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Illumination, critical

(Science: microscopy) The formation of an image of the light Source in the object field.

The Nelsonian method in which the light Source is imaged in the plane of the specimen. A ribbon filament or arc lamp is required to give uniform illumination, the lamp must be focusable, the filament position must be adjustable in all directions. The use of an achromatic condenser is advised.

Synonym: Nelson illumination.