialresolution at the longest possible range under the most adverse atmosphericconditions. An example of a typical surveillance image is the thermogram of ahelicopter taken at night, shown in Figure 5.4.


Figure 5.4: Thermogram of helicopter taken at night


Thermogram of helicopter taken at night


Thermogram of Jet


Aircraft Under IR Trap


Aircraft Under IR Trap


Instruments used for these applications evolved from military programs basedon the need to detect and identify tactical targets through atmosphere in thedark and in bad weather. For this reason, they generally operate in the 8 to 12μm spectral window where the atmosphere has very little absorption.Exceptions to this generality are infrared seeking and homing sensors thatare sensitive to specific target emission signatures. such as rocket engineplumes. These instruments usually operate somewhere in the 3 to 5 μmregion. The same qualitative instruments can be readily adapted to firedetection applications. From the ground or the air, they provide the capabilityof detecting incipient fires and unextinguished portions of forest fires.

The 8 to 12 μm spectral region over which they operate also provides improved visibility (less absorption loss) through smoke and fog.


8 to 12μm spectral region over which they operate also provides improvedvisibility (less absorption loss) through smoke and fog.


8 to 12μm spectral region over which they operate also provides improvedvisibility (less absorption loss) through smoke and fog.


8 to 12μm spectral region for Astrology Stellar cluster and star-forming region M 17


Animal StudiesBody heat allows infrared thermographic studies of animals to be made.Inflammation raises the temperature of infected, diseased or traumatizedportions of the body, as illustrated in Figure 5.5. This shows the thermalcontrast between a bruised equine foreleg (left) and a normal foreleg (right).


Figure 5.5: Injured equine foreleg (left) appears warmer than a normalforeleg


Injured equine foreleg (left) appears warmer than a normal foreleg

http://www.thermomed.org/en/veterinarymedicine.html


5.3 Friction InvestigationsFriction generates heat as energy conversion from mechanical energy tothermal energy. Sliding friction is a force that acts on one body sliding overanother. The maximum force of friction that one body is capable of exertingover another is directly proportional to the normal, or perpendicular force withwhich the bodies are pressed together. This proportionality is called thecoefficient of friction and the equation for sliding friction is:

f = μN

where:f = the maximum force of friction.μ = the coefficient of friction.N = the normal force with which the two bodies are pressed together.


Work energy is expended by frictional force and converted to stored heat.This stored heat is then conducted, convected and radiated to thesurroundings. which can be sensed and measured using thermalinstruments. The heating and resultant damage from excessive friction is one of the most common types of mechanical failure detectable by infrared thermography. Many of the mechanical failures located by thermography occur in rotating machinery. Problems caused by friction include worn, contaminated or poorly lubricated bearings and couplings and misaligned shafts. Typical findings occur in motor bearings such as that shown in Figure 5.6 where the temperature imbalance on a blower fan is because of uneven friction as seen through the end screen. The apparent temperature on the tower section is about 10°C (18°F) warmer than the upper section. Friction investigations applicable to thermography also include air turbulence flow studies in aircraft and spacecraft modeling, machine gear and belt temperature monitoring and effectiveness studies for the cooling of machine tools.


Figure 5.6: Overhead Motor Bearing (bottom)


5.4 Fluid Flow InvestigationsFor successful fluid flow investigations to be performed, a temperature higheror lower than ambient must be induced into the fluid paths. Often, thiscondition already exists but somelimes the investigator must artificiallyintroduce such a fluid. Fluid flow applications include piping, valves, heatexchangers, cooling towers, effluent mapping and ocean mapping. Inpredictive maintenance and plant condition monitoring, many pipe blockageand leakage conditions can be detected using infrared thermography. Ideally,the condition is simple to detect if the valve or pipe section is not covered with insulating material, and if the temperature of the fluid conducted by the valve or pipe section is sufficicnlly hotter or cooler than ambient. when conditions are not ideal, blockages or leakages may be difficult or impossible to detect. Adverse conditions include pipes or valves covered with heavy insulating jackets, particularly those covered with low emissivity metal cladding. Under most measurement conditions, a closed valve will have a distinct temperature gradient across it and a leaky valve will not. For example, when a hot fluid isblocked by a closed valve the temperature difference gradient can be observed thermographically.


Steam traps are special valves that automatically cycle open and closed to remove condensate from sections of steam process lines. If the thermographer has prior knowledge of their normal operation, steam traps can usually be observed thermographically to determine if they are operating properly. Without this prior knowledge, using infrared thermography for steam trap diagnostics may be confusing and misleading. In the image sequence shown in Figure 5.7, the various operating conditions of the valve (top) result in clearly detectable thermal pattern changes. The thermal appearance of the steam trap (bottom) remains essentially the same in all three imagesBlockage of any fluid transfer line can be simple to detect thermographically if the fluid temperature is sufficiently hotter or cooter than ambient. If not, thereare more sophisticated approaches that have had documented success. For example, the injection of uniform transient heat will often result in transienttemperature differentials at the blockage site because of the difference in thermal capacity between the fluid (in liquid form) and the solid blockage. Heat injection techniques are discussed in greater detail in subsequentsections.


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5.5 Thermal Resistance InvestigationsThermal resistance studies are involved in any thermographic applicationwhere the conductive flow of thermal energy is affected by variations inthermal resistance that exhibits a variation in effectjve temperature at thetarget surface. Applications include building and vessel envelope studies. furnaces, refractory linings, hazardous heat leaks and a wide variety ofmaterials testing applications.


■ Building Insulation and Other FactorsAs previously discussed, the conductive heat flow through a laminar structureis related to both the temperature difference from one side of the structure tothe other and the aggregate thermal resistance of the materials encountered.The higher the thermal resistance (insulating properties), the less heat willflow; therefore, when steady state heat flow can be established, mapping thetemperature on the outside of a structure and knowing the thickness and theinside temperature, permits the determination of the insulation properties.

The measurement of conductive heat flow for insulation assessment is onlyone factor; however, in practical heat loss determination, other factors suchas air infiltration and exfiltration, chimney effects. and thermal short circuits orbypasses can be serious enough to completely negate the benefits of goodinsulation. Thermographers have learned to consider the total structure whenevaluating the results of thermographic surveys and to recognize and isolatethermal patterns typically associated with air flow as well as those caused byinsulation deficiencies.


Figures 5.8 and 5.9 illustrate these distinct pattern differences. Figure 5.8 shows the distinct patterns caused by insulation deficiencies on thethermogram of an exterior wall of a structure heated rom within, whereasFigure 5.9. taken of a diffe rent structure under similar conditions. illustratesthe effects of air exfiltration. It should be noted that most structuralapplications of thermography focus on qualitative features, such as thermalpatterns and thermal anomalies. rather than quantitative temperaturemeasurements. The only refe rence to temperature measurements was thestipulation in ANSUASHRAE 101- 1981 that. forthe inspection to be valid,"there shou ld be a minimum (difference) of 10 °C ( 18 °F) between theinside and outside surface tempera tures of the building for at least threehours prior to the survey." This stipulation was made presumably to establishquasi-steady state heat now thereby avoiding any misleading patterns because of struclUral differences in heat capacity and rendering images. which more rel iably represent only resistance di fferences. This standard has been superseded by ASTM C- I06O and ASTM C-1 155.


Figures 5.8 and 5.9 illustrate these distinct pattern differences. Figure 5.8 shows the distinct patterns caused by insulation deficiencies on thethermogram of an exterior wall of a structure heated rom within, whereasFigure 5.9. taken of a diffe rent structure under similar conditions. illustratesthe effects of air exfiltration. It should be noted that most structuralapplications of thermography focus on qualitative features, such as thermalpatterns and thermal anomalies. rather than quantitative temperaturemeasurements. The only refe rence to temperature measurements was thestipulation in ANSUASHRAE 101- 1981 that. forthe inspection to be valid,"there shou ld be a minimum (difference) of 10 °C ( 18 °F) between the inside and outside surface tempera tures of the building for at least three hours prior to the survey." This stipulation was made presumably to establish quasi-steady state heat now thereby avoiding any misleading patterns because of struclUral differences in heat capacity and rendering images. which more rel iably represent only resistance di fferences. This standard has been superseded by ASTM C-106O and ASTM C-1155.


Figure 5.8: Example of missing insulation


Figure 5.9: Example of air exfiltration


Example of air exfiltration


Example of air exfiltration


■ Industrial Roof Moisture DetectionThermal resistance is commonly used to detect industrial roof moisture when there has not been adequate isolation (solar heating) to use the approach based on thermal capacitance. Roof moisture detection by thermal resistance requires that there be a minimum of 10 °C (18 °F) difference between interior and exterior surface temperature for at least 24 h before the survey. This approach is conducted at night with all surfaces clean and dry and with little or no wind (no greater than 15 mph). This approach is based on heat loss rather than solar gain. Saturated roof sections are better heat conductors (poorer insulators) with lower thermal resistance than dry sections, and the temperature difference between the interior and exterior will cause heat to be conducted more rapidly through wet sections than dry sections. Warmer areas on the exterior surface, therefore, indicate water saturation. Because there is a temperature differential between the interior and exterior, this approach is subject to artifacts caused by air flow and thermal conduction through the roof. For validity. the thermographer should be accompanied by supporting intrusive evidence such as roof core samples or by another non intrusive test such as electric capacitance or neutron backscatter.


■ Refractory SystemsIndustrial structures particularly refractory structures, readily lend themselves to thermographic investigations. Damage or wear to a refractory structure invariably results in the breakdown of thermal resistance. Heat escapes through the worn or damaged sections and can be seen on the thermogram. An example of this is illustrated ill Figure 5.10 where the slight vertical crack in the center of the stack results in a distinct temperature increase.


Figure 5.10: heat escaping from a worn refractory structure


Refractory Thermogram


Refractory Thermogram


■ Subsurface Discontinuity Detection in MaterialsSubsurface discontinuity detection in materials is characterized by steady state heat flow. which may be unstimulated or stimulated. Unstimulated steady state heat flow uses process heat such as that produced by buildings. HVAC systems etc. Stimulated steady state heat flow requires the addition of a source of (steady) heat or cold to establish sufficient heat flow through material.


■ The Unstimulated Measurement Approach to Infrared Materials Flaw Detection

The unstimulated measurement approach uses the available heat flowing through the test sample. This occurs when products are being inspected during manufacture and the process being monitored produces or can be made to produce, the desired characteristic thermal pattern on the product surface. It occurs in injection molding, casting and drawing of products. An example of the unstimulated approach is illustrated in Figure 5. 11. On the left, areas of severe refractory breakdown in a boiler wall appear as the result of differences in heat flow because of the heat inherent in the boiler.


Figure 5.11: An example of passive IRNDT- a refractory break down in boiler


■ The Stimulated Measurement Approach to Infrared Materials Flaw Detection

When the desired characteristic thermal pattern on the product surface cannot be made to occur, or when the material samples or products are to be evaluated after manufacture, the stimulated, or thermal injection, approach is necessary. The stimulated approach can also involve thermal extraction, or the removal of heat from the sample, by introducing some form of cooling. Devices used for heat injection or extraction include the sun, air blowers, flood lamps, flash lamps, lasers, refrigerants, hot and cold water, chemical reactions, thermoelectric devices and mechanical heat sinks. In order for the stimulated approach to be effective, it requires the generation of a controlled flow of thermal energy across the Structure of the sample material under test. This is accompanied by thermographic monitoring of one of the surfaces (or sometimes both) of the sample, and the seareh for the anomalies in the thermal patterns so produced that will indicate a defect in accordance with established accept-reject criteria.


The equipment necessary to perform infrared materials discontinuity detection must include thermographic scanning instrumentation and the means to handle the test samples and to generate and control the injection or extraction of thermal energy to or from the samples. These can include hot and cold air blowers, liquid immersion baths. heat lamps. controlled refrigerants. electric current, scanned lasers and induction heating. The goal is to maximize the normal thermal flow, minimize the lateral thermal now (along the materialsurface), cause no permanent damage to the test samples, minimize and carefully meter the test time and generate the most uniform thermal pattern possible across the surface of the test sample. Because the source of energyis finite in dimension, the generation of a uniform thermal pattern on the sample surface is often difficult to accomplish. Using a personal computer with appropriate diagnostic software. a thermographer has access tonumerous image manipulation routines including keyboard controlled image manipulation and subtraction. This image subtraction capability can be quite effective in compensating for limitations in heating pattern uniformity.


Figure 5.12 illustrates a typical infrared materials discontinuity detection configurat ion using the active (heat injection) method under computer control.When uniform heat is applied to one surface of a laminar test sample and an infrared scanner views the opposite surface, two types of defects are detectable. A metal occlusion within the structure has a higher thermalconductivity than the ply material and results in a warm (white) spot on the scanned surface. A void within the structure has a lower thermal conductivity than the ply material and results in a cool (dark) spot on the scanncd surface. The computer software can be used. when necessary, to nornlalize the effective temperature pattern before thermal insertion and to regulate the timing and intensity of the heat source. Available software also facilitates the precision timing and recording of test sequences so that they can be repeated with consistency.


Figure 5.12: Example of active (heat injection) IRNDT for occlusion and voiddetection


Material surface characteristics, as in any other thermographic application.are critical to test effectiveness. Materials with high and uniform surfaceemissivity are ideally suited for evaluation by infrared materials discontinuity detection. When evaluating samples with low or nonuniform emissivity, thethermographer has several alternatives. The first is to apply a removable, thin, high ernissivity coating, Another is to use an image subtraction routine aspreviously discussed in Chaptcr 3. This greatly reduces emissivity artifactswithout affecting the material. Most materials successfully evaluated by infrared materials discontinuity detection are composed of layers of metals. plastics, composites or combinations of all three. The surfaces may be metal or plastic and the core structure may be solid, amorphous or geometricallyconfigured (i.e. a honeycomb structure). Assembled layered sections (i.e.aircraft lapped sections) are also tested thermographically. The surfaces of the test amples facing the scanner are usually uniform in appearance and finish, although emissivity is low and surface scratches are frequently present.

Keywords:Emissivity artifact (Reflectivity)


Typical failure modes of the material samples are(1) voids between layers, (2) disbonds between layers, (3) impurities or foreign material in the laminar interfaces and (4) significam irregularities (damage) to the geometric core structure. Typical defects in assembled sections are loose or damaged welds and rivets and erosion/corrosion between sections. often accompanied by material loss and thinning.

Establishing test protocol involves determining acceptability of each part to beevaluated in terms of minimum size of void to be detected, minimum area ofdisbond that can be said to constitute a defect and any other void or disbondcharacteristic that is deemed significant. For this it is necessary to use knownacceptable and known defective samples. Ideally, the defective samplesfurnished should include known defects of each classification and in theminimum sizes required to be detected and identified. When ideal defectivesamples are not available. it becomes necessary to synthesize flaws to simulate the minimum defects.


Selecting the infrared scanning system requires matching thermographic equipment performance capabilities to test criteria. To be effective,thermographic equipment used should offer resolution, sensitivity and versatility somewhat beyond that envisioned to be necessary to detect and identify the defects, the thermographer expects to encounter. The most critical of the scanner performance characteristics are (1) minimum resolvable temperature, (2) spatial resolution and (3) scan speed.

Figure 5.13 is an example of stimulated thermography that is ideal for thethermographer. Here the deicing mechanism on the wing of a DC·9 aircraft isevaluated. The deicing system also serves as the energizing source and thethermogram indicates areas that are not being heated as cool spots. Thewarm rings represent the instantaneous effect of the deicing mechanism.


Figure 5.13: Test of aircraft deicing element showing unheated areas on the wing of a DC-9


DC - 9


DC - 9


■ Stimulated Thermography Using Pulsed or Thermal Wave InjectionOne of the earliest applications of infrared materials discontinuity detection,performed as carly as 1970 was the detection of flaws in aircraft structures.This application continues to be an important one and most major airframemanufacturers have on going in-house infrared materials discontinuitydetection programs. Innovations in heat injection techniques (i.e. theintroduction of high intensity short-duration thermal pulses) have resulted inimproved capability for detecting small and buried naws. These, coupled withthe imroduclion of high speed focal plane array imagers and improvements incomputer enhancement techniques for isolating and analyzing thermographicpatterns and data, have had an important effect on image understanding anddiscontinuity recognition. The thermal wave technique is illustrated in Figure5.14. Here, high intensity xenon flash lamps are used to irradiate the targetsurface with short duration pulses (on the order of milliseconds) of thermalenergy. In many ways, this pulsed heating is similar to using the sun's heatingcycle for the detection of underground voids. as previously discussed.


Figure 5.14: Conceptual sketch of thermal wave imaging


Xenon Lamp


In this case. however, the heat pulses and the detection intervals are thousands of times faster. While the surface cools, the heat is conducted into the material at a uniform rate until it reaches a thermal barrier or discontinuity, such as a flaw. At this time the temperature at the surface is lower than that at the discontinuity site, and a portion of the heat is conducted back to the surface, simulating a thermal echo. The time it takes from the generation of the pulse to the reheating at the surface, then, is an indication of the depth of the discontinuity. The behavior of the thermal energy moving through the material is similar in many ways to that of a wave of energy propagating through the material and being eflected back to the surface. For this reason the term thermal wave imaging has been adopted by some thermographers to deseribe the process. By using diagnostic software to time-gate the return thermal images, they can estimate the depths of flaws as well as their size and location, often with excellent precision. The term time resolved infrared radiometry is also used to describe the technique of selecting the image that best indicates the detected discontinuity.


Figure 5.15 illustrates a result of thermal wave injection and computer enhanced image analysis. The subject is erosion/corrosion damage in an aircraft skin lap joint. The high speed time-gating of images is essential because of the extremely high thermal diffusivity of the aluminum material.Within the past five years, time resolved infrared thermography has been successful to some extent in locating wall thinning because of erosion and corrosion in pipes and boiler tubes in utilities. Figure 5.16 is a time-resolved thermogram illustrating the resu lts of flash heating of a boiler tube section. The high lighted areas indicate maximum thinning.

Keywords:thermal wave imagingtime resolved infrared radiometry


Figure 5.15: Erosion/corrosion damage in a 737 aircraft lap joints elevated areas indicate erosion/corrosion, depressed areas are rivets


Figure 5.16: Time-resolved thermal Image of boiler wall section showing wall thinning due to corrosion


737 aircraft


5.6 Thermal Capacitance InvestigationsSeveral seemingly diverse applications have in common the fact that datasample timing is critical to accurate detection and analysis. Theseapplications are those that are investigated on the basis of (usuallynonhomogeneous) thermal capacitance and/or thermal diffusivity. Thermalcapacitance and thermal diffusivity are discussed in Chapter 2.

Industrial Roof Moisture DetectionAs in most buildings and infrastructure applications. flat roof surveys areconcerned with detection and identification of thermal patterns rather thanquantitative measurements. These patterns are indications of subsurfacemoisture that is typically absorbed in the insulation. One approach to makingthese measurements depends on solar heating (insolation). This approach isconducted at night with all surface clean and dry and little or no wind (nogreater Ihan 15 mph).


When there has been adequate solar heating of the roof during the day before the survey, stored thermal energy will cause water- saturated sections, with their higher thermal capacitance, to store more heal. At night. the roof radiates thermal energy to the cold sky. At some time during the night, the dry sections, with less stored heat, appear cool. The saturated sections appear warmer and the thermographer can easily locate and identify them. This procedure is particularly effective even when there is no temperature difference between the interior and exterior of the building. Unlike the thermal resistance approach previously discussed, this approach, illustrated in Figure 5.17 is subject to few thermal artifacts due to vent pipes, exhaust fans, etc.

In 1990, ASTM C1153-90, Standard Practice for the Location of Wet Insulation in Roofing Systems Using Infrared Imaging was released by the American Society for Testing and Materials. It outlines the minimum criteria for an acceptable infrared roof moisture survey and clearly stipulates the requirement for both dry and wet core samples. It also defines the minimum performance specifications of thermal sensing and imaging equipment used to perform thermographic surveys.


Figure 5.17: Thermogram with roof with moisture saturation.


Liquid Level DetectionThermal capacitance difference also allows thermographic detection of theliquid levels in storage tanks and other containers. In the thermogram of a fueltank at night. shown in Figure 5.18. the fuel level is clearly evident becausethe fuel has a higher thermal capacitance than the air above it. The heatstored through solar absorption during the day maintains a highertemperature on the tank walls up to the fill level. Conversely. if the entire tankhad cooled, the liquid would warm later than the air and the wall below the filllevel would appear cooler than above.


Figure 5.18: Fuel level in a storage tank


Unstimulated and Stimulated Approaches to Infrared Materials Flaw DetectionMaterials discontinuity detection based on thermal capacitance differences issimilar to that based on thermal resistance differences in that a stimulatedapproach may be used when the desired characteristic thermal pattern on theproduct surface cannot be madc to occur, or when the material samples orproducts are to be evaluated after manufacture. As previously discussed, thiscan involve thermal injection in a variety of forms, but it can also involve thermal extraction, or the removal of heat from the sample by introducing some form of cooling.


Underground Void DetectionThe detection of underground voids is based, for the most pan, on thedifference in thermal capacitance between solid earth and the air cavitiesformed by buried tanks, eroded sewers and storm drains and improperly filledexcavations. Typical programs to detect underground voids are performedusing the sun as a basic source of thermal energy. During the day, the heat from the sun penetrates the earth and heats both the earth and the voids. The voids have a lower thermal capacitance and store less heat than the surrounding earth. On the subsequent thermographer they appear as cool areas. As in roof surveys, apparent findings are usually confirmed by means of other disciplines. Ground penetrating radar has come into use as a confirming discipline for thermographic underground void detection.


Subsurface Discontinuity Detection in MaterialsSubsurface discontinuity detection in materials is characterized by nonsteady(varying) heat flow through the subject, which can be unstimulated orstimulated. Unstimulated nonsteady heat flow uses (unsteady) process heator a cool down after process heating. Stimulated nonsteady heat flowdepends on the use of an (unsteady) source of heating or cooling.



2. b3. c4. a5. b6. a7. b8. c9. d10. a11. d

Q&A


1. A major area of infrared nondestructive material testing is based on the fact that:a. a good structural bond normalizes emittance artifacts.b. uniform structural continuity provides predictable thermal continuity.c. a structural void is a good thermal bond.d. thermal imagers can be made to measure temperature with great accuracy.

2. When analyzing a thermographic image. it is usually possible to distinguish between an overload condition and a loose connection because:a. a loose connection will appear cool compared to its surroundings.b. a loose connection will appear warmer than the wires on either side.c. an overload will cause a sharper thermal gradient.d. one side of a loose connection will appear much warmer than the other.


3. The most significant advantage of thermal wave imaging over conventionalstep stimulation methods of infrared/thermal materials testing is that:a. it can find smaller voids.b. it is simpler to implement.c. it can providc better information regarding the depth of a discontinuity.d. it provides images with better spatial resolution.

4. The diagnostics involved in thermography of electrical switchgear most frequently involves:a. exothermic invesligations.b. thermal resistance investigations.c. security investigations.d. fluid flow investigations.


5. The diagnostics involved in detection of moisture in flat roofs mostfrequently involve:a. exothermic and endothermic investigations.b. thermal resistance and thermal capacitance investigations.c. friction investigations.d. fluid flow investigations.

6. In time resolved thermography (thermal wave imaging) applied to materialsnondestructive testing. the time of the return signal from a void or disband is most closely related to the:a. depth of the discontinuity.b. size of the discontinuity.c. amplitude of the heating pulse.d. spectral characteristics of the heating pulse.


7. In the process monitoring of thin film plastics successful thermographicmeasurement is most closely related to:a. correcting the instrument for background reflections.b. matching the spectral characteristics of the instrument to those of the target material.c. optimizing the speed of response of the measuring instrument.d. optimizing the spatial resolution of the measuring instrument.

8. The unstimulated approach to infrared nondestructive testing can usually be used when evaluating the condition of refractory linings of vessels because:a. refractory materials have high effective emissivities.b. refractory materials have high reflectivity in the infrared.c. a strong. uniform source of heat usually exists within the vessel.d. infrared focal plane imagers are available for these applications.


9. Thermography has been successfully applied to some veterinary medicine applications because, in most cases:a. healthy animals are hotter than sick animals.b. the emissivity of animal hides is high.c. animals have higher body temperatures than humans.d. infection and trauma usually cause the affected portion of the body to become warmer.

10. The use of thermography for the detection of moisture infiltration in airframes is made possible by a combination of thermal capacitance differences and:a. an endothermic effect that causes the infiltrated portions to appear cooler.b. an exothermic effect that causes the infiltrated portions to appear warmer.c. increased friction between the air flow and the infiltrated sections.d. reduced friction between the air flow and the infiltrated sections.


11. Surface thermal patterns can often reveal:a. subsurface material defects.b. delaminations within a structurc.c. impurities within a material sample.d. all of the above.


Appendix AGlossaryThe following are explanations and definitions of terms commonly encountered by the infrared thermographer Many of these terms have multiple definitions and the one provided is the one most applicable to infrared thermography. NOTE: In some cases, the "textbook" definition of a term is replaced by one more explicitly dealing with the practice o/infrared thermography.


1. Absolute zero - The temperature that is zero on the Kelvin or º Rankine temperature scales. The temperature at which no molecular motion takes place in a material.

2. Absorptivity, α (absorptance) - The proportion (as a fraction of 1) of the radiant energy impinging on a material's surface that is absorbed into the material. For a blackbody, this is unity (1.0). Technically, absorptivity is the internal absorptance per unit path length. In tthermography, the two terms are often used interchangeably.

3. Accuracy (of measurement) - The maximum deviation, expressed in percent of scale or in degrees celsius or degrees fahrenheit. that the reading of an instrument will deviate from an acceptable standard reference, normally traceable to the National Institute for Standards and Technology (N IST).

4. Ambient operating range - Range of ambient temperatures over which an instrument is designed to operate within published performance specifications.

5. Ambient temperature - Temperature of the air in the vicinity of the target (target ambient) or the instrument (instrument ambient)

6. Ambient temperature compensation - Correction built into an instrument to provide automatic compensation in the measurement for variations in instrument ambient temperature.


Ambient temperature - Temperature of the air in the vicinity of the target (target ambient) or the instrument (instrument ambient)

instrument ambient

target ambient


7. Anomaly - An irregularity, such as a thermal anomaly on an otherwise isothermal surface; any indication that deviates from what is expected.

8. Apparent temperature - The target surface temperature indicated by an infrared point sensor, line scanner or imager.

9. Artifact ~ A product of artificial character because of extraneous agency; an error caused by an uncompensated anomaly. In thermography, an emissivity artifactsimulates a change in surface temperature but is not a real change.

10.Atmospheric windows (infrared) ~ The spectral intervals within the infrared spectrum in which the atmosphere transmits radiant energy well (atmospheric absorption is a minimum). These are roughly defined as 2 to 5 μm and 8 to 14 μm .

11.Background temperature, instrument - Apparent ambient temperature of the scene behind and surrounding the instrument as viewed from the target. The reflection of this background may appear in the image and affect the temperature measurement. Most quantitative thermal sensing and imaging instruments provide a means for correcting measurements for this reflection. (See Figure A- 1.)

12.Background temperature, target - Apparent ambient temperature of the scene (1) behind and (2) surrounding the instrument, as viewed from the instrument. When the FOV of a point sensing instrument is larger than the target, the target background temperature will affect the instrument reading. (See Figure A-1 .)


Figure A-1

Apparent ambient temperature of the scene (1) behind and (2) surroundingthe instrument


Background Temperature

Instrument Background

Target Background


13.Blackbody, blackbody radiator - A perfect emitter; an object that absorbs all the radiant energy impinging on it at all wavelengths and reflects and transmits none. A surface with emissivity of unity (1.0) at all wavelengths.

14.Bolometer. infrared ~ A type of thermal infrared detector.15.Calibration - Checking and/or adjusting an instrument such that its readings agree

with a standard . 16.Calibration check - A routine check of an instrument against a reference to ensure

that the instrument has not deviated from calibration since its last use. 17.Calibration accuracy - The accuracy to which a calibration is performed. usually

based on the accuracy and sensitivity of the instruments and references used in the calibration.

18.Calibration source, infrared - A blackbody or other target of known temperature and effective emissivity used as in calibration reference.

19.Capacitance, thermal - This term is used to describe heat capacity in terms of an electrical analog, where toss of heat in analogous to loss of charge on a capacitor. Structures with high thermal capacitance change temperature more slowly than those with low thermal capacitance.

20.Capacity, heat - The heat capacity of a material or structure describes its ability to store heat. It is the product of the specific heat (cp) and the density (ρ) of the material. This means that denser materials generally will have higher heat capacities than porous materials.


Capacity, heat (Volumetric Heat Capacity) - The heat capacity of a material or structure describes its ability to

store heat. It is the product of the specific heat (cp) and the density (ρ) of the material. This means that denser materials generally will have higher heat capacities than porous materials.

Heat Capacityvolumetric = Cp x ρfor my ASNT exam


Alas!

Heat Capacity Volumetric =

Cp∙ρ

for my ASNT exam


21.Celsius (Centigrade) - A temperature scale based on 0 °C as the freezing point of water and 100 °C as the boi ling point of water at standard atmospheric pressure; a relative scale related to the Kelvin scale [ 0 °C = 273.12 K; 1ºC (ΔT): 1 K (ΔT) ].

22.Color - A ternl sometimes used to deline wavelength or spectral interval, as in two-color radiometry (meaning a method that measures in two spectral intervals); also used conventionally (visual color) as a means of displaying a thermal image, as in color thermogram.

23.Colored body - See nongraybody.24.Conduction - The only mode of heat now in solids, but can also take place in

liquids and gases. It occurs as the result of (1) atomic vibrations (in solids) and (2) molecular collisions (in liquids and gases) whereby energy is transferred from locations of higher temperature to locations of lower temperature.

25.Conductivity, thermal, (k) - A material property defining the relative capability to carry heat by conduction in a static temperature gradient. Conductivity varies Slightly with temperature in solids and liquids and with temperature and pressure in gases. It is high for metals (copper has a k of 380 W/m∙°C) and low for porousmaterials (concrete has a k of 1.0 W/m∙°C) and gases.

26.Convection - The form of heat transfer that takes place in a moving medium and is almost always associated with transfer between a solid (surface) and a moving fluid (such as air). whereby energy is transferred from higher temperature sites to lower temperature sites.


27.Detector, infrared - A transducer element that converts incoming infrared radiant energy impinging on its sensitive surface to a usefu l electrical signal.

28.Diffuse reflector - A surface that reflects a portion of the incident radiation in such a manner that the reflected radiation is equal in all directions. A mirror is not a diffuse reflector.

29.Diffusivity, thermal, (α) - (Note: same symbol as absorptivity may be confusing.)The ratio of conductivity (k) to the product of density (ρ) and specilic heat (Cp)[ α = k/ρ∙Cp cm2 s-1 ]. The ability of a material to distribute thermal energy after a change in heat input. A body with a high diffusivity will reach a uniform temperaturedistribution faster than a body with lower diffusivity.

30.D* (detectivity star) - Sensitivity figure of merit of an infrared detector - detectivity expressed inversely so that higher D* indicate better performance; taken at specific test conditions of chopping frequency and information bandwidth and displayed as a function of spectral wavelength.

31.Display resolution, thermal - The precision with which an instrument displays its assigned measurement parameter (temperature). usually expressed in degrees, tenths of degrees, hundredths of degrees. etc.

32.Effective emissivity (ε*) - The measured emissivity value of a particular surface under existing measurement conditions (rather than the generic tabulated value for the surface material) that can be used to correct a specific measuring instrument to provide a correct temperature measurement.


33. Effusivity, thermal (e) - A measure of the resistance of a material to temperature change:

e =(kρCp) ½ cm2 ºC-1 S1/2

where:k = thermal conductivityρ = bulk densityCp = specific heat

Comments: compare diffusivityα = (k/ρ)∙Cp cm2 s-1


Thermal EffusivityIn Thermodynamics, the thermal effusivity of a material is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity.

e = (kρCp)½ cm2 ºC-1 S1/2

Here, k is the thermal conductivity, ρ is the density and Cp is the specific heat capacity. The product of ρ and Cp is known as the volumetric heat capacity.

A material's thermal effusivity is a measure of its ability to exchange thermal energy with its surroundings. If two semi-infinite bodies initially at temperatures T1 and T2 are brought in perfect thermal contact, the temperature at the contact surface Tm will be given by their relative effusivities.

This expression is valid for all times for semi-infinite bodies in perfect thermal contact. It is also a good first guess for the initial contact temperature for finite bodies.


34. Emissivity (ε) - The ratio of a target surface's radiance to that of a blackbody at the same temperature, viewed from the same angle and over the same spectral interval; a generic lookup value for a material. Values range from 0 to 1.0.

35. EMI/RFI noise - Disturbances to electrical signals caused by electromagnetic interference (EMI) or radio frequency interference (RFI). In thermography, this may cause noise patterns to appear on the display.

36. Environmental rating - A rating given an operating unit (typically an electrical or mechanical enclosure) to indicate the limits of the environmental conditions under which the unit will function reliably and within published performance specifications.

37. Exitance, radiant (also called radiosity) - Total infrared energy (radiant flux) leaving a target surface. This is composed of radiated. reflected and transmitted components. Only the radiated component is related to target surface temperature.

38. Fahrenheit - A temperature scale based on 32 ºF as the freezing point of water and212 ºF as the boiling point of water at standard atmospheric pressure; a relative scale related to the Rankine scale [ 0 ºF = 459.67 ºR; 1 ºF (ΔT) = 1 ºR (ΔT) ].

39. Field of view (FOV) - The angular subtense (expressed in angular degrees or radians per side if rectangular, and angular degrees or radians if circular) over which an instrument will integrate all incoming radian energy. In a radiation thermometer this denotes the target spot size; in a scanner or imager this denotes the scan angle or picture size or total field of view.


Alas!

Exitance = Rodiosity

for my ASNT exam


40.Fiber optic, infrared - A flexible fiber made of a material that transmits infrared energy, used for making noncontact temperature measurements when there is not a direct line of sight between the instrument and the target.

41.Filter, spectral - An optical element, usually transmissive, used to restrict the spectral band of energy received by an instrument's detector.

42.Focal plane array (FPA) - A linear or two-dimensional matrix of detector elements, typically used at the focal plane of an instrument. In thermography, rectangular FPAs are used in staring (nonscanning) infrared imagers. These are called infrared focal plane array imagers.

43.Focal point - The point at which the instrument optics image the infrared detector at the target plane. In a radiation thermometer, this is where the spot size is the smallest. In a scanner or imager, this is where the instantaneous field of view ([FOV) is smallest.

44.Foreground temptrature (see instrument ambient background) - Temperature of the scene behind and surrounding the instrument as viewed from the target. (See Figure A-1.)


45.Frame repetition rate - The time it takes an infrared imager to scan (update) every thermogram picture element (pixel); in frames per second.

46.Full scale - The span between the minimum value and the maximum value Ihat any instrument is capable of measuring. In a thermometer, this would be the span between the highest and lowest temperature that can be measured.

47.Graybody - An radiating object whose emissivity is a constant value less than unity ( 1.0), over a specific spectral range.

48.Hertz (Hz) – A unit of measurement of signal frequency; 1 Hz = 1 cycle per second.Image, infrared - See Thermogram.

49. Imager, infrared - An infrared instrument that collects the infrared radiant energy from a target surface and produces an image in monochrome (black and white) or color, where the gray shades or color hues correspond respectively to target exitance.

50. Image display tone - Gray shade or color hue on a thermogram.51. Image processing, thermal - Analysis of thermal images, usually by computer;

enhancing the image to prepare it for computer or visual analysis. In the case of aninfrared image or thermogram, this could include temperature scaling, spottemperature measurements, thermal profiles, image manipulation, subtraction andstorage.


52. Imaging radiometer - An infrared thermal imager that provides quantitative thermal images.

53. Indium Antimonide (InSb) - A material from which fast, sensitive photo detector used in infrared scanners and imagers are made. Such detectors usually requiring cooling while in operation. Operation is in the short wave band (2 to 5 μm).

54. Inertia, thermal - See thermal effusivity.55. Infrared - The infrared spectrum is loosely defined as that portion of the

electromagnetic continuum extending from the red visible (0.75 μm) to about 1000 μm (1mm).Because of instrument design considerations and the infrared transmission characteristics of the atmosphere, however. most infrared measurements are made between 0.75 and 20 μm.

56. Infrared focal plane array (lRFPA) - A linear or two-dimensional matrix of individual infrared detector elements, typically used as a detector in an infrared imaging instrument.

57. Infrared radiation thermometer - An instrument that converts incoming infrared radiant energy from a spot on a target surface to a measurement value that can be related to the temperature of that spot.

58. Infrared thermal imager - An instrument or system that converts incoming infrared radiant energy from a target surface to a thermal map or thermogram, on which color hues or gray shades can be related to the temperature distribution on that surface.


Thermal InertiaThermal inertia is a term commonly used by scientists and engineers modelling heat transfers and is a bulk material property related to thermal conductivity and volumetric heat capacity. For example, this material has a high thermal inertia, or thermal inertia plays an important role in this system, which means that dynamic effects are prevalent in a model, so that a steady-state calculation will yield inaccurate results. The term is a scientific analogy, and is not directly related to the mass-and-velocity term used in mechanics, where inertia is that which limits the acceleration of an object. In a similar way, thermal inertia is a measure of the thermal mass and the velocity of the thermal wave which controls the surface temperature of a material. In heat transfer, a higher value of the volumetric heat capacity means a longer time for the system to reach equilibrium.

The thermal inertia of a material is defined as the square root of the product of the material's bulk thermal conductivity and volumetric heat capacity, where the latter is the product of density and specific heat capacity:

e = I = √(kρCp) See also Thermal effusivity

k = is thermal conductivity, with unit [W m−1 K−1]ρ = is density, with unit [kg m−3]Cp = is specific heat capacity, with unit [J kg−1 K−1]e, I = has SI units of thermal inertia of [J m−2 K−1 s−1/2].

http://en.wikipedia.org/wiki/Volumetric_heat_capacity#Thermal_inertia


59. Instantaneous field of Fiew (lFOV) - The angular subtense (expressed in angular degrees or radians per side if rectangular and angular degrees or radians if round) over which an instrument will integrate all incoming radiant energy; the projection of the detector at the target plane. In a radiation thermometer this denotes the target spot size; in a line scanner or imager it representS one resolution clement in a scan line or a thermogram and is a measure of spatial resolution. (D=α∙d)

60. IRFPA imager or camera – An infrared imaging instrument that incorporates a two-dimensional infrared focal plane array and produces a thermogram without mechanical scanning.

61. Isotherm - A pattern superimposed on a thermogram or on a line scan that includes or highlights all points that have the same apparent temperature Kelvin -Absolute temperature scale related to the celsius (or Centigrade) relative scale. The Kelvin unit is equal to 1 °C; 0 Kelvin = - 273.16 °C; the degree sign and the word degrees are not used in describing Kelvin temperatures.


Instantaneous field of View (lFOV)


(care on unit used!)

for my ASNT exam


62. Laser pyrometer - An infrared radiation thermometer that projects a laser beam to the target, uses the reflected laser energy to compute target effective emissivity and automatically computcs target temperature (assuming that the target is a diffuse reflector) - not to be confused with laser-aided aiming devices on some radiation thermometer.

63. Line scan rate - The number of target lines scanned by an infrared scanner or imager in one second.

64. Line scanner, infrared - An instrument that scans an infrared field of view FOV along a straight line at the target plane to collect infrared radiant energy from a line on the target surface, usually done by incorporating one scanning element within the instrument. If the target (such as a sheet or web process) moves at a fixed rate normal to the line scan direction, the result can be displayed as athermogram.

65. Measurement spatial resolution, lFOVmeas - The smallest target spot size on which an infrared imager can produce a measurcment, expressed in terms ofangular subtense (mRad per side). The slit response function (SRF) test is used to measure measurement spatial resolution / IFOVmeas.

66. Medium, transmitting medium – The composition of the measurement path between a target surface and the measuring instrument through which the radiant energy propagates. This can be vacuum, gaseous (such as air), solid, liquid or any combination of these.


Laser pyrometer - Laser pyrometer - An infrared radiation thermometer that projects a laser beam to the target, uses the reflected laser energy to compute target effective emissivity and automatically computcs target temperature(assuming that the target is a diffuse reflector) - not to be confused with laser-aided aiming devices on some radiation thermometer.

Further reading on this subject is necessary.


67. Mercury cadmium telluride MCT (HgCdTe) - A material used for fast, sensitive infrared photodetectors used in infrared sensors, scanners and imagers that requires cooled operation. Operation is in the long wave length region (8 to 12 μm).

68. Micron (micrometer) (μ or μm) - One millionth of a meter; a unit used to express wavelength in the infrared.

69. Milliradian (mRad) - One thousandth of a radian (1 radian = 180/π); a unit used to express instrument angular field of view

70. Minimum resolvable temperature (difference), MRT(D) - thermal resolution; thermal sensitivity - the smallest temperature difference that an instrument can clearly distinguish out of the noise, taking into account characteristics of the display and the subjective interpretation of the operator.

71. Modulation - In general, the changes in one wave train caused by another; inthermal scanning and imaging, image luminant contrast; (Lmax - Lmin)/(Lmax + Lmin).

72. Modulation Transfer Function (MTF) - A measure of the ability of an imaging system to reproduce the image of a target. A formalized procedure is used to measure modulation transfer function; It assesses the spatial resolution of a scanning or imaging system as a function of distance to the targe!.




73. Noise equivalent temperature (difference), NET(D) - The temperature difference that is just equal to the noise signal; a measure of thermal resolution, but not taking into account characteristics of the display and the subjective interpretation of the operator.

74. NIST, NlST traceability - The National Institute of Standards and Technology (formerly NBS). Traceability to NIST is a means of ensuring that reference standards remain valid and their calibration remains current.

75. Nongraybody - A radiating object that does not have a spectral radiation distribution similar to a blackbody and can be partly transparent to infrared (transmits infrared energy at certain wavelengths); also called a colored body. Glass and plastic films are examples of nongraybodies. The emissivity of a colored body has a spectral dependence.

76. Objective lens - The primary lens of an optical system, On an infrared instrument, usually the interchangeable lens that denotes the total field of view.

77. Opaque - Impervious to radiant energy. In thermography, an opaque material is one that does not transmit thermal infrared energy, (τ = 0).

78. Optical element, infrared - Any element that collects, transmits restricts or reflects infrared energy as part of an infrared sensing or imaging instrument.

79. Peak hold - A feature of an instrument whereby an output signal is maintained at the peak instantaneous measurement for a specified duration.


Compare:Minimum resolvable temperature (difference), MRT(D) - thermal resolution; thermal sensitivity - the smallest temperature difference that an instrument can clearly distinguish out of the noise, taking into account characteristics of the display and the subjective interpretation of the operator.

Noise equivalent temperature (difference), NET(D) - The temperature difference that is just equal to the noise signal; a measure of thermal resolution, but not taking into account characteristics of the display and the subjective interpretation of the operator..


80. Photodetector (photon detector) - A type of infrared detector that has fast response (on the order of microseconds), limited spectral response and usually requires cooled operation: photooctectors are used in infrared radiationthermometer. scanners and imagers, because, unlike thermal detector, direct photon interaction obviates 防止 external heating of the detector for the signal tobe sensed.

81. Pyroelectric detector - A type of thermal infrared detector that acts as a current source with its output proportional to the rate of change of its temperature.

82. Pyroelectric vidicon (PEV), also called pyrovidicon - A video camera tube with its receiving elemen! fabricated of pyroelectric material and sensitive to wavelengths from about 2 to 20 μm; used in infrared thermal viewers.

83. Pyrometer - Any instrument used for temperature measurement. (1) A radiationor brightness pyrometer measures visible energy and relates it to brightness or color temperature. (2) An infrared pyrometer measures infrared radiation and relates it to target surface temperature.

84. Radian - An angle equal to 180 degrees/π or 57.29578 angular degrees.85. Radiation, thermal - The mode of heat flow that occurs by emission and

absorption of electromagnetic radialion. propagating at the speed of light andunlike conductive and convective heal flow, capable of propagating across a vacuum; the form of heat transfer that allows infrared tthermography to workbecause infrared energy travels from the target to the detector by radiation.


86.Radiation rererenee source - A blackbody or other target of known temperature and effective emi ssivity used as a reference 10 obtain optimum measurement accuracy. ideally. traceable to NIST.

87.Radiation thermometer – See infrared radiation thermometer.88.Radiosity – See Exitance, thermal.89.Rankine - Absolute temperature scale related to the fahrenheit relative scale. The

Rankine unit is equal to 1ºF; 0 Rankine = - 459.72 ºF ; the degree sign and the word degrees is not used in describing Rankine temperatures.

90.Ratio pyrometer - An infrared thermometer that uses the ratio of incoming infrared radiant energy at two narrowly separated wavelengths to detennine a target's temperature independent of target emittance; this assumes graybody conditions and is normally limited to relatively hot targets (above about 149 ºC, 300 ºF).

91.Reference junction - In a thermocouple. the junction of the dissimilar metals that is not the measurement junction. This is normally maintained at a constant referencetemperature.

92.Reflectivity, (reflectance) (ρ) - The ratio of the total energy reflected from a surface to total incidence on that surface; ρ = 1 - ε - τ; for a perfect mirror this approaches 1.0; for a blackbody the reflectivity ρ is 0. Technically, reflectivity is the ratio of the intensity of the reflected radiation to the total radiation and reflectance is the ratio of the reflected flux to the incident flux. In tthermography, the two terms are oftenused interchangeably. (only subtraction where is the division, ratio?)


93. Relative humidity - The ratio (in percent) of the water vapor content in the air to the maximum content possible at that temperature and pressure.

94. Repeatability - The capability of an instrument to exactly repeat a reading on an unvarying target over a short or long term time interval. For thermal measurcments, expressed in ±degrees or a percentage of full scale.

95. Resistance, thermal (R) – A measure of a material's resistance to the flow of thermal energy, inversely proportional to its thermal conductivity, k. (1/R = k)

96. Response time - The time it takes for an instrument output signal or display to respond to a temperature step change at the target; expressed in seconds. (typically, to 95 percent of the final value and approximately equal to 5 time constants)

97. Resistance temperature detector (RTD) – a sensor that measures temperature by a change in resistance as a funct ion of temperature.

98. Sample hold - A feature of an instrument whereby an output signal is maintained at an instantaneous measurement value for a specified duration after a trigger or until an external reset is applied.

99. Scan angle - For a line scanner, the total angular scan possible at the target plane, typically 90 degrees.

100. Scan position accuracy - For a line scanner. the precision with which instantaneous position along the scan line can be set or measured.


101. Sector - For a line scanner, a portion of the total scan angle over which measurement is made at the target plane.

102. Seebeck effect - The phenomenon that explains the operation of thermocouples; that in a closed electrical circuit made up of two junctions of dissimilar metal conductors, a direct current will flow as long as the two junctions are at different temperatures, The phenomenon is reversible: if the temperatures at the twojunctions are reversed. the flow of current reverses.

103. Sensitivity - See minimum resolvable temperature (difference), MRT(D).104. Setpoint - Any temperature setting at which an activating signal or closure can be

preset so that. when the measured temperature reaches the setpoint, a control signal, pulse or relay closure is generated.

105. Shock - A sudden application of force, for a specific time duration; also the temporary or permanent damage to a system as a result of a shock.

106. Signal processing - Manipulation of temperature signal or image data for purposes of enhancing or controlling a process. Examples for (1) infrared radiation thermometer are peak hold, valley hold, sample hold and averaging. Examples for (2) infrared scanners and (3) infrared imagers are usually referred to as image processing and include isotherm enhancement. image averaging, alignment, image subtraction and image filtering.

107. Slit response function - A measure of the measurement spatial resolution (IFOVmeas) of an infrared scanner or imager.


108. Spatial resolution - The spot size in terms of working distance. In an infrared radiation thermometer this is expressed in milliradians or as a ratio (DId) of the target spot size (containing 95 percent of the radiant energy, according to common usage) to the working distance. In scanners and imagers it is most oftenexpressed in milliradians.

109. Spectral response - The spectral wavelength interval over which an instrument orsensor responds to infrared radiant energy, expressed in micrometers (}lm) - also, the relative manner (spectral response eUlVe) in which it responds over that intelVal.

110. Specular (('Occtor - A smooth refl ecting surface that reflects all incident radiantenergy at an angle complementary (equal around the nomlal) to the angle of incidence, A mirror is a specular refl ector.

111. Spot - The instantaneous size (diameter unless otherwise specified) of the area at the target plane that is being measured by the instrument. In infrared thermometry, this is specifi ed by most manufacturers to contain 95 percent of the radiant energy of an infin itely large target of the same temperature and emissivity.

112. Storage operating range - 1be temperature extremes over which an instrument can be stored and. subsequently, operate within published performance specifications.


113. Subtense, angular - The angular diameter of an optical system or subsystem, expressed in angular degrees or mRad. In thermography, the angle over which a sensing instrument collects radiant energy.

114. Target - The object surface to be measured or imaged. 115. Temperature - A measure of the thermal energy contained by an object; the

degree of hotness or coldness of an object measurable by any of a number or relative scales; heat is defined as thermal energy in transit and flows from objects of higher temperature to objects of lower temperature.

116. Temperature conversion – Convening from one temperature scale to another; the relationships are: Celsius = (Fahrenheit -32) x 5/9, Fahrenheit = 9/5 x Celsius + 32, 1 °C (ΔT) = 5/9 ºF (Δ.T), 0 °C = 273.12 Kelvin: 0 ºF = 459.67 Rankine.

117. Temperature measurement drift - A reading change (error), with time of a target with non-varying temperature that may be caused by a combination of (1) ambient changes, (2) line voltage changes and (3) instrument characteristics.

118. Temperature resolution - See minimum resolvable temperature (difference), MRT(D),


119. Thermal detector, infrared - A type of infrared detector that changes electrical characteristics as a function of temperature; typically. thermal detectors have slow response, (on the order of milliseconds) broad spectral response and usually operate at room temperature: thermal detectors are commonly used in infrared radiation thermometers and in some imagers. (See Photodetector ≡photon detector)

120. Thermal viewer - A non-measuring thermal imager that produces qualitative thermal images related to thermal radiant distribution over the target surface.


121. Thermal wave imaging - A term used to describe an active technique for infrared nondestructive material testing in which the sample is stimulated with pulses of thermal energy and where the timebased returned thermal images are processed to determine discontinuity depth and severity; also called pulse stimulated imaging.

122. Thermistor - A temperature detector. usually a semiconductor, whose electrical resistivity decreases predictably and nonlinearly with increasing temperature.

123. Thermistor bolometer, infrared - A thermistor so configured as to collect radiant infrared energy; a type of thermal infrared detector.


124. Thermocouple - A device for measuring temperature based on the fact that opposite junctions between certain dissimilar metals develop an electrical potential when placed at diffcrent temperatures; typical thermocouple types are;

J iron/constantanK chromeValumelT copper/constantanE chromel/constantanR platinumlplatinum-30 percent rhodiumS platinumlplatinum- I0 percent rhodiumB platinum-6 percent rhodium/platinum-30 percent rhodiumG tungsten/tungsten-26 percent rheniumC tungsten-5 percent rhenium/tungsten-26 percent rheniumD tungsten-3 percent rheniumltungsten-25 percent rhenium


125. Thermogram - A thermal map or image of a target where the gray tones or color hues correspond to the distribution of infrared thermal radiant energy over the surface of the target (qualitative thermogram); when correctly processed and corrected, a graphic representation of surface temperature distribution (quantitative thermogram).

126. Thermograph - Another word used to describe an infrared thermal imager.127. Thermometer - Any device used for measuring temperature.128. Thermopile - A device constructed by the arrangement of thermocouples in

series to add the thermoelectric voltage. A radiation thermopile is a thermopilewith junctions so arranged as to collect infrared radiant energy from a target, a type of thermal infrared detector.

129. Time constant - The time it takes for any sensing element to respond to 63.2 percent of a step change at the target being sensed. In infrared sensing and thermography, the time constant of a detector is a limiting factor in instrumcnt performance, as it relates to response time. (?)

130. Total field of view (TFOV) - In imagers, the total solid angle scanned, usually rectangular in cross section. (TFOV=FOV?)

131. Transducer - Any device that can convert energy from one form to another. In thermography, an infrared detector is a transducer that converts infrared radiant energy to some useful electrical quantity.


132. Transfer calibration - A technique for correcting a temperature measurement or a thermogram for various errors by placing a radiation reference standard adjacent to the larget.

133. Transfer standard - A precision radiometric measurement instrument with NTST traceable calibration used to calibrate radiation reference sources.

134. Transmissivity, (transmiUance) (τ) - The proportion of infrared radiant energyimpinging on an object's surface, for any given spectral interval thai is transmitted through the object. (τ = 1 – ε - ρ) For a blackbody. transmissivity τ = O. Transmissivity is the internal transmittance per unit thickness of a non-diffusing material.

135. Two-color pyrometer - See ratio pyrometer.136. Unity - One (1.0).137. Valley hold - A feature of an instrument whereby an output signal is maintained

at the lowest inslantaneous measurement for a specified duration; opposite of peak hold.

138. Working distance – The distance from the target to the instrument, usually to the primary optic.

139. Zone - In line scanners. a scanned area created by the transverse linear motion of the product or process under a measurement sector of the scanner.


Appendix BCost Benefit Determination


The sample worksheet at the end of this description provides a protocol for estimating cost benefits of any finding. Following the EPRI M&D Center Guidelines for costbenefit detcnnination. the benefits of detecting a failure mechanism at work on asystem or component before failure are quantified in tcnns of probable dollars saved.To do this, the costs of eliminating the failure mechanism in a timely fashion are compared to the likely costs incurred if the failure mechanism was not corrected andthe component or system failed. The approach used in the analysis considers three possible failure scenarios:

1. worst case (catastrophic failure),2. possible case (moderate failure). and3. probable case (minor failure - the failure most likely to occur).

The following three calculations are used to estimate failure scenarios:

1. estimate the percentage likelihood out of 100 percent of each of the three scenarios occurring - with the sum of the three percentages equal to 100 percent;

2. multiply the projected cost of each of the three scenarios by its estimated percent likelihood - the sum of these three products is the weighted estimated savings by not having to do any of them; and

3. estimate the cost benefit by comparing the actual cost of the timely service or repair to thc wcighted estimated savings.


Although the calculations are quite straightforward. the effective use of the guidelines is far from trivial because filling in the blanks can be a challenge. Here your historical database can be of substantial help. Of equal importance is a thorough knowledge ofthe criticality of the component or system to the operation of the facility to project thenature and extent of each of the failure scenarios. If your knowledge in this area islimited, rely on appropriate facility personnel for the information.

The historical database can help you estimate the percent likelihood of each scenario. as well as the associated costs. When preparing cost estimates. remember to include man hours, transportation of parts and equipment, cost of rcplacemem parts and equipment and damage to adjacent equipment. Avoided maintenance may also be included. Another factor in cost benefit determination that is worth considering is the long term savings in excess power that would have been consumed by components and systems restored to optimum operational efficiency by timely service or repair. Overheated componems invariably draw more current than they should either through direct I2R loss or because of excess friction or other inefficiencies. These kilowatts of power lost represent lost revenue – for every every hour that the situation is not corrected, kilowatt hours are lost in the form of dollars that cannot be billed to customers.


These dollars lost are in proportion to the square of the excess current and can be calculated for an electrical component if you know the excess current, I, and the resistance,

1. ΔP(W) = (ΔI)2R

Then divide ΔP by 1000 to convert to kilowatts and multiply by the average rate charged for a kilowatt hour. This will tell you how much every hour of non optimum operation is costing the facility,

2. Dollars lost = (lost KWH) x (Dollars/KWH).

In a rotating component, if you know the rated power consumption (watts) and the rated current, you can calculate the effective resistance and proceed as in step 1above.


Appendix CCommonly Used Infrared Specifications and Standards


Appendix C, Commonly Used Infrared Specifications and Standards






End Of Reading


Peach – 我爱桃子


Good Luck


Good Luck

Charlie Chong/ Fion Zhanghttps://www.yumpu.com/en/browse/user/charliechong

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Understanding Infrared Thermography Reading 3 SGuide-IRT