Köhler illumination is a method of specimen illumination acts to generate an extremely even illumination of the sample and ensures that an image of the illumination source (for example a halogen lamp filament) is not visible in the resulting image.
Köhler illumination is the predominant technique for sample illumination in modern scientific light microscopy although it requires additional optics which less-expensive and simpler light microscopes may not have.
How to adjust Köhler illumination in your microscope:-
Stage 1: adjusting contrast and focusing the image
Step 1: remove the eyepiece and move the iris diaphragm (condenser diaphragm) until ntil the entering light almost coincides with the edges of the field (tube) (i.e. the light octagon is about 90% wide)
Step 2: Open the filed diagram all the way
step 3: raise the sub-stage condenser as high as possible by the sub-stage condenser focusing knob
step 4: carefully place the slide on the stage (take care not to damage any of the lenses) and if necessary move the stage so that the specimen is in the light pass
step 5: move the stage up to it’s highest point by the coarse focus but be careful don’t make the slide touch the objective lens
step 6: use the coarse and fine focuses to focus the image.
Stage 2: adjusting Köhler illumination
step 7 : close the filed diaphragm as far as possible
step 8 : using the sub-stage condenser centering screws to center the octagon of the light.
step 9 : using the sub-stage condenser focusing knob to lower the condenser until the sides of the octagon as clearly in focus as possible.
step 10 : open the field diaphragm until the dark area is just out of the view.
Watch this video to learn more