Germanium (Ge)

Germanium has the highest index of refraction of any commonly used infrared transmitting materials. It is a very popular material for systems operating in the 3-5 or 8-12µm spectral regions. Germanium blocks UV and visible light and in the infrared up to about 2µm. Its high index is desirable for the design of lenses that might not otherwise be possible. Germanium has nearly the highest density of the infrared transmitting materials and this should be taken into consideration when designing for weight restricted systems. Germanium is subject to thermal runaway, meaning that the hotter it gets, the more the absorption increases. Pronounced transmission degradation starts at about 100°C and begins rapidly degrading between 200°C and 300°C, resulting in possible catastrophic failure of the optic.


Property Specification
Transmission Range 2µm to 14µm
Density 5.33g/cm3
Thermal Expansion 2.3 x 10-6 /°K @ 100°K, 5.0 x 10-6/°K @ 200°K, 6.0 x 10-6/°K @ 300°K
Surface Finish Typical specifications for surface quality in the infrared are 40-20 or 60-40 scratch-dig in the 2 to 7µm spectral region & 60-40, 80-50 or 120-80 scratch- dig for the 7-14µm area, depending upon system performance requirements. Diamond turned surface finishes of 120 Å rms or better are typical.
Surface Figure In the infrared, typical surface figure ranges from 1/2 wave to 2 waves @ 0.6328µm depending on the system performance requirements.
AR Coating Options Typical available coatings for Germanium include BBAR for 3 to 5µm, 8 to 12µm, & the 3 to 12µm spectral regions. Many application specialized bands are possible between the 2 & 14µm.
Typical Applications Thermal imaging, FLIR.
Products Manufactured Lenses, Aspheric Lenses, Binary (Diffractive) Lenses, Windows, Optical Beamsplitters, Optical Filters, Wedges.


Wavelength (µm)
Index of Refraction (n)



Ge Typical Curve