Ramsden disk

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Ramsden disk

(Science: microscopy) The small circular patch of light that appears at the eyepoint above the ocular lens. This is the exit pupil of an optical instrument that, in a microscope adjusted for koehler illumination, lies in a plane conjugate with the objective rear focal plane, condenser iris, and light source. Alteration of the ramsden disk (for example, by the observer's iris) modifies the aperture function, diffraction pattern, and direction of view of the specimen.