Metallography is the science and art of preparing a metal surface for analysis by grinding, polishing, and etching to reveal microstructual constituents. A skilled technician is able to identify alloys and predict material properties, as well as processing conditions by metallography alone. Metallographic specimens are typically mounted using a hot compression thermosetting resin, such as a phenolic resin, or with a cast resin, such as an epoxy resin. After mounting, the specimen is wet ground to reveal the surface of the metal. After grinding the specimen, polishing is performed. After polishing, certain micro structural constituents can be seen with the microscope, e.g., inclusions and nitrides.
Otherwise, the micro structural using a suitable reveals constituents of the specimen chemical or electrolytic etch ant. While a number of etch ants may work for a given metal or alloy, they generally produce different results, in that some etch ants may reveal the general structure, while others may be selective to certain phases or constituents. Prepared specimens should be examined after etching with the unaided eye to detect any visible areas that respond differently to the etch ant as a guide to where the microscopical examination should be employed.
Further, certain features can be best observed with the LOM, e.g., the natural color of a constituent can be seen with the LOM but not with EM systems. LOM examination is fast and can cover a large area. Light microscopes are designed with either specimen placement of the polished surface on the stage upright or inverted. Most LOM work is done at magnifications between 50 and 1000X. However, the resolution limit of the LOM will not be better than about 0.2 to 0.3 micrometers. Besides considering the resolution of the optics, one must also maximize visibility by maximizing image contrast.

Image contrast depends upon the quality of the optics, coatings on the lenses, and reduction of flare and glare; but it also requires proper specimen preparation and good etching techniques. Most texts concentrate on resolution and ignore the importance of contrast required for visibility. Most LOM observations are conducted using bright field illumination where the image of any flat feature perpendicular to the incident light path is bright, or appears to be white.
In dark field, the light from features perpendicular to the optical axis is blocked and appears dark while the light from features inclined to the surface, that look dark in BF, appear bright, or self luminous in DF. If the specimen is prepared with minimal damage remaining at the surface, the structure can be seen vividly in crossed polarized light. In some cases, an hcp metal can be chemically etched and then examined more effectively with PL. If it is difficult to get a good interference film with good coloration, the colors can be improved by examination in PL using a sensitive tint filter.
DIC converts minor height differences on the plane-of-polish, invisible in BF, into visible detail. If an ST filter is used along with the Wollaston prism, color is introduced. But, visibility may be better. In OI, the vertical illuminator is offset from perpendicular producing shading effects that reveal height differences. Nevertheless, OI was useful when people needed to know if a second phase particle was standing above or was recessed below the plane-of-polish, and is still available on a few microscopes.