Brightfield (Köhler) illumination

Brightfield microscopy uses transmitted light to view a specimen that contains inherent contrast/colour or is stained. In order to get the best image possible from brightfield, phase contrast, DIC, or polarisation optical set-ups with the light microscope, it is crucial that the light path be set up properly. The method for doing this is called Köhler illumination after August Köhler; the man who invented it. It is also know as double diaphragm illumination because it employs both a field and an aperture iris diaphragm to set up the illumination. The condenser is used to focus parallel rays of light on the specimen, as if coming from infinity, thereby giving you the advantages of an evenly illuminated field, a bright image without glare and minimum heating of the specimen.