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Köhler illumination to reduce reflections

Found in: Photography, Techniques

Köhler illumination reduces stray light and therefore increases contrast when taking pictures.

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Field diaphragm is wide open. Reflections from the side of the tube are very strong.

Field diaphragm is half open. The reflections are less.

Field diaphragm is half open. The reflections are less.

Field diaphragm is closed. Only the direct light is able to reach the camera.

Field diaphragm is closed. Only the direct light is able to reach the camera.

The Köhler (or Koehler or Kohler) field diaphragm is located above the light source. It is responsible for controlling the width of the light beam (but not its intensity). The light source of a microscope without Köhler illumination will illuminate the whole specimen, which may be the source of stray light and excessive heating of the specimen. By closing the field diaphragm, it is possible to limit the beam of light only to the part of the specimen which is actually observed.

Advantages of Köhler illumination for photography

Köhler illumination increases the contrast of a photomicrograph because it reduces stray light and glare caused by reflections inside the microscope. On the right side you can see images taken through a trinocular head with a web cam. The more that the field diaphragm is closed, the less the reflections coming from the side of the tube. The bright spot in the center is the light which comes directly (unreflected) from the light source. In order to see a picture, it would be necessary to remove the lens from the webcam and project the image directly on the sensor of the webcam. In this case, the lens was left on to be able to see the side of the tube.

For more background info on Köhler illumination, you may be interested in the following two posts:

Taking a picture of the tube with a webcam. Any camera with a small lens would also have done the job.

Taking a picture of the tube with a webcam. Any camera with a small lens would also have done the job.

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