Germanium (Ge) is used widely for lenses and windows in thermal imaging and FLIR applications. Its high index of refraction makes it of particular interest. The useful transmission range of Germanium windows is from 2 to 12 μ m. 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°. The higher Knoop Hardness of Germanium (780) makes it ideal for IR applications requiring rugged optics. Germanium's high density (5.33g/cm³) should be considered when designing for weight-sensitive systems.