ABSTRACT
Optical windows and lens are most frequently made of amorphous materials, such as glasses and plastics, because these materials are relatively inexpensive and can be made in large quantities with excellent reproducibility of material properties. The optical crystals, by the conventional definition, are transparent with respect to visible light. Classification of crystalline materials based on optical properties is well established in the field of optical mineralogy. All the optical crystals are classified into three categories: Isotropic crystals, Anisotropic crystals and uniaxial crystals. For many applications the transmission at longer wavelengths is more important than the transmission at short wavelengths. The values cited are approximate and are intended to as a general guide because many factors such as impurities, imperfections, temperature, crystallographic orientations, and compositional variations can affect the values. Crystals with tetragonal, hexagonal, and trigonal symmetry exhibit a unique index of refraction when light vibrates parallel to the c-axis.
