There are numerous glass types available for the visible spectrum, but there are only a small number of materials that can be used in the MWIR (mid-wave infrared) and LWIR (long-wave infrared) spectral bands.
Germanium is a crystalline material. It is one of the most common infrared materials, and it can be used in the MWIR and the LWIR band. Germanium has a very high refractive index (n = 4.0243), which makes elements with long radii feasible. It has a large dn/dT (396 ppm/K) which can cause large focus shifts with temperature changes. This could make athermalization difficult.
Fabrication methods for Infrared materials such as germanium, silicon, zinc sulfide, and zinc selenide are in general similar to normal glass optics. Many of the crystalline materials, however, are hygroscopic (absorbs water), which can be challenging to work with.
Some Infrared materials - like germanium, silicon (though it is difficult), zinc sulfide, zinc selenide and the fluorides - can be single-point diamond turned. Sapphire cannot be diamond turned (see Sapphire Domes). Diamond turning becomes important when you need to incorporate aspheric surfaces into a design.
Good antireflection coatings are required, due to the very high index of refraction, which causes Fresnel reflections to increase significantly, and thus lowers transmission. Proper coatings are also needed to protect these materials from moisture damage.
Germanium is most widely used for lenses and windows in IR systems operating in the 2 µm - 12 µm range. Its transmission is very temperature sensitive, becoming opaque near 100°C. Environment does not cause problems because Germanium is inert, mechanically rugged, and fairly hard.
Raytheon ELCAN leverages experience in the design and manufacture of high precision infrared optics for defense to meet the challenging tolerances and specifications required for optical fabrication with IR materials such as germanium.