Light microscopy – reflectance

About this technique

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Reflected light microscopy, sometimes referred to as incident light, episcopic illumination, or metallurgical microscopy, and is used for the examination of opaque specimens. It can be used to study the detailed structure of a vast array of samples from the biological to the metallugical. Metals, ores, ceramics, many polymers, semiconductors (unprocessed silicon, wafers, and integrated circuits), slag, coal, plastics, paint, paper, wood, leather, glass inclusions, and a wide variety of specialised materials can all be studied using this technique.

Reflected light microscopy uses top and down lighting to illuminate the specimen and the light is reflected back from the specimen to the viewer. The light is directed on the surface and eventually returned to the microscope objective by diffused reflection. The geometrical profile, topography, and morphology of the specimen are all important factors to consider in the choice and configuration of illumination so that the desired information will be revealed. The sample must be flat because of the low depth of field of reflected light microscopy. Samples are either cut to size or mounted in a resin mould. The samples need to be polished (with rotating discs) to a fine finish, normally one micrometre diamond paste or better.

For many samples, etching is also performed to differentially reveal various structural elements within the material such as the crystals within a metal. Common etching techniques include chemical, electrolytic, thermal, plasma, molten slat or magnetic. The two most common techniques are chemical and electrochemical etching. Chemical etching is typically a combination of either an acid or base with an oxidising or reducing agent in a solute such as an alcohol. This is quicker and easier than electrochemical etching but there is considerably less control over the outcome. The combination of chemical etching with a biased voltage provides greater flexibility in the outcome.