THE OPTICAL (LIGHT) MICROSCOPE It is the most important tool to study of microstructure, despite the fact that more sophisticated electron metallographic instruments, such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), have been evolved. Electron microscopy should be used in conjunction with optical microscopy, rather than as a substitute. All examinations of microstructure should begin with use of the optical microscope, starting at low magnification, such as 100×, followed by progressively higher magnifications to assess the basic characteristics of the microstructure efficiently. Most microstructures can be observed with the optical microscope and identified based on their characteristics. Identification of questionable or unknown constituents may be aided by observation of their hardness relative to the matrix, by their natural colour, by their response to polarized light, and by their response to selective etchants. These observations are compared to known details about the physical metallurgy of the material being examined. If doubt still remains or if the structure is too fine to observe, more sophisticated techniques must be implemented. Polished or etched metallographic specimens are examined by optical microscope. Some constituents such as inclusions, nitrides, certain carbides, and intermetallic phases can be readily observed without etching. Specimens that respond to polarized light, such as materials with non-cubic crystal structures, are also examined without etching. Other phases may be more easily examined if some relief is introduced during final polishing. The specimen must be adequately prepared free from artifacts for correct observation Etching must be performed carefully to observe the microstructure. A general-purpose etchant is normally used first to reveal the grain structure and the phases present, followed by selective etchants that attack or colour specific phases of interest. Selective etchants are widely used for quantitative metallography, particularly if performed using an automated device. Microscope Components Reflected light is used for the study of metals. Transmittedlight microscopes are used to study minerals and polymers, which can also be examined using reflected light. Optical microscopes are also classified as "upright" or "inverted"; these terms refer to the orientation of the plane of polish of the specimen during observation. Figure 1 illustrates the light path in the two designs. The simplest optical microscope is the bench type (usually upright). Photographic capabilities can be added to some units depending on the rigidity of the stand. Figure 2 illustrates basic bench microscopes, and Figure 3 shows research-quality bench microscopes suitable for photographic work. Fig. 1 Light paths in (a) an upright incident-light microscope and (b) an inverted incident-light microscope. (E. Leitz, Inc.; C. Zeiss, Inc.) Fig. 2 (a) Upright bench microscope. (b) Inverted bench microscope Fig. 3 Research-quality optical microscopes. (a) Upright. (b) Inverted. (E. Leitz, Inc.; Unitron Instruments, Inc.) Fig. 4 Moderately priced inverted metallograph. The small box to the right is an automatic exposure control. (Nikon, Inc.) Fig. 5 Research-quality metallograph with a projection screen for group viewing. (E. Leitz, Inc.) Illumination System. A variety of light sources for optical microscopy are available. The low-voltage tungsten filament lamp used primarily with bench microscopes has adequate intensity for observation, but not for photography. Altering the current to the bulb controls light intensity. Carbon-arc illumination systems, once common on metallographs, have been replaced by arc or filament light sources. The xenon-arc light source is prevalent because of its high intensity and daylight colour characteristics. Light intensity, however, can be adjusted only by the use of neutral-density filters. Tungsten-halogen filament lamps are also widely used for their high intensity and high colour temperature. Light intensity can be controlled by varying the current or by use of neutral-density filters. Other light sources, such as the zirconium-arc, sodium-arc, quartziodine, or mercury-vapor lamps, are less common. Condenser. An adjustable lens free of spherical aberration and coma is placed in front of the light source to focus the light at the desired point in the optical path. A field diaphragm is placed in front of this lens to minimize internal glare and reflections within the microscope. The field diaphragm is stopped down to the edge of the field of view. A second adjustable-iris diaphragm, the aperture diaphragm, is placed in the light path before the vertical illuminator. Opening or closing this diaphragm alters the amount of light and the angle of the cone of light entering the objective lens. The optimum setting for this aperture varies with each objective lens and is a compromise among image contrast, sharpness, and depth of field. Opening this aperture increases image sharpness, but reduces contrast; closing the aperture increases contrast, but impairs image sharpness. The aperture diaphragm should not be used for reducing light intensity. It should be adjusted only for contrast and sharpness. Light filters They are used to modify the light for ease of observation, for improved photomicroscopy, or to alter contrast. A green or yellow-green filter is widely used in black-and-white photography to reduce the effect of lens defects on image quality. Most objectives, particularly the lower cost achromats, require such filtering for best results. Polarizing filters are used to produce plane-polarized light (one filter) or crossed-polarized light (two filters rotated to produce extinction) for examination of noncubic (crystallographic) materials. Materials that are optically anisotropic, such as beryllium, zirconium, α-titanium, and uranium, can be examined in the crossed-polarized condition without etching. A sensitivetint plate may also be used with crossed-polarized light to enhance coloration. The objective lens It is used to form the primary image of the microstructure and is the most important component of the optical microscope. The objective lens collects as much light as possible from the specimen and combines this light to produce the image. The numerical aperture (NA) of the objective, a measure of the light-collecting ability of the lens, is defined as: NA = n sin α (Eq 1) where n is the minimum refraction index of the material (air or oil) between the specimen and the lens, and α is the half angle of the most oblique light rays that enter the front lens of the objective. Light-collecting ability increases with α. The setting of the aperture diaphragm will alter the NA of the condenser and therefore the NA of the system. The most commonly used objective is the achromat, which is corrected spherically for one color (usually yellow-green) and for longitudinal chromatic aberration for two colors (usually red and green). Therefore, achromats are not suitable for color photomicroscopy. Use of a yellow-green filter and orthochromatic film yields optimum results. However, achromats do provide a relatively long working distance, that is, the distance from the front lens of the objective to the specimen surface. Working distance decreases as magnification of the objective increases. Most manufacturers make long-workingdistance objectives for special applications, for example, in hot-stage microscopy. Achromats are strain free, which is important for polarized light examinations. Because they contain fewer lenses than other more highly corrected lenses, internal reflection losses are minimized. Semiapochromatic or fluorite objectives provide a higher degree of correction of spherical and chromatic aberration. Therefore, they produce higher quality images than achromats. The apochromatic objectives have the highest degree of correction, produce the best results, and are more expensive. Plano objectives have extensive correction for flatness of field, which reduces eyestrain, and are often found on modern microscopes. Figure 6 illustrates three plano-type objectives. Each is coded as to the type of objective, its magnification, and numerical aperture. With parfocal lens systems, each objective on the nosepiece turret will be nearly in focus when the turret is rotated, preventing the objective front lens from striking the specimen when lenses are switched. Fig. 6 Plano-type objective lenses and cross sections through each. The lens shown in (c) is a 14-element oilimmersion objective, with a numerical aperture (NA) of 1.32. Because the lens and specimen must be cleaned between each use, oil immersion is rarely used; it does provide higher resolution and a crisper image, which is valuable for examining low-reflectivity specimens. (E. Leitz, Inc.) The eyepiece (ocular), It magnifies the primary image produced by the objective; the eye can then use the full resolution capability of the objective. The microscope produces a virtual image of the specimen at the point of most distinct vision, generally 250 mm (10 in.) from the eye. The eyepiece magnifies this image, permitting achievement of useful magnifications. The standard eyepiece has a 24-mm-diam field of view; wide-field eyepieces for plano objectives have a 30-mm-diam field of view (Fig. 7), which increases the usable area of the primary image. The simplest eyepiece is the Huygenian, which is satisfactory for use with low- and medium-power achromat objectives. Compensating eyepieces are used with high NA achromat and the more highly corrected objectives. Because some lens corrections are performed using these eyepieces, the eyepiece must be matched with the type of objective used. Eye clearance is the distance between the eye lens of the ocular and the eye. For most eyepieces, the eye clearance is 10 mm or less--inadequate if the microscopist wears glasses. Simple vision problems, such as nearsightedness, can be accommodated using the fine focus adjustment. Vision problems such as astigmatism cannot be corrected by the microscope, and glasses must be worn. High-eyepoint eyepieces are available to provide the eye clearance of approximately 20 mm necessary for glasses (Fig. 8). Fig. 7 Cross sections of typical eyepieces. (a) Standard (24-mm) field of view. (b) Wide (30-mm) field of view. The widefield eyepiece increases the usable area of the primary image. (E. Leitz, Inc.) Fig. 8 Comparison between the position of the eye with (a) a standard eyepiece and (b) a high-point eyepiece. Eye clearance with a standard eyepiece is approximately 10 mm (0.4 in.); a high-point eyepiece allows clearances of approximately 20 mm (0.8 in.) (E. Leitz, Inc.) Eyepieces are commonly equipped with various reticles or graticules for locating, measuring, counting, or comparing microstructures. The eyepiece enlarges the reticle or graticule image and the primary image.