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Köhler illumination is a microscope alignment technique , most often used for transmitted light, that ensures images are evenly illuminated and at maximum resolution. The microscope should be adjusted for Köhler illumination before transmitted light imaging, phase contrast and differential interference contrast (DIC).
Before adjusting for Köhler illumination you need to know where the relevant condenser components are on the microscope. This varies between manufacturers and models. The image below gives some examples of where to find the field diaphragm, iris aperture, etc. If in doubt, please as ask a member of light microscopy staff.
Condenser adjustment for Köhler illumination
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- Focus on a transmitted light image of the specimen using a 10x air lens
- Rotate the condenser turret to the bright field position. Adjustment can either be manual or done through buttons or a touch pad, depending on the microscope model
- Fully close the field diaphragm and adjust the condenser focus so the image of the iris field diaphragm is in sharp focus
- Centre the image of the iris field diaphragm using the two centring screws
- Open the field diaphragm until it is just outside the field of view
- Remove an eyepiece and look into the empty opening
- Adjust the iris aperture until it is about 70% open and then replace the eyepiece. The microscope is now adjusted for Köhler illumination
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