Question

How does Kohler Illumination affect resolution – be specific in your answer?

(hint: do NOT simply describe how to do Kohler Illumination, remember what Kohler illumination does for your ability to resolve your specimen).

Answer 1

Kohler illumination acts to generate an even illumination of the sample and ensures that an image of the illumination source is invisible in the resulting image. Kohler illumination is the predominant technique for sample illumination in modern scientific light microscopy. It requires additional optical elements that are more expensive and may be or may not be present in more basic light microscopes.

In Kohler illumination the collector lens or field diaphragm collects light from the illumination source and focuses it at the front focal plane of the sub-stage condenser’s aperture diaphragm which, in essence, projects an image of the lamp filament onto the lens.

The condenser transmits the light to illuminate the specimen. Often, the condenser must be adjusted to guarantee that the filament image appears in the focal plane and fills the aperture. The image of the filament must fill the aperture diaphragm and the field diaphragm and they must share the same conjugate image planes as the specimen.

This results in extremely even illumination because the filament in the specimen image planes is completely defocused and forms a clear image of the specimen.

Closing the field diaphragm does not reduce the brightness of the image but merely controls the width of the light beam being transmitted to the condenser and restricts the light to the part of the specimen which is actually being observed.

Adjusting the condenser aperture affects the angle of the light being transmitted to the specimen. Setting this diaphragm and the aperture of the objective determines the actual working numerical aperture of the microscope. Resolution and contrast increase as the condenser diaphragm is opened.