Germanium (Ge) is the preferred lens and window material for high performance infrared imaging systems in the 8–12 μm wavelength band. Its high refractive index makes Ge ideal for low power imaging systems because of minimum surface curvature. Chromatic aberration is small, often eliminating the need for correction.

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.

Germanium is a high index material that is used to manufacture Attenuated Total reflection (ATR) prisms for spectroscopy. Its refractive index is such that germanium makes an effective natural 50% beamsplitter without the need for coatings. It is also used extensively as a substrate for production of optical filters. 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.

Application as window, lens, beamsplitter, ATR prism or filter in spectrometers and in thermal imaging.

Main Properties

Crystal properties
Crystal Growth Method			Czochralski pulling techniques
Maximum Size			<Φ250mm
Optical properties
Transmission Range			2.0~17um
Reflection Loss, for two surfaces at 11µm			53%
Refractive Index			See below
Wavelength (um)	Refractive Index (n)	Wavelength (um)		Refractive Index (n)
2.0	4.1079	9.0		4.0047
3.0	4.0446	10.0		4.0040
4.0	4.0242	11.0		4.0031
5.0	4.0153	12.0		4.0029
6.0	4.0106	12.5		4.0024
7.0	4.0076	13.02		4.0021
8.0	4.0053	15.0		4.0017
Transmission Curve		See below