Koehler Lighting in Microscopes - What is it and is it necessary?

Q: What is Koehler Illumination? I keep hearing this term in the reference to compound microscopes.

A: Most modern compound microscopes prior to 2011 use halogen or tungsten lamps as a source of illumination in the base of the microscope. That lamp contains a tiny wire inside used to complete the electrical circuit and illuminate the lamp. That wire is referred to as a "filament". That thin wire can sometimes be seen when you look through your microscope.
Microscopes that use LED illumination do not need Koehler illumination as there is no filament in the light source. An example of this is the Nikon 55i Microscope

Koehler Illumination is a way to solve that problem by evenly distributing the light throughout the field of view. This is done by focusing and centering the filament in the back of the objective focal plane and the front of the condenser focal plane. By keeping the filament focused at the meeting of those two planes, the fllament becomes "defocused" in the plane of the image specimen.

Most high quality microscopes like Nikon come with precentered lamps as in the Nikon E 200, thus making Koehler illumination unnecessary. The use of a field diaphragm in the Olympus allows for Koehler illumination.