Optical Microscopy
Authors: Connell Richard G. 
Published: October 2003

Among the numerous investigative techniques used to study materials, optical microscopy, with its several diverse variations, is important to the researcher and/or materials engineer for obtaining information concerning the structural state of a material. In the field of metallurgy, light optical metallography is the most widely used investigative tool, and to a lesser degree its methods may be extended to the investigation of nonmetallic materials. The state of microstructure of a metal or alloy, or other engineering material, whether ceramic, polymer or composite, is related directly to its physical, chemical, and mechanical properties as they are influenced by processing and/or service environment. To obtain qualitative, and in many cases quantitative information about the microstructural state of a material, it is often necessary to perform a sequence of specimen‐preparation operations, which will reveal the microstructure for observation.

Optical microscopy as related to the study of engineering materials can be divided into the categories of
reflected‐light
microscopy
for opaque specimens and
transmitted‐light
microscopy
for transparent specimens. On the whole, reflected light microscopy is of more utility to materials scientists and engineers because the vast bulk of engineering materials are opaque. Within each of the categories, there are a number of illumination variations (
bright
field
,
dark
field
,
polarized
light
,
sensitive
tint
,
phase
contrast
, and
differential
interference
contrast
), each providing a different or enhanced observation of microstructural features. It is fortunate that a majority of illumination variations of reflected‐light microscopy parallel those of transmitted‐light microscopy.

In general, one does not set out to employ optical microscopy to measure a material property or set of properties. Information gained using optical microscopy is more often than not of a qualitative nature, which, to a skilled observer, translates to a set of correspondences among a material's physical, mechanical, chemical properties as influenced by chemical composition, processing and service history. One can use the measurement methods of quantitative microscopy to follow the progress of microstructural evolution during processing or during the course of exposure to a service environment. Such techniques may be used to follow grain growth, the coarsening of precipitates during a process, or the evolution of microstructure that would lead to catastrophic failure under service conditions.

The light‐optical microscope is the instrument of choice for a majority of studies. The reasons are as follows: (1) instrument cost is relatively low, (2) less technician training is required, (3) specimen preparation is relatively straightforward, and (4) the amount of information that can be obtained by an experienced microscopist is large. Because the maximum resolution is limited by the wavelength of light, useful magnification is limited to ∼1600 times. At these upper limits of resolution and magnification, microstructural features as small as ∼325 nm may be observed. Most of the other instruments are better suited to much higher magnifications, but the area of specimen surface is severely limited, so sampling of the microstructure state is better using optical microscopy.