Designation:E 883–02
Standard Guide for
Reflected–Light Photomicrography 1
This standard is issued under the fixed designation E 883;the number immediately following the designation indicates the year of original adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.A superscript epsilon (e )indicates an editorial change since the last revision or reapproval.
1.Scope
1.1This guide outlines various methods which may be followed in the photography of metals and materials with the reflected-light microscope.Methods are included for prepara-tion of prints and transparencies in black-and-white and in color,using both direct rapid and wet processes.
1.2Guidelines are suggested to yield photomicrographs of typical subjects and,to the extent possible,of atypical subjects as well.Information is included concerning techniques for the enhanced display of specific material features.Descriptive material is provided where necessary to clarify procedures.References are cited where detailed descriptions may be helpful.
1.3This standard does not purport to address all of the safety concerns,if any,associated with its use.It is the responsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory requirements prior to use.Specific precau-tionary statements are given in X1.7.
1.4The sections appear in the following order:
Referenced documents 2Terminology
3Significance and use 4Magnification
5Reproduction of photomicrographs 6Optical systems 7Illumination sources
8Illumination of specimens 9Focusing
10Filters for photomicrography 11Illumination techniques 12Instant-processing films 13Photographic materials 14Photographic exposure 15Photographic processing 16Keywords
17Suggestions for visual use of metallographic microscopes X1Guide for metallographic photomacrography X2Electronic photography
X3
2.Referenced Documents 2.1ASTM Standards:
E 3Methods of Preparation of Metallographic Specimens 2
E 7Terminology Relating to Metallography 2E 175Terminology of Microscopy 3
E 768Practice for Preparing and Evaluating Specimens for Automatic Inclusion Assessment of Steel 2
E 1951Guide for Calibrating Reticles and Light Micro-scope Magnifications 2
3.Terminology
3.1Definitions —For definitions of terms used in this guide,see Terminologies E 7and E 175.
4.Significance and Use
4.1This guide is useful for the photomicrography and photomacrography of metals and other materials.
4.2The subsequent processing of the photographic materi-als is also treated.
5.Magnification
5.1Photomicrographs shall be made at preferred magnifi-cations,except in those special cases where details of the microstructure are best revealed by unique magnifications.5.2The preferred magnifications for photomicrographs,are:253,503,753,1003,2003,2503,4003,5003,7503,8003,and 10003.
5.3Magnifications are normally calibrated using a stage micrometer.Calibration procedures in Guide E 1951should be followed.
6.Reproduction of Photomicrographs
6.1Photomicrographs should be at one of the preferred magnifications.A milli-or micrometre marker shall be super-imposed on the photomicrograph to indicate magnification,in a contrasting tone.The published magnification,if known,should be stated in the caption.
6.2Photomicrograph captions should include basic back-ground information (for example,material identification,etchant,mechanical or thermal treatment details)and should briefly describe what is illustrated so that the photomicrograph can stand independent of the text.
6.3Arrows or other markings,in a contrasting tone,shall be used to designate specific features in a photomicrograph.Any marking used shall be referenced in the caption.
1
This guide is under the jurisdiction of ASTM Committee E04on Metallogra-phyand is the direct responsibility of Subcommittee E04.03on Light Microscopy.Current edition approved Nov 10,2002.Published April 2003.Originally published as E 883–82.Last previous edition E 883–99.2
Annual Book of ASTM Standards,V ol 03.01.
3
Annual Book of ASTM Standards,V ol 14.02.
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7.Optical Systems
7.1Microscope objectives are available in increasing order of correction as achromats,semiapochromats (fluorites)and apochromats (see Terminologies E 7and E 175).Plan objec-tives are recommended for photographic purposes because their correction provides a flatter image.The objective lens forms an image of the specimen in a specific plane behind the objective called the back focal plane.(This is one of several possible real image planes,called intermediary planes,where reticles may be inserted as optical overlays on the image.)7.2The eyepiece magnifies the back focal plane (or other)intermediary image for observation or photomicrography.Eye-pieces are sometimes also used to accomplish the full correc-tion of the objective’s spherical aberration and to improve the flatness of field.
7.2.1The pupil of the observer’s eye must be brought to coincidence with the eyepoint of the visual eyepiece to view the entire microscopical image.High-eyepoint eyepieces are necessary for eyeglass users to see the entire image field.7.2.2Most microscopes have built-in photographic capa-bilities that use an alternate image path through the microscope leading to a camera attachment port or to a viewscreen.A projection eyepiece delivers the image to the camera port or screen.
7.3Intermediate lenses (relay or tube lenses)are often required to transfer the specimen image from the intermediary plane of the objective to that of the eyepiece.They may also add their own magnification factor,either fixed or as a zoom system.
7.4The objective,the eyepiece,and the compound micro-scope (including any intermediate lenses)are designed as a single optical unit.It is recommended to use only objectives and eyepieces which are intended for the microscope in use.7.5The resolution of the microscope depends primarily on the numerical aperture of the objective in use (1)4.The term empty magnification is used to describe high magnifications (above approximately 1100times the numerical aperture of an objective),which have been shown to offer no increase in image resolution.Nevertheless,some types of information,such as the distance between two constituents,may be more easily obtained from microstructures examined at moderate empty magnifications.
8.Illumination Sources
8.1Metallographic photomicrography typically uses Köhler illumination.To obtain Köhler illumination,an image of the field diaphragm is focused in the specimen plane,and an image of the lamp filament or arc is focused in the plane of the aperture diaphragm.Specific steps to obtain Köhler illumina-tion vary with the microscope used.The manufacturer’s instructions should be followed closely.
8.2For incandescent lamps,the applied voltage determines the unit brightness and the color temperature of the source.Evaporated tungsten blackens the envelope,resulting in dimin-ished brightness and color temperature as the lamp ages.
Tungsten-halogen lamps minimize envelope blackening,
main-taining constant brightness and color temperature for most of their life.The high brightness and 3200K color temperature of these lamps makes them especially suitable for color photomi-crography.
8.3With arc sources,brightness per unit area is substan-tially higher than that from any incandescent source.Their spectral output contains high energy spikes superimposed on a white-light continuum.They also contain significant ultraviolet (UV)and infrared (IR)emissions that should be removed for eye safety (and for photographic consistency,with UV);see 8.4,11.3.1,and 11.5.2.
8.3.1Xenon arcs produce a spectral quality close to daylight (5600K),with a strong spike at 462nm.Strong emissions in the IR should be removed.Xenon arcs that do not produce ozone are recommended.
8.3.2Carbon arcs have a continuous output in the visible portion of the spectrum,with a color temperature near 3800K and a strong emission line at 386nm.
8.3.3Mercury arcs have strong UV and near-UV output,and are particularly useful to obtain maximum resolution with a blue filter.The color quality is deficient in red;it cannot be balanced for color photomicrography.
8.3.4Zirconium arcs have strong spectral output lines in the near IR,requiring filtration.Within the visible region,they are rated at 3200K color temperature.
8.4Arc lamps require heat protection for filters and other optical components,and certainly for eye safety.Infrared removal may be obtained by:“hot”mirrors in the illumination beam to reflect IR while transmitting visible light;heat-absorbing filters to transmit visible light while absorbing IR,for example,solid glass filters or liquid-filled cells.
8.5A detailed discussion of illumination sources and the quality of illuminants is given by Loveland (2).
8.6Some advice on using metallographic microscopes for visual observation has been compiled in Appendix X1.9.Illumination of Specimens
9.1Photomicrographs are made with a compound micro-scope comprised at least of an objective lens and an eyepiece with a vertical illuminator between them.Field and aperture diaphragms,with a lamp a
nd lamp condenser lenses,are integral parts of the system.The microscope should allow sufficient adjustment to illuminate the field of view evenly and to completely fill the back aperture of the objective lens with light.
9.2The vertical illuminator is a thin-film-coated plane glass reflector set at 45°to the optical axis behind the objective.It reflects the illumination beam into the objective and transmits the image beam from the objective to the eyepiece.In some microscopes prism systems are used to perform this function.9.3The field diaphragm is an adjustable aperture which restricts the illuminated area of the specimen to that which is to be photographed.It eliminates contrast-reducing stray light.The field diaphragm is also a useful target when focusing a low-contrast specimen.
9.4The aperture diaphragm establishes the optimum bal-ance between contrast,resolution,and depth of field.It should be set to illuminate about 70%of the objective’s aperture
4
The boldface numbers in parentheses refer to the list of references at the end of this standard.
diameter.This can be observed by removing the eyepiece and inspecting the back of the objective,eith
er directly or with a pinhole eyepiece.The aperture diaphragm should never be used as a light intensity control.
9.5See Fig.1for an illustration of a typical vertical illumination system.
10.Focusing
10.1Sharp focus is necessary to obtain good photomicro-graphs.
10.2There are two systems for obtaining sharp focus:ground-glass focusing and aerial image focusing.
10.2.1For ground-glass focusing,relatively glare-free sur-roundings and a magnifier up to about 33are required.To focus,the focusing knob is oscillated between underfocus and overfocus in succeedingly smaller increments until the image is sharp.
10.2.2There are four possible variations for focusing an aerial image.
10.2.2.1The simplest case is a transparent spot on a ground-glass containing a fiduciary mark in the film plane.The specimen image is focused to coincide with the fiduciary mark,using a magnifying loupe of about 33to 53.When the focus is correct,the specimen image and the fiduciary mark will not move with respect to each other when the operator’s head is moved.
10.2.2.2A second case uses a reticle fixed within the optical system at an intermediary plane.Focusing is a two-step process:focus the eyepiece on the reticle;bring the image into focus against the reticle figure.
10.2.2.3In the third case,a reticle is inserted into a focusing eyepiece.Depending on equipment used,this can be either a two or three-step process:focus the reticle within the eyepiece;next,set the proper interpupiliary distance,if required (some equipment requires a specific interpupiliary distance for eye-piece focus to coincide with camera focus);then focus the image coincident with the reticle.
10.2.2.4The fourth case uses a single-lens reflex camera body,where the camera focusing screen is the plane of
reference.An eyepiece magnifier for the camera is an
impor-tant accessory for this case.An aerial image focusing screen is preferred.
10.3The critical focus point is affected by both the principal illumination wavelength in use and the size of the aperture diaphragm.Final focusing should be checked with all filters,apertures,and other components set for the photomicrograph.11.Filters for Photomicrography
11.1Photomicrographs require filtration of the light source.This section describes filter types and their uses.
11.2Each filter selectively removes some wavelengths from the transmitted beam of light.Two types of filters,interference and absorption,can be used for this purpose.
11.2.1Interference filters act as selective mirrors.By means of coatings on a glass substrate,they selectively transmit certain wavelengths while reflecting all others.These filters may be used in high-energy light beams.The mirrored side of the filter should face the light source.(The hot mirrors in 8.4are interference filters.)
11.2.2Absorption filters are dyed substrates of glass,plas-tic,or gelatine.They absorb some wavelengths of light and transmit the balance.Through their absorption,they can become overheated a
nd damaged if placed in high-energy light beams without protection.The usual protection is either an interference filter or a liquid-filled cell placed in the beam before the absorption filter.Wratten gelatine filters are used below as examples (3).Many similar glass and plastic filters are also available.
11.3Certain general purpose filters have application in both color and black-and-white photomicrography.
11.3.1Ultraviolet light can be removed with an interference filter,a glass or gel filter from the Wratten #2series,or a liquid cell filled with a sodium nitrite solution (2%NaNO 2is used for a 1-cm path.It should be proportionately stronger or weaker for other cell path lengths).Ultraviolet light must be removed from arc lamps for eye safety,and should be removed for color photomicrography,as explained in 11.5.2.
FIG.1Vertical Illuminating System for a Metallurgical Microscope
11.3.2Gray neutral density filters reduce the intensity of a light beam equally across the visible spectrum.They are made in interference and absorption types in many different densi-ties,for example,the Wratten #96series.They are useful for eyepiece work with an arc source,and to modify the brightness of any tungsten source without changing its color temperature.11.4Filters for Black and Whit
e Photomicrography :
11.4.1Generally,a monochromatic filter is used to optimize the resolution of the objective.With achromats,a green centered around 550nm is used;for apochromats and semi-apochromats,a blue centered around 486nm provides slightly better resolution,but with a penalty of more difficult visual focusing.
11.4.2Cases arise where the visual contrast can be im-proved to emphasize a colored feature in the microstructure.The color will reproduce darker in the photomicrograph if a filter is used with a color complementary to that of the feature (for example,a cyan filter for reddish copper plating;a blue for yellow carbonitride particles).When maximum detail in a colored phase must be shown,choose a filter with the same color as the phase.
11.5Filters for Color Photomicrography :
11.5.1Color photomicrography generally requires filtration to balance the light at the image plane to the color temperature specified by the film’s manufacturer.Most transparency and negative color films are balanced for use with daylight at 5600K.Some films are balanced for tungsten source lighting at either 3200K or 3400K.
11.5.2Color films record ultraviolet light as blue.Since different metals reflect varying amounts of ultraviolet light,the simplest solution is to remove all ultraviolet light,as in 11.3.1,and rebalance by adding compensatory blue filters.
11.5.3Table 1lists filter recommendations appropriate for color photomicrography.These include strong conversion fil-ters (the blue 80series and the orange-yellow 85series)and weaker light-balancing filters (the yellow 81series and the blue 82series).Because of individual variations in equipment and other filtration (for example,IR and UV removal),some fine tuning is usually required with color correction filters .These filters are commonly used in color printing,and are available in sets containing various strengths of red,yellow,green,cyan,blue,and magenta.
11.5.4The correct color balance for any color film can be determined using a first-surface mirror as the specimen (see Note 1).After the recommended filtration from Table 1has been inserted,a series of test exposures of the mirror is made with several color correction filters,until a neutral gray result is obtained.(Because of differences in manufacturing,different
films with the same color temperature ratings may
require slightly different groups of filters to achieve the correct color balance.)
N OTE 1—It is important to have a standard to balance the effective illumination of the system to photographic neutrality.Aluminum is photographically neutral throughout the visible and UV wavelengths.A first-surface aluminum mirror can be used as a repeatable standard.(A protective chromium overcoating destroys the neutrality,but a thin silicon monoxide protective layer is acceptable.)
12.Illumination Techniques
12.1Metallographic specimens should be illuminated to reveal significant structural details with optimum contrast and resolution,and with sufficient brightness for accurate photo-graphic recording.
12.2With bright field illumination,polished areas of the specimen that are perpendicular to the light path reflect incident vertical illumination back into the objective lens and appear bright (see 9.2and Fig.1).Fe
atures such as inclusions and etched grain boundaries have edges that are inclined to the polished surface and reflect light away from the objective lens,making them appear dark.
12.3Oblique illumination is similar to bright field,but is nonspecular,with the light impinging on the specimen at an oblique angle to the optical axis.It is obtained by decentering the aperture diaphragm,or by tilting the specimen slightly (4).The technique is useful to enhance specimen surface relief and to determine if specific features are pits or projections,since shadows are cast by nonplanar features.Resolution decreases as the illumination is made more oblique.(It is important that the decentered diaphragm be completely imaged in the rear focal plane of the objective to keep the illumination reasonably uniform across the field.)
12.4Dark field illumination is obtained by directing light to the specimen along the outside of the objective,blocking out the illumination passing through the lens.These outside rays are diverted onto the specimen plane obliquely by a conical reflector.No specular reflections enter the objective.Only features that are tilted with respect to the surface (for example,grain boundaries,pits,and inclusions)will reflect light into the objective.These features will appear bright against a dark background.Image contrast is higher in dark field illumination than in other modes and will frequently reveal specimen detail that would be completely obscured with other kinds of illumi-nation.
12.5Polarized light reveals grain structure and twinning in metals with a hexagonal lattice structure,such as beryllium,tin,titanium and zinc.Polarized illumination is produced by optical components consisting of calcite prisms or Polaroid y filters.They selectively provide an image consisting of polar-ized light reflected from a specimen surface and scattered depolarized light from nonplanar surface features.Polarized light reacts differently when reflected from isotropic and anisotropic material lattices.For a cubic material,the micro-scope field appears dark because most of the light reflected from the specimen is absorbed by the system.With an anisotropic material,the plane polarized beam reflected from the specimen surface either becomes elliptically polarized or the polarization plane is rotated.In both cases,the system now
TABLE 1Suggested Filtration for Color Photomicrography
Film Color Balance Daylight 3200K
3400K Light Source Wratten Filter Number
Tungsten
80A +82A 82A 82C Tungsten-halogen 80A None 82A Zirconium arc
editor evaluating revision80A None 82A Carbon arc,4.5amp 80C
81C 81A Carbon arc,10amp 82C +82C 81EF 81C Xenon arc
None
85B
85
passes a portion of the reflected light through to the viewing system.Polarized light is also used with optically inactive cubic metals that are treated to produce an anisotropic surface film on the substrate.It is also useful to identify optically active inclusions and phases and in defining domains in ferromagnetic materials.
12.6Sensitive Tint—Many metals and nonmetallic crystals are birefringent.Plane polarized light is reflected from them as elliptically polarized light,which has a component not extin-guished by the system.If a quartz or gypsum sensitive tintfilter (also known as a l compensator,a full-wave plate or a first-order compensator)is used,a magenta color is seen with cubic metals and all birefringent metals appear in vivid color contrasts.Nodular cast iron demonstrates this effect particu-larly well,if a rotatable
stage is used.
12.7Differential Interference Contrast—(DIC or Nomarski illumination)This illumination technique shows edges of discontinuities on specimens as variations in brightness.Color contrast can be added as an additional indication of level variation.The method is termed differential because very minor discontinuities are emphasized,whereas slightly angled slopes are displayed almost as if they were perfectly normal to the optical axis;for example,a cylindrical phase looksflat with fairly sharp edges.A modified Wollaston prism located at the rear focal plane of the objective splits the illumination beam into two parallel beams,separated in phase by one-quarter wavelength.Any alteration of the optical path by the specimen, by either path length(feature height)or refractive index, produces an interference pattern in the image beams.As the beams return through the DIC prism,they are reunited and the interference effect appears as a variation in brightness and color.Most microscopes allow translation of the DIC prism to produce different color displays as well.DIC has several unique advantages:Since the full back aperture is illuminated, the full resolution of the objective is utilized;the interference plane is very shallow,keeping out-of-plane detail from inter-fering;there is an oblique appearance as an additional clue to level differences.Useful applications of DIC are:judging adequacy of specimen preparation for automated microscopy, as in Practice E768;display of surface relief,including changes of a few nanometres at abrupt edges.
13.Instant-Processing Films
13.1These materials yield photographic images within seconds after exposure(8).Both color and black-and-white versions are available.All use variations of the diffusion-transfer process,with each frame developed individually after exposure.
13.1.1Instant materials should be exposed so that the lightest(white)tone in the image is reproduced as the brightest tone in the picture.(Shorter exposures will produce muddy whites,while longer ones will give insufficient separation between the lighter tones.)
13.1.2The majority of the instant materials are of peel-apart construction,where the positive print is detached from the processing packet after development and the rest of the unit is discarded.A useful variation of this provides a transparent negative as well,for multiple print production by wet-process darkroom methods.Excellent photographic prints can be made from the negative instantfilms with great degrees of
enlarge-ment possible.(In order to optimize the exposure of the negative with a positive/negativefilm,the positive will be overexposed,and therefore not considered an acceptable print.)
13.1.3The monopack materials,both black-and-white and color,require adapters that are unlike those normallyfitted to metallographic equipment.No processing control is possible. With some exceptions,monopack prints should not be cut.The use of the more adaptable peel-apart materials may be the better choice for metallography.
13.2Processing Instant Films—All instant materials,both peel-apart and monopack,are processed by pulling the picture unit through an accurately-spaced roller pair.A chemical pod at the leading edge is ruptured and the contents spread throughout the unit by the rolls,starting the development action.The reaction goes to completion according to the time-temperature relationship supplied with thefilm.
13.2.1The picture unit must be pulled in a straight line through the rollers,at a constant speed,to secure uniform processing.A pull-time of1⁄4to1⁄2second is suggested for peel-apart materials.Monopack instant materials permit no control over processing time,since the self-developing reaction is controlled by thefilm holder and proceeds to completion without attention from the user.
13.2.2The roller pair should be kept clean,since evenfine debris on the rolls will cause uneven reagent spreading and picture nonuniformity.
13.2.3Black-and-white peel-apart materials are best pro-cessed for at least the recommended time for the type involved. Extended processing up to three minutes is permissible.Be-yond this,problems may be encountered as the units are peeled.
13.2.4Improved contrast and color saturation can be achieved with color peel-apart materials by processing to a two-minute standard,rather than the recommended one minute. The color balance will shift toward cyan,which can be corrected by adding some red to thefilter pack.
14.Photographic Materials
14.1Wet-Process Materials:General and Black-and-White—Conventional photographic materials provide an al-most unlimited choice of conditions for recording an image. Many of the readily available products can be usefully em-ployed in metallography.References(4-6)are recommended reading to learn the complete photographic characteristics of the products,as well as the terminology used to describe them.
14.2The essential construction of a photographic material consists of a carrier base with a light-sensitive layer of silver halides in gelatine,commonly called the emulsion.Negative and projectable emulsions are on transparentflexible acetate or polyesterfilm bases,while reflection print materials have white paper or paper/plastic composite bases.
14.3The most common materials are negative-acting,that is,exposure to light and subsequent chemical processing displays an image on afilm wherein the tonal values of the original scene(microscopicalfield)are reversed.This is subsequently printed by light exposure through the negative
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