Köhler illumination

Köhler illumination arranges a microscope's illumination optics so that the light source is imaged at the condenser aperture and the field diaphragm is imaged at the specimen plane. These distinct conjugate planes let the source illuminate the specimen without its structure being sharply projected onto it. Proper adjustment produces a broadly even field and helps limit illumination artifacts and glare.

The field diaphragm limits the illuminated area; closing it to just beyond the viewed field helps reduce stray light. The condenser's aperture diaphragm controls the angle of the illumination cone and therefore influences the effective numerical aperture of the imaging system. Opening it generally supports greater resolution and light collection, while closing it can increase contrast but reduce resolution. The best setting depends on the specimen and imaging purpose.

A typical transmitted-light setup begins with the specimen in focus and the field diaphragm partly closed. The condenser is raised or lowered until the diaphragm edge is sharply focused, then centered using its adjustment controls. The field diaphragm is opened until its edge is just outside the field of view. Finally, the condenser aperture is adjusted to suit the desired balance of contrast and resolution. The exact controls vary among microscope models.

The method is named after German microscopist August Köhler, who described the illumination approach in an 1893 publication. It became a standard arrangement in modern light microscopy because it supports consistent specimen illumination and makes practical use of separate field and aperture adjustments.