Diascopic illumination

One of the most critical aspects of observation using any optical microscope is the specimen illumination. Two illumination systems are commonly used in optical microscopy transmitted light and reflected light (Fig. 2). Transmitted, also called diascopic illumination, requires the specimen to be transparent. It is used primarily to examine thin sections of biological or material samples. Reflected light, or episcopic illumination (epi-illumination), is most commonly used for fluorescence microscopy, where fluorphores inside the specimen are excited to produce fluorescent light. The fluorescence is then emitted, or reflected back to the objective and collected by the detector (eyes, or camera). The reflected light is also used to study the surfaces of opaque specimens, which is the focus of this session. 

Fig. 4. Flow diagram illustrating the application of image analysis to IGSS using a conventional diascopic white light/episcopic polarized light double-illumination light microscope system. Adapted from ref. 28 with permission.

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