Optical paths for condensers yielding Critical and Köhler Illumination
Critical Illumination. Conjugate planes are the illuminating bulb filament and sample plane (O). When adjusted correctly, the image of the filament is seen coincident with the sample image. A diffusing glass filter (d) is used to blur the filament image.

FD: Field diaphragm
CD: Condenser diaphragm

Köhler Illuminating: Conjugate planes are the illuminating bulb filament and Condenser diaphragm. Second conjugate planes are the Field diaphragm and the sample plane. When adjusted correctly, the image of the field diaphragm and the sample are coincident. The filament is out of the plane of focus, and thus uniformly diffuse.