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.
2µm to 14µm
2.3 x 10-6 /°K @ 100°K, 5.0 x 10-6/°K @ 200°K, 6.0 x 10-6/°K @
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.
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.
Thermal imaging, FLIR.
Lenses, Aspheric Lenses, Binary (Diffractive) Lenses, Windows,
Optical Beamsplitters, Optical Filters, Wedges.
Index of Refraction (n)