Germanium is popular for its high index of refraction (around 4.0from 2-14μm). Due to its high index, an antireflection coating is required for sufficient transmission in the region of interest. Our Germanium optics are available from stock with an AR coating designed for 8-12μm and a 3-5μm AR coating is available on a custom basis. Germanium is subject to thermal runaway, meaning that the transmission decreases as temperature increases. As such, they should be used at temperature below 100oC. Germanium’s high density (5.33g/cm3) should be considered when designing for weight-sensitive systems. Germanium is typically used in thermal imaging and FLIR applications.


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 Quality 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 Accuracy 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.
Wavelength (µm) Index of Refraction (n)
2.5 4.046
3 4.044
4 4.025
8 4.007
10 4.005
12 4.004
14 4.003


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