Kohler illumination was developed in 1893 by August Koehler. Koehler illumiation is the optimal illumination of a specimen by creating homogeneous light without the light source visible in the field of view. Koehler illumiation helps reduces the amount of stray light.
In order to center for koehler illumiation your microscope must be equipped with a field diaphragm. Many new LED microscopes do not have a field diaphragm because centering for koehler is not necessary with an LED light source.
To center for kohler illumination follow the steps below:
1. Place your specimen on the microscope and focus your specimen under the 10x objective.
2. Close your field diaphragm. When closed you will see a small illuminated area.
3. Focus your condenser so that the edge of this illumninated area is in focus, you are actually focusing on the blades of the diaphragm.
4. You will now need to adjust the condenser position, in order to do so you may need to locate your centering tools if the condenser rack does not have two centering thumbscrews . The image below shows the two centering thumbscrews.
5. Rotate the condenser centering thumbscrews so that the illuminated area moves to the center of the field of view.
6. Gradually open the field diaphragm, as you do the edges of the diaphragm should be just at the edge of the field of view.
You have now centered your microscope for koehler illumination.