When a conventional phase-contrast microscope is available, relief phase contrast can be achieved simply using a Zernike condenser; alternatively a brightfield condenser can be modified for phase contrast by placing
special slides into the light path. In these cases, the condenser aperture diaphragm has to be closed to achieve correct alignment of the illuminating mask in the condenser and the phase ring in the objective.
Closing the aperture diaphragm can improve the focal depth and sharpness in the same manner as is usual in brightfield. In conventional phase contrast, the quality of the microscopical images is not influenced by
the aperture diaphragm. The resulting three-dimensional appearance can contribute to the gobal quality of relief phase contrast images as well as the diminution of existing spherical aberrations. It can be regarded
as an additional advantage of these modifications, that objectives made by different manufacturers can be used simultaneously also in cases of misalignments in conventional phase contrast caused by non-compatibility.
The resolving power is reduced when the aperture diaphragm is closed. The lower the diameter of the phase ring in relation to the diameter of the lenses, the lower the width of the aperture diaphragm and the
remaining resolving power when relief phase contrast is achieved.
When ring-shaped masks are directly modified to crescent- or punctate- shaped masks, the condenser aperture iris diaphragm can be fully open.
The special features of the specimen and the individual relative diameter of the existing phase ring determines which option (large or small aperture diaphragm) leads to better optical results.
Relief phase contrast can be regarded as a complementary method which can improve the quality of conventional phase contrast images.
When expensive high-end lenses are not available, relief phase contrast can still lead to excellent image quality when only less well objectives are used.
These improvements of quality are visible as well in life microscopy, photographic images or movies.
Copyright: Joerg Piper, Bad Bertrich, Germany, 2007