Germanium (Ge) has high density and has excellent transmission features of blocking UV but transmitting infrared from 2µm-14µm. Germanium has high refractive index and low dispersion and is used as IR-transmitters in these wavebands so desirable for IR lens design where its high refractive index allows otherwise impossible specifications to be met.
Germanium transmits over 45% between 2-14µm up to 45°C so BBAR coatings are advisable as use of AR coatings increases transmission to over 80% between 3µm-14µm. Diamond like coating (DLC) are also applied for harsh environments.
Germanium lenses are typically used in thermal imaging cameras. Germanium lenses are useful for wide-angle lenses. Germanium is also used for FTIR (Fourier Transformed Infrared) and FLIR (Forward Looking Infrared) and spectroscopy systems.