Monday 3 February 2020

Köhler Illumination

Köhler Illumination is a form of microscope sample illumination, invented by August Köhler, (see below), that provides even light across the field of view, and a light cone that matches the numerical aperture of the objective lens. It is suitable for either viewing or photography. It normally consists of a collector lens near the light source, together with a field iris, and a sub-stage condenser with a condenser iris. The focus of both the collector and condenser lenses is adjustable, as is the aperture of each of the irises.

The Cooke microscope light you see here has both an adjustable collector lens (the big lever pointing straight up), and an adjustable field iris (the small lever you see further to the right). You can click on the image to see it enlarged.

It also has a knob to adjust how bright the light is, and slots where filters can be inserted. This particular model of light is designed by Cooke to be used with a Cooke microscope, but it can easily be used with any conventional microscope.

The microscope I've shown in the picture above is by Charles Baker. It has a sub-stage condenser and iris, and a sub-stage mirror. The mirror has a plane face and a curved face. The plane face of the mirror is used in conjunction with Köhler Illumination, since the focusing of the illuminating light is achieved by the collector lens and the sub-stage condenser.

Using Köhler Illumination

The collector lens is focused to form an image of the light source in the plane of the sub-stage condenser iris. The condenser lens is focused to form an image of the field iris in the sample plane. The two irises are adjusted to provide the required light cone. Thus, when the sample is in focus the image of the light source is completely de-focused, resulting in even illumination.

Some microscopes have built-in Köhler illumination. In this case, the principle is the same, but the practicalities of the adjustments may be different.

August Köhler

August Köhler, (1866-1948), was a German scientist who made several innovations in microscopy. He is best known for his invention of Köhler Illumination in 1893. In 1900 he began to work for Carl Zeiss, where he continued to work until 1945, three years before his death.

All content and images © Tony Benson

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