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Optical Materials 221

Figure 10.8 Index and transmission of calcium fluoride (CaF 2 ) for various wavelengths.

and infrared, which make it valuable for instrumentation purposes.
In addition, its partial dispersion characteristics are such that it can
be combined with optical glass to form a lens system which is free of
secondary spectrum. Its physical properties are not outstanding since
it is soft, fragile, resists weathering poorly, and has a crystal structure
which sometimes makes polishing difficult. In exposed applications,
the fluorite element can sometimes be sandwiched between glass ele-
ments to protect its surfaces. The table of Fig. 10.8 lists selected index
and transmission values for fluorite. Natural fluorite has been used in
microscope objectives for many, many years. The FK glasses, especially
FK51, FK52, and FK54, share many of fluorite’s characteristics and
are very useful in correcting the secondary spectrum.

10.5 Plastic Optical Materials
Plastics are rarely used for high precision optical elements. A great
deal of effort was made to develop plastics for optical systems during
the Second World War, and a few systems incorporating plastics were
produced. Since then, the technology of fabrication of plastic optics has
advanced significantly, and today, in addition to novelty items such as
toys and magnifying glasses, plastic lenses can be found in a multitude
of optical applications, including inexpensive, disposable camera lenses,