In the visible portion of the spectrum almost all lenses and windows are made of silicate glass. It is transparent, strong, hard, resists chemical attack, and is a wonderful material overall. Unfortunately, silicate glass transmits only to about 2 mm. Therefore many different materials must be used in different parts of the infrared spectrum.
The choice between the use of lenses and mirrors leans more toward mirrors in the infrared than in the visible. There are several reasons for this, which will be discussed. In addition, the choice criteria and some of the shorts and longs of each of these will be discussed. The reader is cautioned to use this information as a beginning guide and to review properties and performances with individual vendors.
There exist infrared glasses, although they do not have any silica in them. Single crystals are used in some applications, even though their size is limited. Polycrystalline materials and even oligocrystals (just a few crystallites) are used, and there is controversy regarding the use of single and multiple crystals. Hot-pressed compounds, first introduced by Eastman Kodak as Irtrans, have been used, even though they generate some scatter. They can be made as large as the hot presses can hold. Recent advances have generated large crystals made by chemical vapor deposition (CVD).
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