Köhler Illumination

  1. Focus on the specimen. This step is most important.
  2. Decrease the size of the diaphragm located nearest the light source (field iris) so that you can see its edges.
  3. Bring the edges of the field iris into focus by raising or lowering the condenser focusing knob. Both the specimen and the iris should be in focus.
  4. Center the image of the field iris using the condenser-centering knobs (usually facing you on the condenser)
  5. Open the centered and focused field iris so the edges lie just beyond the field of view.
  6. Adjust the condenser iris to increase or decrease image contrast. The optimum opening depends on the specimen. Never use this aperture to control light intensity.
  7. Adjust light intensity with the light power supply or with neutral density filters (for color photography).

In a correctly adjusted Köhler illuminated microscope specimen contrast is obtained by adjusting the condenser diaphragm. Illumination intensity is varied by adjusting the voltage to the light source or by placing neutral density filters in front of the illuminator.