Germanium  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. The environment does not cause problems because Germanium is inert, mechanically rugged, and fairly hard.
Optical Properties
Transmission range in µm @10% min. 1.8 - 24
Transmission range in µm @50% min. -
Refractive index @633nm 4.10417
(4.0026 @11µm)
Reflection losses in % at 1 surface 36.04 @10µm
Reflection losses in % at 2 surfaces 53 @10.6µm
dn/dT in 1/K 396 · 10-6
Physical Properties
Density in g/cm³ 5.327
Melting point in °C 938.25
Specific heat capacity in J/(kg · K) 310
Thermal Conductivity in W/(m · K) 58.61
Thermal Expansion in 1/K 5.9 · 10-6 @293K
Dielectric constant 16.6
Solubility in water in g/100g unsoluble
Mohs hardness 6
Knoop hardness in kg/mm² 800
Material type Single crystal, synthetic
Crystal type cubic face-centered, diamond structure
Crystal structure Fd3m
Lattice constant in Å a = 5.62
Elastic coeficients in GPa C11 = 129
C12 = 48.3
C44 = 67.1
Young's Modulus (E) in GPa 102.7
Shear Modulus (G) in GPa 67
Bulk Modulus (K) in GPa 77.2
Rupture Modulus in MPa 110
Apparent elastic limit in MPa 8926
Poisson ratio 0.2
Spectral Properties