Critical illumination focuses an image of a light source on to the specimen for bright illumination.

Prior to Köhler illumination critical illumination was the predominant technique for sample illumination.

The primary limitation of critical illumination is the formation of an image of the light source in the specimen image plane.

Critical illumination acts to form an image of the light source on the specimen to illuminate it, this image is formed by the condenser or collector lens.

Critical illumination therefore gives uneven illumination of the sample; bright regions in the filament image illuminate those region of the sample more strongly.

Critical illumination or Nelsonian illumination is a method of specimen illumination used for transmitted and reflected light (trans- and epi-illuminated) optical microscopy.

Köhler illumination has largely replaced critical illumination in modern scientific light microscopy although it requires additional optics which less expensive and simpler light microscopes may not have.

Critical illumination generally has problems with evenness of illumination as an image of the illumination source (for example a halogen lamp filament) is visible in the resulting image.

By adjustment of the field diaphragm the amount of light entering the sample can be freely adjusted without altering the wavelengths of light present, in contrast to reducing power to the light source with critical illumination.

Critical illumination has the major limitation that the image of the light source (typically a light bulb) falls in the same plane as the image of the specimen, i.e. the bulb filament is visible in the final image.

Because of the large value of R, the total resistance is mainly determined by R. The dynamic range of the photoresistor with respect to illumination is equal to the ratio of the critical illumination Φ to the sensitivity threshold Φ.