INFRACRVENA TERMOGRAFIJA INFRARED THERMOGRAPHY...SVEUČILIŠTE U ZAGREBU FAKULTET STROJARSTVA I BRODOGRADNJE Laboratorij za toplinu i toplinske uređaje I. Luči ća 5, 10000 Zagreb

S VE UČ I L I Š T E U ZAGRE B U

FAKUL T E T S T RO J AR S T V A I B RO DO GRA DNJ E

L a b o r a t o r i j z a t o p l i n u i t o p l i n s k e u ređa j e I . L u č i ć a 5 , 1 0 0 0 0 Z a g r e b T e l . : ( 0 1 ) 6 1 6 8 2 2 2 , F a x . : ( 0 1 ) 6 1 5 6 9 4 0

INFRACRVENA TERMOGRAFIJA INFRARED THERMOGRAPHY

STUDIJ: MEĐUNARODNI POSLIJEDIPLOMSKI STUDIJ

INTERNATIONAL MASTER OF SCIENCE PROGRAMME: SUSTAINABLE ENERGY ENGINEERING

USTANOVA: FAKULTET STROJARSTVA I BRODOGRADNJE

SVEUČILIŠTA U ZAGREBU FACULTY OF MECHANICAL ENGINEERING AND NAVAL ARCHITECTURE UNIVERSITY OF ZAGREB

Pripremili: Prof.dr.sc. Srećko Švaić, dipl.ing. Doc.dr.sc. Ivanka Boras, dipl.ing. Fakultet strojarstva i brodogradnje

Sveučilište u Zagrebu




1.0.0 INTRODUCTION Infrared (IR) Thermography IR thermography is a contact less temperature and surface temperature distribution measuring method. It is based on the measurement of IR radiation intensity from the observed surface. The result of thermographic measurement is a thermogram giving the temperature distribution at the surfaces of the observed object in grey-scale or in a colored code. The temperature distribution gives information of different states of the surface itself or is the consequence of the structure and internal state of the object. Electromagnetic Radiation All bodies emit continuously electromagnetic radiation, traveling through vacuum at speed of light – 3 ⋅ 108 m/s. Experiments have proven that radiation is behaving like particles in interaction with matter and like waves when traveling through space. Thus electromagnetic waves are of dual nature: corpuscular and wave. The radiation wavelength λ is connected to the wave frequency ν and the wave propagation velocity c through the expression: λ⋅= vc (1) Although bodies glow at high temperatures, visible light is not the only radiation they emit. Emission spectra of solid bodies are continuous and consist of all wavelengths. The energy distribution at particular wavelengths depends on the temperature and physical properties of the emitting surface. Fig. 1 represents the electromagnetic spectrum. Thermal effects are bound to radiation in the wavelength range from 0,1 to 100 µm. The visible part of the spectrum is a very narrow band of the thermal radiation range, i.e. the visible spectrum is only a part of the thermal radiation spectrum that may be perceived by the human eye. It covers the wavelength range from 0,4 to 0,7 µm. Following the increasing wavelengths, the thermal radiation range may be divided into three subdomains: ultraviolet, visible and IR range.

IR Thermography 2/13




Figure 1 The electromagnetic spectrum (gamma, Roentgen, ultraviolet, visible, IR, microwaves, radio, visible, IR shortwave, longwave, µm)

Figure 2 Photograph in visible spectrum and thermogram in IR spectrum In most solids and liquids the neighboring molecules absorb the radiation of a particular molecule. Therefore the radiation of liquids and solids is emitted or absorbed only by the molecules near to the surface: in metals this is a layer only a few molecules thick and in non-metals a few micrometers. In such materials emission and absorption may be regarded as surface phenomena. On the other hand, mixtures of gases which include water vapor particles, carbon dioxide or even solids partially transmissive to radiation, the absorption is deep and the emitted radiation may come from any part inside the observed gaseous body.





2.0.0 BLACKBODY A blackbody is an ideal body, which absorbs the entire incoming radiation, regardless to its wavelength and incidence angle, thus reflecting nothing. The evident consequence of this definition is that the entire radiation coming from a blackbody is emitted radiation and that at a given temperature and wavelength the emission of a blackbody is the largest. A blackbody has no preferred direction of radiation; the radiation is diffuse. E*

1 ⋅ E*

Fig. 3 The blackbody absorbing the entire incoming radiation Blackbodies emit in the entire range of the spectrum wavelength. In the case of monochromatic emission of a blackbody, i.e. radiation energy emitted from unit surface area at a certain wavelength (W/m2µm), the spectral distribution of radiated energy is described by Planck's law:

1/

51

2 −⋅

= ⋅−

TCb eCE λλ

λ (2)

where λ is the wavelength in µm, T the absolute temperature in K, and the constants W⋅µm

81 10742,3 ⋅=C

4/m2 and µmK. The maximum of the spectral radiation density is shifted to shorter wavelengths with the raise of temperature. Wien's law gives the relation between the temperature and wavelength at maximum spectral radiation density:

42 104389,1 ⋅=C

2898max =⋅Tλ µmK (3) which explains the change of color of surfaces from red to white at heating. The emission of a blackbody is the energy emitted from its surfaces at all wavelengths. Its amount is proportional to the fourth power of the body absolute temperature, according to the Stefan-Boltzmann law: W/m4TEb ⋅=σ

2 (4) where σ = 5,6697 ⋅ 10-8 W/m2K4 is the Stefan-Boltzmann constant. 3.0.0 REAL BODIES The radiation coming to the surfaces of a real body is partially absorbed, partially reflected and partially transmitted: (5) **** EdErEaE ⋅+⋅+⋅=





E*

r ⋅ E*

d ⋅ E*

a ⋅ E*

Fig. 4 Absorbed, reflected and transmitted radiation The ratios of the absorbed, reflected and transmitted radiation respectively and the received radiation are called absorptivity (a), reflectivity (r) and transmissivity (d). Equation (5) yields: dra ++=1 (6) The majority of surfaces interesting in engineering do not transmit radiation (d = 0), except for some materials as glass and plastic films. In that case the radiation is either absorbed or reflected, so eq. (6) becomes: ra +=1 (7) The portion of the incoming radiation, which will be absorbed or reflected, depends on the material and state of the body surface, the radiation wavelength and the incidence angle. It may also depend on the temperature. For engineering practice it is suitable to use average values of absortivity and reflectivity. The emission of real bodies is essentially different from the emission of blackbodies and has a different distribution of radiation intensity in the wavelength spectrum. Emissivity ε is defined as the ratio of the real body emission to the emission of a blackbody at equal temperatures:

)()(

TETE

b

=ε (8)

The emissivity of real bodies depends on the temperature and the state of the surface, and significantly on the angle of the radiation to the surface normal. The emissivity ε of the overall radiation will differ from the emissivity of radiation perpendicular to the surface εn. It may be calculated as:

2,1≅nεε for low emitting polished metal surfaces

98,0≅nεε for high emitting non-metal surfaces.

Accordingly, the Stefan-Boltzmann law for real bodies becomes: (9) 4TE ⋅⋅= σεThe Kirchoff law defines the equality of emissivity and absorptivity: )()( λλε a= (10) It becomes evident that the emission spectra of real bodies, where the emissivity depends on the wavelength, will not be equal to the radiation spectrum of the blackbody.





4.0.0 OPERATING PRINCIPLE OF THE THERMOGRAPHIC SYSTEM The thermographic system consists of the IR camera and the thermogram processing unit (PC). The camera includes the IR optics, IR sensor, unit for conversion of electrical into video signals, display and memory card. Thermograms are processed in the PC using special software, and the PC stores data from the camera memory card. Because the characteristics of electromagnetic radiation are the same throughout the entire spectrum, the optics of IR cameras is shaped as in usual photographic devices, but it is produced from different materials, which must be transparent to IR radiation. These are germanium, zinc-selenide and zinc-sulphide for longwave IR, and silicon, sapphire, quartz or magnesium for mediumwave IR radiation.

Fig. 5 Operating principle of a modern thermographic system The IR sensor measures the amount of incident energy at its surface, which corresponds to the radiation intensity of a defined IR spectrum range. The energy radiated to the sensor of the camera Ecam equals the sum of energies radiated from the observed body, consisting of proper and reflected radiation (E+rE*), radiation transmitted through the body dE** and radiation from the environment Eenv: ( ) envcam EEdErEE +⋅+⋅+= *** (11)

E*

d ⋅ E*

a ⋅ E*

d ⋅ E**

r ⋅ E*

E**

E = Eb ⋅ ε

Eenv

a ⋅ E**

r ⋅ E**

Eenv

Fig. 6 Energy impinging the IR sensor at thermographic recording of a body





In order to calculate the correct temperature of the observed body from the radiation received by the camera sensor, the properties of the body surface, temperature of the surrounding objects, camera to object distance, temperature and the humidity of air must be known. All this parameters must be set as input data to the camera software. The influence of the ambient radiation should be minimized, especially if the observed object is at a temperature similar to the ambient and/or has low emissivity. The basic purpose of the camera software is to determine the temperature distribution at the surface of a body of known emissivity. However it offers other possibilities, e.g. it may be used to determine the emissivity at the basis of all the mentioned parameters and known temperature of the body. When it is necessary to eliminate the transmitted radiation, various filters opaque to wavelengths to which the observed object is transparent may be inserted in front of the camera optics. 5.0.0 ACTIVE AND PASSIVE THERMOGRAPHY According to the measurement approach and data processing, thermography may be active or passive and qualitative or quantitative. Active thermography is based upon observing the dynamic behavior of the object surface exposed to thermal stimulation. This is accomplished in various manners as impulse, periodical, lock-in, vibration stimulation etc. The common aim to all of them is to send a certain amount of energy to the observed object and to analyse the object response to thermal stimulation in form of the temporal development of the temperature distribution. The subsequent analysis yields conclusions of the inside structure of the material, possible inhomogeneities, cracks or processes occurring below the surface.

24,6°C

48,1°C

25

30

35

40

45

LI01

LI02

LI03

Fig. 7 Active thermography: Measurement of phenol resin sample, t = 300 s At passive thermography objects are observed in a stationary state. The recorded IR radiation differences coming from the object surface are consequence of temperature and/or property differences.





Fig. 8 Passive thermography: Photograph and thermogram of a wall at the Croatian National and

University Library at Zagreb The processing of thermograms stored in the PC may be qualitative, which that only differences in the shading (grey scale or colour code) are analysed (Fig. 9), or quantitative, which includes the estimation of temperature values, temperature differences or emissivities at distinct locations of the thermogram (Figs. 10, 11 and 12)

Fig. 9 Areas of higher and lower temperatures are easily spotted

Fig. 10 Thermal load of machine parts, an analysis with the "Isotherm dual above" tool





Fig. 11 Thermal image of a vessel with a vertical temperature profile line

Fig 12 Evaluation of the state of a building using the histogram analysis of two thermogram areas 6.0.0 THERMAL IMAGING SYSTEM ThermaCAM 2000 Camera basic data Measurement accuracy +/- 2 % Thermal sensitivity < 0,08 oC at 30 oC Field of view(FxV) / min.focus distance 24 o x 18 o / 0,5m Detector type FPA 320 x 240 pixel (uncooled bolometer) Spectral range 7,5 – 13 µm Video output VH Display colour LCD PC card drive type II or type III Image storing real time, 14 bit digital Battery system ACU Nickel-metal hydride Size 209 x 122 x 130 mm Weight 2,43 kg Visual camera 640 x 480 pixels





Object temperature measurement range - 40 oC – 120 oC 0 oC – 500 oC 350 oC – 1500 oC

Menus and choice possibilities The menu FILE with its submenus enables the opening of thermograms formerly saved to the disc, individual or periodical thermogram saving to chosen or new directories, erasing of thermograms and input of various notes to individual thermograms, as sound or text data. The ANALYSIS menu offers with series of submenus the definition of important characteristics of the observed object and its surrounding: emissivity, ambient temperature and air humidity. The submenus Spot, Area, Isotherm and Profile enable an immediate analysis of the recorded object through spot temperature metering, line temperature profile and temperature analysis of particular areas. The IMAGE menu contains submenus enabling the choice among IR and video recording, selection of temperature range, adjusting the temperature level and temperature range of thermograms, freezing the displayed image, automatic focus and colour adjustment, and setting of markers in video mode, which helps at the analysis of thermograms. The SETUP menu and its submenus enable setting of options at spot, area or isotherm metering (colour, size etc.), changing of picture parameters, manual or automatic adjustment of thermograms, choice of colours, correction of noise and indication of temperature saturation. A series of options to define the thermogram organization is offered, along with the choice of measurement units, language, date, saving mode, text, sound, save format and general selection of information appearing at the thermogram. 6.1.0 RUNNING THE ThermaCAM-Researcher 2002 SOFTWARE The basic purpose of the ThermaCAM-Researcher 2002 software is processing of IR recordings (thermograms) coming from the camera in real time. However, the software may receive and process thermograms from other media as PC hard disc or memory card. The program handles fast/medium/slow thermal processes and, depending on the set configuration, it may display thermograms or save them on disc and analyse them later. The thermograms as measurement results may be processed using the following tools: isotherm, spot, area or line. The results obtained using these tools are displayed at the monitor along with the thermogram as windows showing the temperature profile, histogram, basic result data table or drawing. The measurement results can also be linked and processed using various subprograms. The standard application in this program is the adjustment of the image marked with "lock". This facilitates locking of the temperature scale, object parameters or the zoom factor. This means that a previously defined specific temperature scale, adjusted to the user's wishes may be used. The present and the following displays will be shown using this specific temperature scale, although they are saved with another. After unlocking, each thermogram will be displayed with the original temperature scale.





Program screen layout There are several layout options available. These are controlled by tabs in the bottom part of the ThermaCAM Researcher window. You can see combinations of the IR image, the profile, the histogram, the plot and result table windows. All tabs have an IR image with a temperature scale in the top left corner.

Fig. 13 One of the possible interfaces of the ThermaCAM Researcher Tools enabling the thermogram processing are located at the following tool bars: ■ standard tool bar (creating, opening, saving etc.), ■ play images tool bar, ■ recording tool bar, ■ image directory tool bar, ■ analysis tool bar ■ scaling tool bar. In order to get a good image from the camera, you should establish a connection, select an appropriate measurement range, auto adjust it and focus it. No matter if you have a live image, a frozen image or a disk image you should now consider the object parameters (emissivity, ambient temperature, atmospheric temperature, relative humidity of the air, the distance and the external optics transmission and temperature). They describe the physical properties of the body of interest and its environment and the atmosphere between the object and the camera. You can reach them via Settings in the Image menu or this button:





Figure 14 Settings in the Image menu It is important that these parameter values become correct. Otherwise the scale temperatures and displayed colours will be wrong. The image parts for which the object parameters are wrong will get incorrect temperatures and colours. (The measurement functions have object parameters of their own which are used to handle the case when there are two different targets in the same image.) If the colours of the image are inappropriate, you can change them. The selection Palette tool button will bring up a dialogue window with the palettes available. How to use the analysis tools to get numerical temperatures and statistical information out of a single image The analysis tools will show their results in the result table, plot, profile or histogram window or directly inside the IR image. Results are also available through the OLE functions, such as Copy Value. Both absolute measurements (i.e. the result is a real temperature) and relative measurements (i.e. the result is a difference temperature) can be made. The relative measurements are made relative to the reference temperature that you can enter in the dialogue window Image Settings (in the Image menu), the Object Parameters tab. The analysis tools work both with live images and recorded images. The isotherm tool An isotherm is a marker in an infrared image that highlights areas where the radiation from the object is equal. The name isotherm can be misleading, since it implies that equal temperatures are highlighted. This is only true if the emissivity of the object is the same all over the image. If you bring up the menu on this button, you will see that there are five types of isotherms in ThermaCAM Researcher. The most commonly used one is the interval isotherm. It will highlight a temperature interval with a certain (selectable) width. The spot meter tool This tool measures the temperature in one spot on the image and shows the result in the result table or beside its symbol in the IR image. The results are also available through OLE. You can obtain the following values: Temperature, Temperature relative to the reference temperature, Emissivity, Object distance and the image co-ordinates of the spot meter.





The flying spot meter This tool only measures the temperature at the mouse cursor and displays it beside the cursor in a tool tip window. There is just one single flying spotmeter. The area tool This tool measures the maximum, minimum, average and standard deviation temperature within a chosen part of the image and presents these values in the result table window or beside its symbol in the image. Results can also be displayed graphically in the histogram window. The line tool This tool measures the minimum, maximum, average and standard deviation temperature along a straight or bendable line within the image. The temperature in one spot, the line cursor, can also be measured. These values are presented in the result table or beside the line symbol in the image. The line temperatures can also be graphically presented in the profile window. The Formula tool This tool is used for adding and editing formulas. A formula can contain all common mathematical operators and functions, such as +, -, *, / square root, etc. Also, numeric constants such as 3.14 can be used. Most importantly, references to measurement results, formulas and other numerical data can be inserted into formulas. Object's parameters Frequently, the object emissivity or distance is varying between different parts of the IR image. All analysis tools (except the isotherm) can be forced to use their own values on these object parameters. 7.0.0 CONCLUSION Every experimental method has the capabilities and limitations. For thermography we could say that the advantages are:

■ Contact less technique: no physical contact, no interaction with specimen ■ Fast, surface inspection ■ Ease of interpretations of thermograms ■ Great versatility of application ■ Ease of numerical thermal modeling

And the limitations are: ■ Variable emissivity ■ Cooling losses (convection/radiation causing perturbing contrast) ■ Absorption of infrared signals by the atmosphere ■ Difficult to get uniform heating (for active procedure) ■ Limited contrasts and limited signal/noise ratio, causing false alarms ■ Observable defects generally shallow ■ Works only if thermal contrast naturally present

For correct quantitative and qualitative analyses of thermograms it is necessary that in measurements are included trained person knowing the problem.


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INFRACRVENA TERMOGRAFIJA INFRARED THERMOGRAPHY...SVEUČILIŠTE U ZAGREBU FAKULTET STROJARSTVA I BRODOGRADNJE Laboratorij za toplinu i toplinske uređaje I. Luči ća 5, 10000 Zagreb