When mirror objectives are available, constructed according to the Cassegrain- Schwarzschild-type (1, 2), the backside of the centric convex mirror in the middle of the objective can act as a light absorbing element. In this case, specific advantages of mirror systems are also relevant for high optical quality in luminance contrast. Thus, images are completely free from any chromatic aberration and great visual fields are illuminated homogeneously without any relevant spherical aberration. Fig. 6 demonstrates the optical principles of  a mirror objective and the various variants of luminance contrast with regard to the specific light path in these systems.

Fig. 6:
Light path in mirror objectives
(schematic)
ad = aperture diaphragm
ss = specimen slide
cs = cover slip

a: bright field
b: luminance dark field
c: luminance phase contrast
d: luminance phase contrast, using a special double diaphragm with a centric perforation and a peripheral light ring (see fig. 7b)

Copyright: Joerg Piper, Bad Bertrich, Germany, 2007

[Introduction]
[Resolution and focal depth]
[Principles of luminance contrast]
[Modification of lenses]
[Mirror objectives]
[Illuminating apparatus]
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