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CHAPTER 5

The Metallurgical Microscope

THE METALLOGRAPHER’S most important tool is the met-allurgical microscope. Every metallographic laboratory has atleast one metallurgical microscope for observing microstructures.This microscope is different from the conventional microscope,which uses transmitted light for transparent material, for example,stained biological specimens. Metallographic specimens areopaque to light, and therefore, a metallurgical microscope needs asource of reected light. This source of light is discussed in thefollowing sections. Both kinds of microscopes are commonlycalled optical microscopes. This term is not used in this book,because it is more appropriate to use the name of the source of theincident “beam” being used to illuminate the specimen. Forexample, a controlled beam of light is used in both the metallur-gical and biological microscopes. Thus, they are called lightmicroscopes or light optical microscopes. If a beam of electrons isemployed, the microscope would be called an electron micro-scope, and if a beam of ions is employed, it is called an ionmicroscope. Electron microscopes are valuable tools for themetallographer and are discussed in the next chapter.

When discussing microscopes, one is entering a eld of physicscalled optics, and many terms, concepts, and mathematicalexpressions are used that are generally unfamiliar to the metal-lographer. This chapter describes only those items that are

necessary for the metallographer to develop a basic understandingof the microscope. Some of the basic terms described in thischapter include resolving power, the virtual image, bright- anddark-eld illumination, numerical aperture, focal length, imagecontrast, depth of eld, and spherical and chromatic aberration.For more detailed technical descriptions, there are several refer-ences listed at the end of the chapter. The metallographer mustobviously know the basics of the microscope in order to use itproperly. These are sophisticated scientic instruments, and themetallographer must have the required working knowledge inorder to obtain the optimal benet from the microscope. Themodern-day metallographer is very involved with metallographicinterpretation and must ne-tune the microscope to obtain theultimate image for accurate microstructural interpretation. Also, ametallographer may be in the position to recommend or purchasea microscope or metallograph (a dedicated microscope withbuilt-in camera for taking micrographs). A full understanding of the various features of a metallurgical microscope is necessary inorder to intelligently procure this type of instrument. Thesefeatures include such things as apochromatic objectives, hyper-plane oculars, vertical illuminators, counting reticles, wideeld

oculars, polarization lters, eld diaphragms, interferometers, antungsten-halogen lamps. This chapter discusses all these feature

In addition to developing a basic understanding of the metlurgical microscope, the metallographer must also develop a basunderstanding of methods to record the microstructural imagThe latter part of this chapter is devoted to the metallographmetallurgical microscope that is dedicated to micrography, that irecording the microstructure. First, the metallographer muunderstand the microscope.

The Microscope

The term “microscope” is derived from the Greek words (small) and skopein (to see). The words were combined and gia Latin form by Giovanni Faber, a Roman scientist, in 1625. Tmicroscope is thus an instrument that can see small things. TDutch eyeglass maker, Zacharias Janssen, has been credited witdeveloping the principles of the compound microscope in 1590. the mid-1600s, Anthony Van Leewenhoek, a Dutch amatescientist, was the rst to observe microscopic life in pond watand has been called the father of the microscope. He constructa simple microscope (not a compound microscope) with a pow

of 270 , which, at the time, was the most powerful microscoever built. An early example of a compound microscope is seen Fig. 5.1. This type of microscope was used by Robert Hooke, English microscopist, in 1665.

Although the microscope has been used for over 300 years,was only in the latter part of the 19th century that the microscowas rst used for observing metals. As mentioned in Chapter Sir Henry Clifton Sorby, the father of metallography, used tmicroscope to observe the microstructure of a polished and etchesteel specimen.

The Basic Metallurgical Microscope

In this book, only the metallurgical microscope is discusseExamples of this type of microscope are seen in Fig. 5.2 and 5There are two types of metallurgical microscopes: the upright aninverted microscope. In the upright microscope (Fig. 5.2), tspecimen is positioned below the objective, and in the invertmicroscope (Figure 5.3); the specimen is upside down above thobjective. Each type of microscope has advantages and disadvantages, some of which are listed subsequently:

. .



Upright microscope Inverted microscope

The beam of light can be seen on thespecimen surface.

Difficult to see the beam of lighton the specimen surface

The specimen must be leveled inorder to maintain focus whilemoving specimen.

Leveling of the specimen is notrequired.

x-y movement not limited by stage(entire specimen surface can beobserved)

x-y movement not limited by stageopening (diameter of hole limits thearea of specimen to be seen)

Polished specimen surface does notcontact stage surface

Polished surface may contact the stagesurface (potential for scratches)

Specimen thickness generally limitedby distance between objectiveand stage

Ideal for large specimens

Limited stage weight capacity Usually built with ample weightcapacity

Specimen surface can be scribed witha circle while on the stage forspecial identication (specialscribing device needed)

Scribing not possible

The basic upright metallurgical microscope shown in Fig. 5.2consists of many important components (the same components areon the inverted metallurgical microscope). These components canbe roughly divided into having three main functions: the mechani-cal system, the optical system, and the illumination (light) system.All these systems are supported and aligned by the stand, thetructural frame or body of the microscope, as shown in Fig. 5.2.

This microscope has a far different appearance than microscopesof the past. The older metallurgical microscopes were simplyadaptations of the standard biological microscope. Up until a fewdecades ago, most microscope manufacturers produced the stan-dard upright tube-type microscope. Because the metallographerequires a microscope with reected light, special attachments

were made to t onto the standard microscope. Today, microscopemanufacturers make dedicated metallurgical microscopes thatmeet the special needs of the metallographer. Each manufacturerhas its own unique design and special features. This book does not

address the differences between manufacturers or describe anyparticular manufacturer’s design. However, the basic featurescommon to all metallurgical microscopes are described subse-quently.

The Mechanical System

The mechanical system includes those components that arerequired for moving the specimen beneath or above the lighbeam and for focusing the image of the microstructure.

The stage is a movable at platform that supports the specimenA top view of a typical stage on an upright microscope can be seenin Fig. 5.4. On the stage, one can see the specimen directly belowthe objective of the microscope. All stages have mechanicalmovement in two horizontally perpendicular directions. Thisallows the metallographer to move the specimen from left to right

Fig. 5.1 A sketch of an early 17th century microscope used by RobertHooke in 1665

Fig. 5.2 An upright metallurgical microscope

Fig. 5.3 An inverted metallurgical microscope

110 / Meta ograp er’s Gu e



( x -movement) and front to back ( y-movement). Coaxial knurledknobs, that is, one knob within the other, are used to make thesehorizontal adjustments. These knobs can be seen in the center of Fig. 5.5. The amount of x - and y-movement of the stage can bemeasured by graduated scales along the edge of the stage. Thesescales are shown along the edges of the stage in Fig. 5.6. Somestages also allow rotation of the specimen. This is generally thecase for an inverted microscope. Also, as described subsequently,all metallurgical microscope stages have a third, up-and-down z-motion that is used for coarse and ne focus of the specimen.

Some metallographers use the scales on the stage to obtainrough measure of the thickness of certain features in the specimesuch as the thickness of a coating or the length of a crack. Tscale is usually calibrated in 0.1 mm divisions. For more accurameasurements, an eyepiece reticle is used. The reticle isgraduated scale within the eyepiece that can be focused along withe image of the microstructure. Reticles are discussed later in thchapter.

The Coarse- and Fine-Focus Knobs. On older tube-type mscopes, the coarse- and ne-focus knobs adjusted the barrel of th

tube up and down, and the stage was at a xed vertical positioOn modern-day microscopes, the upward and downward movments of the stage are controlled by coaxial knurled knobs easi

Fig. 5.4 The stage of an upright metallurgical microscope with a mountedspecimen directly beneath the incident beam of light

Fig. 5.5 Coaxial knurled knobs used to move stage in x - and y -dir

Fig. 5.6 Scales on the stage that indicate the amount of x - and y- motion

Fig. 5.7 Coaxial knurled knobs for ne (small, inside knob) and (large knob) focus. Note the graduated scale on the ne

knob, which indicates the amount of vertical ( z -axis) movement.

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accesssible to the metallographer. In metallurgical microscopes,hese are the coarse- and ne-focus knobs similar to those shownn Fig. 5.7. For coarse focus, the large-diameter knob is used, andor ne focus, the small-diameter knob is used. Most microscopes

have sets of knobs on both sides of the microscope stand for bothight- and left-handed operators. In Fig. 5.7, the ne-focus knob

has a graduated scale from 0 to 180, with each incrementepresenting a movement of 0.001 mm. The metallographer can

use the graduations on the ne-focus knob to measure depth of aeature within a specimen. For example, the depth of a pore can be

measured by magnifying the pore so that both its edge and bottomare in the eld of view. With the eld aperture of the light sourceully open, the metallographer rst focuses on the edge of the pore

and records the graduated location on the barrel of the ne-focusknob. The knob is then moved to focus on the bottom of the pore.By subtracting the location at the bottom of the pore from that athe top of the pore, the depth is determined. Generally, the

graduations on the ne-focus barrel are 0.001 to 0.005 mm,depending on the microscope manufacturer. This procedure isdescribed in more detail in the section “Special Procedures for theMetallurgical Microscope” at the end of this chapter. Also, theproper way to focus the microscope on a specimen is described in

hat section.

The Optical System

The optical system consists mainly of an objective (the lensassembly close to the object or specimen) and an eyepiece orocular (the lens assembly close to the eye). They are called lensassemblies, because each objective and eyepiece contains morehan one lens element, that is, compound lenses. Some objectives

contain up to 14 different lenses. Older microscopes are con-tructed with a xed distance between the objective and the

eyepiece, called the mechanical tube length. The tube length is

measured from the top of the tube to the last thread of the ob-ective, that is, at the point where the unthreaded portion of theobjective meets the microscope or nosepiece. Each microscopemanufacturer has a xed tube length that generally variesbetween 160 and 250 mm. For this reason, most objectives couldnot be interchanged between microscopes of different manu-acturers. However, more and more manufacturers are developingnnity-corrected objective lenses that depend less on tubeength. These lenses are discussed in a later section.

The Objective

The objective is the most important component of the opticalystem, because it denes the quality of the magnied image. Forhis reason, the metallographer, when purchasing a microscope

and particularly a metallograph, should always obtain the bestobjectives possible, even if some other feature of the microscopeneeds to be sacriced because of cost.

The objective lens controls six important properties of theoptical system of a microscope. These are the magnication,numerical aperture, resolving power, depth of eld, workingdistance, and light-gathering capability.

Magnication. The main function of the objective is to creaa magnied image. This magnied “real” image is created withinthe tube length of the optical system at a point called the imageplane of the eyepiece. Figure 5.8 shows a simplied ray diagramof a metallurgical microscope. In this diagram, the reected lightfrom the specimen passes through the objective in the form of amagnied image of the specimen. This magnied image isprojected onto the image plane of the eyepiece (called the realimage). At this point, the image is again magnied by the eyepieceand observed through the lens of the human eye. What the eye

actually sees is an image called the virtual image. The virtualimage appears about 250 mm (10 in.) from the eye (see Fig. 5.9)The image is inverted, because each time the image passesthrough a lens system, that is, through the objective, through theeyepiece, and through the lens of the eye to the retina, the imageis inverted.

Many metallurgical microscopes are purchased with a set of 5,10, 20, 50, and 100 objectives. A 40 or 60 objective canused in place of the 50 objective. Some microscopes can bpurchased with objectives as low as 1 and as high as 200 .symbol “ ” is a shortened way to represent the number of timein diameter the image is magnied, that is, 50 means 50 tim

the diameter, or 50 times magnication. Most modern-day objec-tives have the magnication boldly engraved on the barrel, asshown in Fig. 5.10. This is a 100 bright-eld objective with

Fig. 5.8 Sketch of the ray diagram for the typical upright metallurmicroscope showing the location of the human eye, the eyepie

the real image, the objective, and the polished specimen




numerical aperture (discussed subsequently) of 0.90. Other mark-ings indicate that the objective is to be used “dry.” This means thatit is used with air between the tip of the objective and thespecimen (the normal situation). Some objectives cannot be usedin air, as in the case of an oil-immersion objective, where a specialoil is placed between the tip of the objective and the specimen.The oil-immersion objectives provide more resolving power thana dry objective. The oil-immersion objective is discussed later inthis chapter. Also in Fig. 5.10, the number “210” indicates that thisobjective must be used in a microscope with a tube length of 210

mm, and the “0” indicates that the objective is not corrected for acover slip or cover glass (a thin, glass sheet that is placed on topof the specimen in a biological microscope). If a cover slip isnecessary, the objective will indicate its required thickness inmillimeters instead of a “0.” Cover slip correction is discussedlater.

The nal magnication in the microscope is usually themagnifying power of the objective times the magnifying power of the eyepiece. For example, a 100 objective with a 10 eyepieceyields a total magnication of 1000 . This assumes that the tubefactor is 1 . Some older microscopes can have tube factorsgreater or less than 1 (a range from 0.8 to 1.25 ). The tube

factor is usually engraved on the microscope tube. A tube factor isnecessary when there is an intermediate lens in the light pathbetween the objective and the eyepiece. Newer microscopes thatuse innity-corrected objectives have a special built-in tube lens(discussed later), and a tube factor is not necessary.

Magnication can be achieved by any combination of objetives and eyepieces that produce the required magnication. Fexample, for a magnication of 250 , the metallographerchoose any of the following combinations:

Objective Eyepiece

10 2525 1050 5

Even though the magnication of each combination is identica

the quality of the image produced is different. As discusspreviously, the objective denes the quality of the image, and the three examples listed previously, for the best quality imagthe metallographer should usually choose the objective with thhighest magnication (numerical aperture), in this example, th50 objective with the 5 eyepiece. With this combination, mof the total magnication is obtained with the 50 objectivthe higher-power objective will also have the greatest resolvinpower (discussed subsequently).

Numerical Aperture. As can be seen in Fig. 5.10, thereother words and numbers engraved on the barrel of the objectivThese include the type of lens and the numerical aperture (NA

The objective shown in Fig. 5.10 has a NA of 0.90. The NA imeasure of the light-gathering ability of the objective le

Fig. 5.9 Sketch of the same ray diagram in Fig. 5.8, but showing thelocation of the virtual image

Fig. 5.10 Photograph of a 100 bright-eld objective with a numaperture of 0.90. The “dry” indicates that the objective i

without oil between the objective lens and the specimen; the “210” indithe required 210 mm tube length of the microscope; and the “0” indicatesthe objective is used without a cover slip (cover glass).

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assembly. The higher the number, the greater the amount of lightcollected. Mathematically it is represented by the formula:

NA sine

where is the aperture half-angle of the most tilted (oblique) lightays entering the objective lens, and µ is the refractive index of the

medium (i.e., air where µ 1.0) in front of the lens. In Fig. 5.11,which is a schematic representation of light rays being reectedrom the specimen to the objective lens, the aperture half-angle s shown as 18°. From the previous equation, this objective would

have a NA of 0.30. Figure 5.12 has been included to furtherllustrate the importance of the light-gathering ability of an

objective. In these gures, ray diagrams represent objectives witha NA of 0.70 (Fig. 5.12a) and a NA of 0.25 (Fig. 5.12b). In bothcases, the objectives are assumed to be correctly focused on thepecimen. In Fig. 5.12(a), the aperture half-angle is 45°, and alleven reected rays pass through the objective lens. In Fig.

5.12(b), with a aperture half-angle of 15°, only three of the sevenays pass through the objective lens. In both gures, the angularelationship of the seven rays are identical. Therefore, the objec-ive in Fig. 5.12(a) has the greatest light-gathering ability andhus, the highest NA.

With air having a refractive index of 1.0, the maximum practicalNA is about 0.95. However, when a special oil is placed betweenhe lens and the specimen, the NA can be increased to 1.40. Most

oils that are supplied by microscope manufacturers have a refrac-ive index of 1.515, which matches the refractive index of the

glass in the lens. Special objectives, called oil-immersion objec-ives, are used with oil. With this almost-perfect matching, thepecimen is actually coupled to the lens, and the lens can gather

more reected light. This is discussed subsequently in the sub-ection “Oil-Immersion Objective.”

Resolving Power. The higher the numerical aperture, thegreater the resolving power or resolution of the microscope.

Resolving power is the ability of the objective to distinguishfeatures that are closely spaced. The resolution can be determinedby the following formula:

d (0.61 )/NA

where d equals the distance between two points or lines beingresolved and is the wavelength of the light from the illuminatiosource (i.e., tungsten-lament lamp, etc.). Thus, the resolvingpower is larger when the NA is large and the wavelength of thelight is short. With a good oil-immersion lens (high NA), the limitof resolution is about 0.2 µm.

To show the effect of NA on the quality of a microstructuralimage, Fig. 5.13 shows micrographs of the same eld of pearlitein a gray cast iron taken with 40 0.55 NA and 0.65 objectives. Both micrographs were taken at the same magnica-tion of 600 . Note the sharper image in Fig. 5.13(b), thmicrograph taken with the 0.65 NA objective, especially theresolution of cementite lamella in the pearlite (see arrows).

Filters can also be used to enhance the resolving power of anobjective. For light, the shorter wavelengths are at the violet-blue-green end of the spectrum, and the higher wavelengths are at theorange-red end of the spectrum. The wavelength of the light beam

can be controlled by lters and the actual light source itselfFigure 5.14 shows coarse pearlite in a pearlitic gray cast iron. The

Fig. 5.11 Sketch of an objective and a specimen showing the cone of lightthat can enter the objective lens from reection from the

pecimen; is the half-angle and is used to calculate numerical aperture. Theworking distance is indicated as the distance between the bottom of the lensand the top of the specimen.

Fig. 5.12 Sketches showing two different objectives with half-angles 45° and (b) 15° and numerical apertures of 0.70 and 0

respectively




micrographs in Fig. 5.14 were taken without a lter and with agreen lter. Note the improved resolution of the pearlite in Fig.5.14(b), the micrograph taken with the green lter (see the arrow).

Useful magnication is a term that applies to resolving power.In the microscope, magnifying an image beyond the resolvingpower of the lens system produces a magnied image butno furtherdetail in the image. The magnication range up to the point of op-timal detail is useful magnication (also called signicant or mean-ingful magnication). Any magnication above this point is calledempty magnication. From theory, empty magnication is pro-

duced above a magnication of 1000 times NA. Therefore, usinthis rule, an objective with a NA of 0.95 would produce a usefmagnication limit of 950 , and from experience, most metagraphic work is conducted at or below a magnication of 1000

Fig. 5.13 Micrographs of pearlitic gray cast iron taken with 40 objec-tives of different numerical apertures. Micrograph (a) taken with

an objective with NA 0.55 and micrograph (b) taken with NA 0.65. Notethe better resolution in (b) (see arrows). The white constituent is ferrite, thelamellar constituent is pearlite, and the dark gray akes are graphite. 4%picral etch. 600 . Courtesy of J. Wright, Lehigh University

Fig. 5.14 Micrographs of pearlitic gray iron taken (a) with and (b) a green lter. Note the improved resolution in the micro

taken with the green lter (see arrows). The light gray speckled constisteadite (iron phosphide eutectic), the lamellar constituent is pearlite, andark gray akes are graphite. 4% picral etch. 600 . Micrographs by J. Lehigh University




Depth of Focus. Another important property of an objectiveens is depth of focus (also called depth of eld), which is the

maximum amount of vertical movement of the stage or objectivewithout any noticeable change in focus on the specimen. Thedepth of eld is inversely proportional to the square of the NA aseen in the following equation:

Depth of eld µ

2NA 2

where µ is the index of refraction of the medium between

objective and specimen, for example, µ 1 for air; and is thewavelength of the light (mm), for example, green 548 nm. Thismeans that an objective with a lower NA will allow a greaterdepth of focus. Thus, for rough or heavily etched surfaces, a lowNA lens must be used.

Working Distance. The working distance is the distancebetween the tip of the objective lens and the specimen, asllustrated in Fig. 5.11. The working distance decreases as the

power of the objective increases and generally, as the NAncreases. For example, the working distance of a 10 objective

may be about 10 mm, whereas the working distance of a 100objective may be less than 0.5 mm. With short working distance

objectives, the metallographer must take extreme care in order toavoid damaging the tip of the objective by chipping, cracking, orcratching the glass lens by contact with the specimen. Some

objectives with short working distances are spring loaded, whichallows the bottom portion of the objective to retract into theobjective barrel if the tip is inadvertently pressed against thepecimen. This is a safety feature to minimize damage to the

exposed objective lens. However, there is no reason to ever touchhe specimen surface with the objective lens.

As is seen later in this chapter, there are specially designedobjectives with long working distances. For example, a longworking distance objective is required when observing a specimenhrough a glass or quartz window of a hot stage (a special

attachment that heats the specimen under vacuum or inert gas).Basic Characteristics of the Objective. The following basic

ules apply to all objective lenses (within their class).As magnifying power increases:

Working distance decreasesLens diameter decreasesField of view decreasesDepth of eld decreases

As NA increases:

Resolving power increasesUseful magnication increasesLight-gathering increases

The Nosepiece

For convenience, most metallurgical microscopes have a rotat-ng nosepiece where three or more objectives are mounted. Figure

5.15 shows a nosepiece with ve objectives. The objectives aremounted in progressive rotational order. As the metallographerrotates the nosepiece, there are obvious mechanical stops builtinto the nosepiece that allow for proper optical alignment with thebeam of light. This alignment with the optical axis means that theobjective is parcentric. During rotation of a nosepiece with veobjectives, the rst objective in rotation might be the 5objective, followed by the 10 objective, the 20 objective,50 objective, and nally, the 100 objective. The nosepiemounted on a microscope shown in Fig. 5.16 is rotated clockwise,

looking at the nosepiece from the specimen, to achieve the

Fig. 5.15 A typical nosepiece from a metallurgical microscope.

nosepiece accomodates ve objectives.

Fig. 5.16 Nosepiece mounted on a typical upright metallurgical micscope




previously mentioned order of magnication. When the nosepieceis rotated from objective to objective, the eld remains inapproximate focus. This is known as being parfocal. The onlyadjustment may be to slightly adjust the ne-focus knob. How-ever, the central features of the microstructure remain centered. Of course, the eld is much larger at the lower magnications. Thisis shown in Fig. 5.17 where the same eld is photographed after

rotation of the nosepiece to 5, 10, 20, 50, and 100 (using eyepiece). In this example, a white cast iron is shown withdominant feature of massive cementite platelets (white constituent). The rotating nosepiece is an important advantage in metalography, because the metallographer can quickly nd the area ointerest at low magnication and then rotate the nosepiece tohigher magnication for better detail of the features in the eld.

Fig. 5.17 Micrographs taken of the same eld of a white cast iron by rotating a ve-objective nosepiece through (a) 5 , (b) 10 , (c) 20 , (d) 50 ,100 . The white constituent is cementite, and the dark constituent is pearlite. 4% picral etch. Final magnication (a) 50 , (b) 100 , (c

(d) 500 , and (e) 1000




act, this should be normal procedure when examining a specimenor the rst time. Always scan the specimen at low magnication,hat is, using the 5 objective, to nd the eld of interest and thenotate the nosepiece to progressively higher magnications. It is

not advisable to rotate directly from a 5 objective to a 100

objective, because changing the magnication from 501000 is too large a change. Also, there may be a chance ohitting the specimen with the lens of the highest magnicationobjective (the longest objective with the shortest working dis-tance), especially if the objective is not fully screwed into thenosepiece. This loosening of objectives can actually happen whenthe nosepiece is rotated by using the objectives as a handle insteadof properly using the knurled ring on the nosepiece for rotation.

If the metallographic laboratory has more than one microscope,always place the objectives in the same rotational order in the

nosepiece. This simple procedure will prevent damage to theexpensive higher-power objectives with the very short workingdistances.

Optical Defects in ObjectivesMicroscope manufacturers strive to produce the perfect objec-

tive. This is almost impossible, because even if the glass lens isground to perfection, there are errors or aberrations that arepresent due to the inherent nature of refraction when light passesthrough air and into a glass lens, and due to the differenwavelengths within the visible light spectrum. Therefore, alllenses need to be corrected for these aberrations so that thmetallographer will observe a near defect-free image of themicrostructure, that is, an image that is as close as possible to theactual microstructure. In order to understand how these aberra-tions arise, the metallographer must understand how light interactswith a lens. In most cases, we are dealing with convex lenseswhere the lens surfaces are curved outward, as seen in Fig5.18(a), as opposed to concave lenses where the lens surfacescurve inward, as seen in Fig. 5.18(b). The convex lens is magnifying lens that is necessary to produce the highly magniedimage observed in the microscope (most compound objectivescontain both convex and concave lenses).

When a beam of light travels through air and then through the

glass lens, the speed of the beam of light decreases. The speedchange creates a phenomenon called refraction. A simple exampleof refraction is observed when a pencil is placed into a containerof water, as seen in Fig. 19. The portion of the pencil in the wateappears to bend at an angle to the portion of the pencil in the aiabove the water. In a lens, this bending of light, or refraction, is

Fig. 5.17 Micrographs taken of the same eld of a white cast iron byrotating a ve-objective nosepiece through (a) 5 , (b) 10 , (c)

20 , (d) 50 , and (e) 100 . The white constituent is cementite, and the darkonstituent is pearlite. 4% picral etch. Final magnication (a) 50 , (b) 100 ,c) 200 , (d) 500 , and (e) 1000 Fig. 5.18 Sketches of a (a) convex and a (b) concave lens




the feature that makes the lens work. However, refraction cancreate aberrations. Because the beam of light (called white light)is composed of various primary colors (red, orange, yellow, green,blue, and violet), the colors observed in a rainbow, and each colorhas a different wavelength, the speed of each color is slowed at adifferent rate as it passes through the glass. For example, red hasa wavelength of 656 nm, green a wavelength of 548 nm, blue awavelength of 486 nm, and violet a wavelength of 405 nm. Thesedifferences in wavelength can be seen when a beam of light passesthrough a prism and the beam splits into its primary colors. This

effect creates two kinds of lens aberrations: chromatic aberrationand spherical aberration.

Chromatic aberration is created by the different refractionangles (different wavelengths) of the various colors of the lightbeam passing through the lens. Axial (longitudinal) chromaticaberration is illustrated in Fig. 5.20 where light rays passingthrough a convex lens separate into blue, yellow-green and redcolors along the optic axis. This aberration must be corrected,because all the colors should have one common focal point andnot a separate focal point for each color. The lens designer must

correct for this disparity in focus. There is also a lateral (tranverse) chromatic aberration, where the light of one color hasgreater magnication than another color. Without correction, thimage as seen in the microscope will have color fringes.

Spherical aberration is created by the different bending anof the light rays as they pass through a convex lens. Thisillustrated in Fig. 5.21. The light rays entering the outer diametof the lens (the thinnest part of the lens) are called marginal rayThese rays have a different focal point than the axial rays thenter and exit the central portion of the lens (the thickest part

the lens). The lens must be corrected in order to create ocommon focal point.

Coma is another lens defect that produces comet tails images of points in the specimen. It is more pronounced in tholight rays that pass through the lens further away from the leaxis. Coma can be corrected by grinding the proper surfacurvature of the lens.

Astigmatism is a lens defect that creates lines in the image frpoints in the specimen. The word astigmat means “not a pand, as with coma, it can be corrected by grinding the propcurvature.

Curvature of eld is a serious defect in images produced

convex lenses. In the metallurgical microscope, the image must bas at as possible so that the center of the image is in as sharfocus as the outer edges. Curvature of eld creates blurred outedges. It is very difficult to design an objective without havinsome degree of curvature of eld, and thus, these speciadesigned objectives are very expensive. In some microscopes, thcurvature of eld created in the image of the objective can corrected by the eyepiece.

Distortion is another defect that cannot be tolerated in metallurgical microscope. The image created by the objective cabow in or out at the outer edges. In other words, if examiningsquare grid of perpendicular lines, the grid at the center (axwould represent the proper spacing, but near the edge of image, the lines become curved and no longer represent the trgrid spacing. This can be seen in Fig. 5.22 where it is greaexaggerated. Distortion is produced by a lens having a differemagnication at the central axis than at the outer edges of the lenMost modern metallurgical microscopes have been corrected fo

Fig. 5.19 A photograph of a pencil immersed in a dish of water showingthe effect of refraction of light, that is, the apparent bending of

the pencil in water due to a difference in refractive index between waterand air

Fig. 5.20 Sketch of a ray diagram of light passing through a convex lenswith chromatic aberration. The rays passing through the outer

portion of the lens split into different focal lengths of blue, yellow-green, andred light because of the different wavelengths.

Fig. 5.21 Sketch of a ray diagram of light passing through a convwith spherical aberration. The rays passing through the ce

the lens (axial rays) have a different focal point than the rays passing tthe edge of the lens (marginal rays).




parallax, which eliminates distortion. Therefore, it is very rare toee distortion in modern microscopes.

Types of Objectives

The selection of an objective can be quite confusing for thebeginning metallographer. This is because there are many differ-ent types of objectives. Some have corrections for various lensaberrations. These corrections, discussed previously, includepherical aberration, coma, astigmatism, distortion, and chromatic

aberration. Some of these same aberrations can be found in theenses of the human eye, and eyeglasses are used to correct forhese errors. Some objectives have glass lenses; some have lenses

made from calcium uoride (uorite). Lens designers even usedifferent types of glass, for example, int glass or crown glass.Some objectives are produced for exceptional atness of eld,ome are produced with a longer focal length, and others are

produced for dark-eld applications. The metallographer shouldbe familar with the various types of objectives available for themicroscopes in the metallographic laboratory. Some of theseobjectives are briey described subsequently.

Achromatic Objective . This is the most commonly used and

east-expensive objective for the metallurgical microscope. Theobjective is partially corrected for chromatic aberration (discussedpreviously) by correcting for two colors, usually green and red.This means that the red and green colors of the spectrum of lightare brought into the same focal point. An achromatic objective, orimply an “achromat,” is also partially corrected for spherical

aberration for one color, usually the green color or the yellow-green color. Therefore, when using an achromat, it is best to usea green or yellow-green lter to take into account this latercorrection. Because they are only partially corrected, they are not

suitable for color micrography but are well suited for black andwhite micrography. An advantage of the achromatic objective isthat it has a larger working distance than the more highlycorrected objectives, for example, the apochromatic objective.This is because there are fewer lens elements in the objective. Alarger working distance provides for more clearance between thetip of the objective and the specimen surface. With a largeclearance, there are fewer chances of chipping, scratching, andcracking the exposed lens. Some objectives are spring loaded tominimize such damage. An achromatic objective is generally not

identied with a code or symbol engraved on the barrel of theobjective, whereas most all the other objectives have specialidentication.

Semiapochromatic (Fluorite) Objective . These objectivcalled uorites, employ uorite instead of glass for the lenselements. Fluorite is the mineral uorspar (calcium uoride) andcan be ground into lenses from either natural or synthetic crystals.These uorite objectives are more expensive than the achromats.They are corrected for chromatic aberration for two colors, the redand blue or the red and green colors, and are corrected fospherical aberration in two colors. This means that these objec-tives can be used for color micrography as well as black and white

micrography. Usually these objectives are identied by a “Fl” or“Fluor” symbol that is engraved on the barrel.Apochromatic Objective. These objectives are more expe

sive than the achromats and uorites, because they are morehighly corrected. Therefore, these are the nest objectives avail-able and are particularly suited for higher-magnication metal-lography. They are chromatically corrected for three colors, theprimary colors red, blue, and green, and spherically corrected forat least two colors, generally green and blue. This means that anapochromatic objective will perform best with a green or bluelter. These objectives are used for color micrography. They arealso very suitable for black and white micrography. Apochromaticobjectives usually have higher NAs than the achromatic anduorite objectives and thus have the potential for higher resolvingpower. Generally, these objectives are only used for magnica-tions of 500 and above. Even though these objectives archromatically corrected for three colors and spherically correctedfor two colors, they are still not optimally corrected. Furthercorrections of the apochromatic objective can be made by theproper selection of a compensating eyepiece. Therefore, whenusing apochromatic objectives for optimal correction, it is impor-tant to couple the objective with the compensating eyepiece of thesame microscope manufacturer. Generally an apochromatic objective is identied with an “Apo” engraved on the barrel. Onemanufacturer uses “CF,” or chromatic-free, on the barrel of their

apochromatic objective. All the aberrations have been correctedfor in the objective, and they do not depend on the eyepiece foadditional corrections. This type of objective is ideal for charge-coupled device (CCD) digital cameras that do not require aneyepiece.

Plano Objective. The plano objectives are corrected fatness of eld and therefore are sometimes called at-eldobjectives. The previous three types of objectives are generallynot corrected for atness of eld unless they are engravedplan-achromatic, planuorite, or plan-apochromatic. If the objec-

Fig. 5.22 Sketch of a distorted image. (a) Barrel-type image and (b)pincushion-type image




tive is engraved with just “plano,” “plan,” or “pl,” it is aplan-achromatic objective. A plan-apochromatic objective may beengraved as “Pl apo.” The plan-apochromatic objective is veryexpensive and is the most highly corrected objective available forthe microscope. These expensive objectives may not be neededfor a microscope that is only used for observation, because themetallographer can accept some curvature of eld and simplyfocus and refocus on the eld of interest. However, if theinstrument is used for micrography, a plano objective will producethe atness of eld necessary for the entire micrograph.

Phase Contrast Objective. When the optical system is func-tioning under phase contrast conditions (transmitted light), specialphase contrast objectives are used with a special vertical illumi-nator (condenser). A phase contrast objective may have “Phaco,”“Ph,” “DL,” or “DM” engraved on the barrel of the objective.Phase contrast illumination is not used for specimens of steel orcast iron but is widely used in transmitted light for biologicalspecimens.

Strain-Free Objective. For ferrous specimens, differentialinterference contrast, or the Nomarski technique, is more com-monly used than phase contrast (previously mentioned). In thiscase, a strain-free objective is used, and this objective may be

engraved with an “N” for Nomarski. Differential interferencecontrast is discussed later in this chapter. Also, strain-free objec-tives are used for interference illumination, described later in thischapter. These objectives can be engraved with an “IK” forinterference contrast, “NIC” for Nomarski interference contrast,“DIC” for differential interference contrast, “POL” for polarizedlight, or “Pol. Interf.” for polarization interference contrast. A

strain-free lens may also be engraved with the letters “SF” fstrain-free.

Dark-Field Objective. Most objectives are producedbright-eld illumination. Bright eld is the normal operatiillumination of the metallurgical microscope, where the incidenlight passing through the objective is perpendicular to the specmen surface. However, there are certain applications whedark-eld illumination is required. In these cases, the incidelight radiates from the objective at oblique angles to the specimesurface. The dark-eld objective illuminates the specimen with

360° circle of light around the periphery of the objective barrthus allowing only the reected light to pass through the glalenses of the objective. Dark-eld and bright-eld objectives ashown in Fig. 5.23. Note the annular opening surrounding tcentral lens of the dark-eld objective (left). This circular openinallows the illumination light to pass around the outer portion the objective (just inside the objective barrel) while the reectlight passes through the lens of the objective. More details dark-eld illumination can be found later in this chapter. dark-eld objective usually has a “D,” “DF,” or “BD” engraved oits barrel and is larger in diameter than a bright-eld objective, seen in Fig. 5.23.

Long-Working-Distance Objective (Quartz CorrectSpecial objectives with a long working distance are produced fhot-stage microscopy. The long working distance is necessarbecause the objective must be kept at a safe distance from theated specimen. Generally, the working distances range from to 16 mm. Also, these objectives may be corrected for quartz. This because there is usually a clear quartz window between tspecimen and the objective. As described previously, these objectives are corrected for the quartz window. An objective used fthe hot stage may display an “H” (hot) or “L” (long workidistance) engraved on the barrel. Sometimes the barrel of tobjective will be engraved with the word “Quartz.”

Oil-Immersion Objective. Special oil-immersion objectare designed to give the highest resolution obtainable with a ligmicroscope. These objectives are used with a drop of special othat is placed between the tip of the objective and the specimeThe oil has the same refractive index as the glass used in tobjective lens (µ 1.515). Because the NA is determined byrefractive index and aperture half-angle of light entering the tip the objective (explained previously in the section “NumericAperture”), an oil-immersion objective can have a much highNA than a dry objective. In all the other objectives describpreviously, the light must pass through air with a refractive indeof unity (µ 1). In passing from the glass lens with µ into air with µ 1.0, the rays of light are bent by the phenome

of refraction (see Fig. 5.19). This also occurs when the same ligis reected off the specimen back through air into the glass lens the objective. This bending of light causes a portion of the light be lost. If the bending of light is eliminated, very little light is loand therefore, most of the light passes to the specimen and bathrough the lens. Earlier in this chapter it was mentioned that tmore light gathered by the objective, the higher the NA. Oimmersion objectives can have a NA up to about 1.40. The upplimit for dry objectives is NA 0.95. Oil-immersion objective

Fig. 5.23 Photograph of a 40 dark-eld objective (left) and a 40bright-eld objective (right). Both objectives have a numerical

aperture of 0.65, a required tube length of 210 mm, and are used dry. In thedark-eld objective, note the annular opening around the central objectivelens for the incident light path.




160 are available for high-resolution requirements. Figure 5.24hows a comparison of the image produced with an oil-immersion

objective (NA 1.3) and a dry objective at the upper limit of NANA 0.95). Both micrographs were taken at the same magni-

cation of 1300 . In this fully pearlitic microstructure, one canee the improved resolution using the oil-immersion objective.

Generally, oil-immersion objectives are only rarely used, becausethey are not required for the vast majority of metallographicspecimens. Oil-immersion objectives require special care andmust be cleaned after each use. The special procedure used for anoil-immersion objective can be found in the procedures section atthe end of this chapter. An oil-immersion objective is usuallyengraved with “Oil,” “Oel,” or “HI” (homogeneous immersion).Also, any objective with a NA greater than 1.0 is usually anoil-immersion objective.

Innity-Corrected Objectives. Newer metallurgical mic

scopes have objectives that are “innity” corrected. In a conven-tional microscope, the rays of reected light emerging from theback lens of the objective lens are not parallel. With an innity-corrected objective, the reected light rays emerging from theback lens of the objective are parallel (projected toward innity).To use these objectives, a tube lens is mounted in the path betweenthe objective and the eyepiece (above the vertical illuminator).This lens brings the parallel rays from the objective into conver-gence (focus) onto the focal plane of the eyepiece diaphragm(described in the section “Types of Eyepieces” later in thischapter). The advantage of an innity-corrected lens is that thelight rays are parallel when passing through the vertical illumi-

nator (bright-eld reected mirror) region of the microscope tube.This means that optical components, that is, polarizers, can beinserted into the column of parallel light rays without introducingoptical errors (aberrations) and at an exact magnication of 1

Some innity-corrected objectives have the symbol “ ”graved on the barrel. These objectives are less restricted to a xedtube length of the microscope and in some special microscopes,allow movement of the objective rather than the stage for focus.

Retractable Objectives. Some objectives with a short workindistance are spring loaded to prevent damage to the exposed lensof the objective when inadvertently pressed against the specimen.(Note that there is no circumstance where any objective shouldphysically touch the specimen.) A spring-loaded assembly allowsthe bottom part of the objective to retract into the objective barrel.This type of objective may be useful for students who are nofamiliar with a metallurgical microscope. Experienced metallog-raphers have less need for this feature.

Cover-Glass-Corrected Objectives. It must be remembenot to mix objectives and eyepieces of different microscopemanufacturers and even from different microscopes of the samemanufacturer. Generally, when a metallurgical microscope ispurchased, the set of objectives and eyepieces will only be usedfor that model of microscope. This is because of different tubelengths, tube factors, and other basic characteristics of themicroscope. It should also be pointed out that the objectives for a

biological microscope should not be used on a metallurgicalmicroscope. This is because the objectives for the biologicalmicroscope are corrected for a cover glass (also called cover slip).The cover glass is a thin glass sheet that is placed over thbiological specimen (blood stain, biopsy tissue slice, etc.). Thestandard cover glass thickness is 0.17 mm (other thicknesses areavailable). One way to distinguish a cover-slip-corrected objectivefrom an uncorrected objective is by “0.17” engraved on the barrelof the objective. For example, in Fig. 5.25 a cover-glass-corrected

Fig. 5.24 Micrographs of an AISI/SAE 1080 steel showing the same eld of pearlite taken with (a) a dry objective of NA 0.90 and (b) and

oil-immersion objective of NA 1.30. Note the improved resolution in themicrograph taken with the oil-immersion objective. 4% picral etch. 1300 .Courtesy of S. Lawrence, Bethlehem Steel Corporation




objective is shown on the right, where one can see the cover glass

thickness alongside the tube length as “170/0.17.” The 10uncorrected objective at the left in Fig. 5.25 has a dash, “-”, nextto the tube length. The dash means with or without cover glass.The 100 cover-glass-corrected objective in this photograph is anoil-immersion lens (“Oel”) with a NA of 1.30. The uncorrectedobjective is a dry objective with a NA of 0.25. Some microscopemanufacturers engrave a “NCG,” “NC,” or “0” on the objective toindicate no cover glass.

Types of Eyepieces

The eyepieces or oculars of the metallurgical microscope arenot as complex as the objectives. The function of the eyepiece isto magnify the image produced by the objective. Therefore, theimage of the microstructure is magnied twice: rst by theobjective and then again by the eyepiece. Thus, a 20 objectivewould magnify the image 20 times, and a 10 eyepiece increasesthe magnication to 200 times (assuming no other lenses arebetween the eyepiece and the objective). This nal image in theeye appears as a “virtual image” at a distance of about 250 mm(10 in.) from the eye (as shown in Fig. 5.9). Eyepieces usually aremanufactured with magnications of 5, 6.3, 8, 10, 12.5, 20, 25,and 30 . The magnication is engraved on the top ring of theeyepiece. For most metallographic work, the 10 eyepiece is

appropriate. The eyepiece ts into the eyepiece tube of themicroscope. Many metallurgical microscopes have two eyepiecesthat t into a binocular attachment to the main tube of themicroscope, as seen in Fig. 5.26. The image from the objective issplit into the two eyepieces by a prism. With two eyepieces, thereis less eye strain for the operator. The eyepiece tubes are adjustedfor the correct interpupillary distance, that is, the distance betweenthe pupils of one’s eyes. For most people, this is between 60 and70 mm. Usually, an interpupillary scale is located between the two

eyepieces, as seen in Fig. 5.27. In a metallographic laboratowith many people using the microscope, the metallographshould remember his or her unique interpupillary distance so ththe binocular attachment can be reset each time the microscope used. Some microscopes have another eyepiece adjustment callethe diopter scale, as seen in Fig. 5.28. This adjustment compesates for differences in focus of the left and right eyes. In F5.28, the diopter is set at the zero position. The metallographshould know his or her unique diopter settings.

Fig. 5.25 Photograph of two objectives. The objective on the left, marked“170/-,” is for a tube length of 170 mm to be used without a

cover slip (cover glass). The objective on the right, marked “170/0.17,” is fora tube length of 170 mm but must be used with a 0.17 mm thick cover slip.

Fig. 5.26 A binocular attachment for a metallurgical microscope

Fig. 5.27 A binocular attachment showing the interpupillary scaltween the eyepieces)


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