Köhler illumination is a method of specimen illumination used for transmitted and reflected light (trans- and epi-illuminated) optical microscopy. Köhler illumination acts to generate an extremely even illumination of the sample and ensures that an image of the illumination source (for example a halogen lamp filament) is not visible in the resulting image. Köhler illumination is the predominant technique for sample illumination in modern scientific light microscopy although it requires additional optics which less expensive and simpler light microscopes may not have.
The primary advantage of Köhler illumination is the extremely even illumination of the sample. This reduces image artifacts and provides high sample contrast. Even illumination of the sample is also critical for advanced illumination techniques such as phase contrast and differential interference contrast microscopy.
By adjustment of the field diaphragm the amount of light entering the sample can be freely adjusted without altering the wavelengths of light present, in contrast to reducing power to the light source with critical illumination. Adjusting the condenser diaphragm alters sample contrast. Furthermore altering the size of the condenser diaphragm allows adjustment of sample depth of field by altering the effective numerical aperture of the microscope. The role of the condenser diaphragm is analogous to the aperture in photography although the condenser diaphragm of a microscope functions by controlling illumination of the specimen, while the aperture of a camera functions by controlling illumination of the detector.
Check out the Köhler illumination setup instructions at this link: http://microscopy.berkeley.edu/courses/TLM/condenser/kohler.html
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