Germanium (Ge) is the preferred lens and window material for high performance infrared imaging systems in the 8-12 µm wavelength range. It is a high index material that is used to manufacture ATR (Attenuated Total reflection) prisms for spectroscopy. Its high refractive index makes Ge ideal for low power imaging systems because of minimum surface curvature.
Germanium is grown by Czochralski pulling techniques. It is relatively expensive. The refractive index changes rapidly with temperature and material becomes opaque at all wavelengths a little above 293K.
Chromatic aberration is small, often eliminating the need for correction. Germanium covers the whole of the 8-14 micron thermal band and is used in lens systems for thermal imaging. It can be AR coated with diamond producing an extremely tough front optic. 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.