Properly Setting up Koehler Illumination

You have heard of the term Koehler illumination. What is it and what does it do for the microscope?

Koehler Illumination is a process which provides optimum contrast and resolution by focusing and centering the light path and spreading the light evenly over the field of view. All microscopes do not have the ability to be setup for Koehler. They must have a field diaphragm and controls to adjust the substage condenser to complete the alignment.

Using the following setup, Koehler Illumination can greatly compliment the operation of your microscope by improving the specimen contrast.

a. Rotate to the 10X objective.
b. Open the condenser aperture diaphragm by turning the control to the right.
c. Raise the condenser with the condenser focus knob.
d. Close the field diaphragm to it's smallest opening with the field diaphragm control.
e. Focus the field diaphragm image with the condenser focus knob until the edges on the octagonal images are sharp.
f. Center the diaphragm image with the condenser centering screws. This centers the light path onto the specimen plane.

g. Switch to desired objective.
h. Enlarge the field diaphragm image toward edge of the field of view with the field diaphragm control.
i. Recenter the field diaphragm image.
j. Further enlarge the field diaphragm image until it is just outside the field of view. This provides enough light to fully and evenly cover the observation area.
k. Remove the right eyepiece and look down the eyepiece tube. The circular beam of light is the image of the condenser.
l. Adjust the condenser diaphragm with the condenser diaphragm control until the beam fills 3/4 of the tube (1/4 of the area will be dark and 3/4 will be light).
m. Replace the eyepiece.
n. Adjust the light intensity with the brightness control.

Setting up Koehler Illumination on your microscope will help provide the best image in an illuminated light microscope like the Meiji MT4200H or the VanGuard 1421BRI.

National Microscope Exchange has been in business since 1991, selling and servicing abbe refractometers, refractometers, inverted microscope and microscopes. The service staff has 30 years of experience with microscopes, and is the authorized United States service facility for Atago brand refractometers.