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 Î¼m. 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.
10.1 Types of Materials
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).
10.2 Selection Criteria
The relative importance of the properties of materials to be considered depends on the application. One set of criteria applies to windows that do not form images but provide a protective shield for the system, and another to the elements inside, usually lenses. There are some considerations to be applied if the lenses are to be cooled to cryogenic temperatures or heated to scorching temperatures, or must undergo relatively wide operational temperature swings. Thus, for windows, one considers the following properties, in the following order: transparency as a must, strength and hardness for protection, thermal expansion, thermal conductivity, specific heat for high-temperature applications, refractive index for reflection loss, and sometimes rain-erosion resistance.
For a lens, one considers first of all its transparency; the refractive index and its change with wavelength and temperature (sometimes called the thermorefractive coefficient) are next. The thermal properties are secondary, usually because the inside of the system is under control. Hardness is to be considered for manufacture and less importantly for scratch resistance.