Germanium is used widely for lenses and windows in thermal imaging and FLIR applications. The useful transmission range of Germanium windows is from 2 to12um.Germanium is opaque in the visible.
Germanium has the property of thermal runaway, meaning that the Transmission decreases as temperature increases. As such, Ge window should be used at temperatures below 100. Germanium’s high density (5.22g/cm3) should be considered when designing for weight-sensitive systems.