Köhler Illumination
Köhler Illumination provides uniform illumination across the field of view of a microscope. (Historical note: Köhler illumination is named for August Köhler, b. 1866, who invented this optical configuration.)
Explanation of optics for Köhler illumination, from "Optical Microscopy Primer."
This website is an excellent resource for understanding microscopes and their history.Simplified explanation of optics for Köhler illumination, from Wikipedia.
(The following instructions apply to common bright-field microscopes, including the multiview-microscopes in Lindegren Room 206. Condenser adjustment may be simplified on student microscopes.)
Initial set up: Turn on the lamp, place a slide on the stage, focus on the specimen, adjust interocular distance to match that of your own eyes.
(Quick shortcut.)
1. Focus the condenser. (Why? And what is a condenser?)
Close the field diaphragm located on the base of the microscope. This will reduce the illuminated field of view, as seen through the eyepieces, to a small circle of light. Then adjust the substage condenser up or down until the edge of this circle appears sharp.
2. Center the condenser. (Why?)
Open the field diaphragm until the illuminated circle almost fills the viewing area. Then adjust the screws at either side of the condenser until this circle is precisely centered in your field of view. Now open the field diaphragm completely to restore illumination over the entire field of view.
3. Adjust the condenser diaphragm. (Why?)
Remove one eyepiece and look into the open tube. You should see an illuminated field of view whose size is controlled both by the objective lens aperture and by the diaphragm within the condenser. Adjust the condenser diaphragm until its edge just barely encroaches on the illuminated field of view. (This matches the width of the illuminating beam to the diameter of the objective lens aperture.) Then replace the eyepiece.
This adjustment should be repeated whenever the objective lens is changed (i.e., whenever magnification is changed).
4. Adjust illuminator brightness.
Use the voltage regulator on the base of the microscope to adjust image brightness to suit your eyes. (Why not just use the condenser diaphragm?)
5. Whenever you change objectives, repeat steps 3 and 4 .
Congratulations. Your microscope should now be adjusted for proper Köhler illumination.
Quick shortcut.
Condenser.
The condenser is the package of optics located directly beneath the microscope stage. (The stage is the platform which holds the slide.) The condenser "condenses" light into a parallel beam that then uniformly illuminates the specimen.
Explanation of optics for Köhler illumination, from "Optical Microscopy Primer."
This website is an excellent resource for understanding microscopes and their history.Simplified explanation of optics for Köhler illumination, from Wikipedia.
If the condenser is not properly adjusted, image detail can be obscured by false contrast or by diffuse glare. Diffraction can produce artefactual optical effects.
The condenser has its own focussing knob (usually located beneath stage) and an adjustable diaphragm used to control the diameter of the light beam, matching the illuminating beam to the aperture of the objective lens.
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Centering the condenser.
Centering screws position the condenser directly under the objective lens. If the condenser is not properly centered, specimen details will appear to move from side to side during focussing.
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Adjusting the condenser diaphragm.
The condenser diaphragm controls the size of the beam of light entering the objective lens. If the beam entering the objective is broader than the objective is designed for, the extra light (glare) can cloud the image. On the other hand, if the beam is too small, the resolving power of the objective will be reduced (in effect, turning a fine expensive objective into cheap low performance one).
The objective lens is the primary lens of the microscope, the one immediately above the specimen. Most microscopes have several objective lenses, of varying power. You choose magnification by swivelling the nosepiece to choose an objective. Ideally, the condenser diaphragm should be adjusted after each change of objective.
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INAPPROPRIATE condenser use.
Do NOT use the condenser diaphragm to adjust brightness.
Although closing down the condenser diaphragm can indeed dim the image, this action also reduces resolution and introduces false contrast. (Ideally, brightness is adjusted with neutral filters near the light source.)
Shortcut to adequate adjustment.
For a quick adjustment (usually good enough for casual looking, but not for photography):
- Adjust the condenser upward as close to your specimen as it will go.
- Open the condenser diaphragm all the way, then close it slowly until the view shows the first hint of dimming.
This will not be perfect, but it's a lot better than no adjustment at all.
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Comments and questions: dgking@siu.edu
SIUC / School
of Medicine / Anatomy / David
King
https://histology.siu.edu/intro/Köhler.htm
Last updated: 4 June 2022 2021 / dgk