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 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, (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.
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