Critical illumination
Encyclopedia
Critical illumination or Nelsonian illumination is a method of specimen
illumination
used for transmitted and reflected light (trans- and epi-illuminated) optical microscopy. Critical illumination focuses an image of a light source on to the specimen for bright illumination. 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. 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.
or the sun
give more even illumination. Alternatively a ground or opal glass diffuser can be used to homogenise the light source, although a significant amount of light will be scattered away from the sample.
Specimen
A specimen is a portion/quantity of material for use in testing, examination, or study.BiologyA laboratory specimen is an individual animal, part of an animal, a plant, part of a plant, or a microorganism, used as a representative to study the properties of the whole population of that species or...
illumination
Illumination
Illumination, an observable property and effect of light, may also refer to:* Illumination , the use of light sources* Illumination , the use of light and shadow in art...
used for transmitted and reflected light (trans- and epi-illuminated) optical microscopy. Critical illumination focuses an image of a light source on to the specimen for bright illumination. 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. Köhler illumination
Köhler illumination
Köhler illumination is a method of specimen illumination used for transmitted and reflected light optical microscopy. Köhler illumination acts to generate an extremely even illumination of the sample and ensures that an image of the illumination source is not visible in the resulting image...
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.
Optical principles
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. This illumination is bright but not always even as any structure in the light source (for example the filament of a light bulb) will be visible in the resulting image. Homogenous light sources such as a flameFlame
A flame is the visible , gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone...
or the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
give more even illumination. Alternatively a ground or opal glass diffuser can be used to homogenise the light source, although a significant amount of light will be scattered away from the sample.