Koehler Illumination Procedure
Since then, manufacturers have outfitted the modern microscope with the necessary hardware for Koehler Illumination: A so-called collector lens system, a field stop iris diaphragm and condensers housing an aperture stop diaphragm. Usually, the collector lens system is built into the base of the microscope. The collector lens system will project an enlarged and focused image of the light source (e.g. halogen lamp filament, LED-array) into the plane of the aperture diaphragm of a properly height positioned condenser. The correct height of the condenser is achieved when the image of the field stop appears with maximum sharpness in the focused object plane. By doing so, the light source can never become focused into the specimen plane (instead the light source image is projected into the objective’s back focal plane). As a result, the focused object plane is illuminated very homogenously. The proper adjustment of the field stop diameter will reduce the amount of contrast- diminishing straylight. Closing or opening the condenser aperture diaphragm controls the angle of the light rays emerging from the condenser. By doing so, the optimum ratio between maximum resolution and strongest contrast can be individually set.
Now, this interactive tutorial explores how to establish Koehler illumination on a transmitted light microscope.