ABSTRACT
Crystalline materials included in this section are insulators and semiconductors that have a transparent region within the range from the vacuum ultraviolet (from ~I 00 nm) to the infrared (up to 100 11m) portion of the electromagnetic spectrum. Crystals with wide band gaps are transparent from the ultraviolet through the visible region; crystals with a narrower band gap may appear opaque but are transparent in the infrared region. Using this broad transparency definition of optical crystals, virtually all known crystals can be included. Coverage, however, is limited to those crystals which either occur in nature or are produced in the laboratory for optical use or with potential for such use. For this reason hydrate or hydroxide crystals are generally excluded because they are thermally less stable and have limited tranmission range due to OH absorption. Highly hygroscopic materials are also excluded because of the obvious difficulty of handling, unless they have already been used, such as urea, KDP, CD* A, etc. Only pure compounds are considered. Compounds containing elements having intrinsic absorptions due to incompletely filled d or f shell electrons are also avoided.
