Optical microscopes have a wide variety of applications; they are very powerful tools for inspecting the microstructure of a great range of materials. It is important to use the appropriate mode for the specimen, choosing from reflected-light or transmitted-light modes.
Reflected-light microscopy is used for a range of materials, including metals, ceramics and composites. Contrast between different regions when viewed in reflected light can arise from variations in surface topography and differences in reflectivity (e.g. of different phases, different grain orientations, or boundary regions). These features are revealed by a series of specimen preparation techniques which, when carried out with care, can produce useful, high quality images.
Transmission mode can be used when the specimen is transparent. The specimen is usually in the form of a thin slice (e.g. tens of microns thick). Contrast arises from differences in the absorption of light through different regions. This method is used for the examination of minerals and rocks, as well as glasses, ceramics and polymers. In addition, the transmission mode can often be further enhanced with use of polarised light.
Polarised light microscopy is a specialised use of the transmission mode, and contrast is due to differences in birefringence and thickness of the specimen. This can allow the observation of grains, grain orientation and thickness.