Rheinberg illumination, a form of optical staining, is a striking variation of low to medium power darkfield illumination using colored gelatin or glass filters to provide rich color to both the specimen and background. This tutorial explores the changes in specimen appearance when the colors are varied in the annular and central filters used in Rheinberg illumination.

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The tutorial initializes with the Annular Filter slider set to a value of 636 nanometers in the red region of the visible spectrum. This setting affects the color of transparent specimens and illuminates them with light passing through the colored filter. The Central Filter slider is initialized at 456 nanometers, in the blue region, and is the color seen as a background when observing the specimen through the microscope eyepieces. In order to vary specimen and background color, translate the sliders and observe a simulated transparent crystal specimen near the lower right corner of the tutorial window. The actual filter colors are depicted in the lower left corner of the window. As the sliders are moved with the mouse cursor, the specimen and background colors change to produce varying degrees of contrast. Visitors are encouraged to adjust the sliders and observe how contrast can be increased and decreased with changes to the central and annular filter colors.

In Rheinberg illumination, which is related to darkfield microscopy, several kinds and shapes of filters are used. The oblique or outer light rays coming through a wide-open bright-field condenser pass through an annular (doughnut-shaped) filter of one or more colors; the central rays of light pass through another spot-shaped filter fitting into the circular opening of the annular-shaped filter. The objective is used at full aperture.