In this sense, the Plan Apochromatic lens correction represents the highest level of color reproduction. Here the focal point for several wavelengths has been brought together by a clever combination of different lenses, resulting in a lack of colored fringes around phenomenon borders.
|Apochromatic lens (APO)|
Especially for oblique light rays, this is a difficult task.
The level of color correction can be checked best by using the classic “Abbe-Testplatte”. Here a pattern of vertical bars is combined with a wedge-shaped cover slip, imitating the possible thickness deviation of the cover glass.
By principle, the illumination has to be somewhat oblique to check the chromatic aberration for the most demanding case of peripheral illumination. A black piece of metal (see below) with a cake-shaped sector is placed onto the light exit to create a “segment illumination” (to be compared with modern LED ring lights for stereo microscopes).
As this equipment is almost extinct, the user may take a regular stage micrometer (“Objektmikrometer”) as a test sample and replace the metal plate by a similar shaped piece of black cardboard.
The chromatic aberration of a certain objective can be seen by checking the border of those micrometer bars. Bluish and reddish/brownish fringes are visible. Working with a camera, a maximum digital magnification up to pixelization of the image will display the different quality level. The more “defined” a black bar is, the better the color correction.
All Plan Apochromats surely play in the highest price league. Good to know that this “Rolls Royce” is not needed for most samples, and, due to lots of glass elements inside, these objectives are not recommended in case high transmission rates are needed (e.g. Fluorescence). In pathology and histology, where best color reproduction is essential for medical diagnosis and life-saving therapy, these optics will keep its relevance.