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  • Koehler illumination

A two-stage illuminating system for a microscope in which the source is imaged in the aperture of the substage condenser by an auxiliary condenser, and the substage condenser in turn forms an image of the auxiliary condenser on the object. An iris diaphragm is often placed at each condenser. The first iris controls the area of the object to be illuminated, and the second iris varies the numerical aperture of the illuminating beam.
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