critical illumination

optics

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July 14, 2026

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Critical illumination is a technique used in microscopy to provide bright, uniform lighting for observing specimens. It involves focusing the image of a light source directly onto the specimen's plane. This method is part of the illumination system in optical microscopes, which also includes a light source and a condenser lens. The condenser concentrates light, ensuring the object is well-lit and the objective lens's aperture is filled with light. While critical illumination is effective, an alternative called Köhler illumination focuses the light source's image onto the condenser, which then illuminates the objective's entrance pupil, averaging out any non-uniformities in the light source.1

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The illumination system
…the specimen, a technique called critical illumination. Alternatively, the image of the source is focused onto the condenser, which is in turn focused onto the entrance pupil of the microscope objective, a system known as Köhler illumination. The advantage of the latter approach is that nonuniformities in the source are…
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…the specimen, a technique called critical illumination. Alternatively, the image of the source is focused onto the condenser, which is in turn focused onto the entrance pupil of the microscope objective, a system known as Köhler illumination. The advantage of the latter approach is that nonuniformities in the source are…
Read More