Back Button


Critical or 'Nelsonian' Illumination

19th to 20th C.

Critical Illumination, or Nelsonian Illumination as it is sometimes called, is a method of optimizing illumination of objects seen under an achromatic microscope. Until Koehler developed his method, Critical illumination was the best form of illlumination. Unlike Koehler, it requires only one diaphragm or stop in the condenser, while Koehler requires two diaphragms. Critical illulmination has the disadvantage of optimal illumination of only part of the field of view so it does not lend itself to photomicrography well. Critical illumination requires a thin intense source of light such as the narrow edge of a flat lamp wick, or the edge of an electric ribbon filament.

The following is a description of the technique taken from a contemporary reference: