When using transmitted illumination with an optical microscope equipped with a condenser lens, there are three types of lighting: Koehler illumination, diffuse illumination, and critical illumination.

Koehler illumination

Gathers light to the backside of the objective lens. This is the most commonly used method for transmitted illumination and features bright and less-varied illumination. This method is indispensable for observation at high magnification.

Diffuse illumination

A light diffusing plate placed on the reflector enables uniform illumination, but passing through the plate makes the light slightly less bright.

Critical illumination

Gathers light at the surface of a sample. This methods generates bright illumination, but has a higher chance of being unevenly distributed.