Germanium, a semi-metallic element (atomic number 32, symbol Ge) is highly suitable for midwave and longwave infrared optics even though it is obviously opaque to visible band light.
Another concern to the thermographic process is the transmittance of radiation by the lens optic elements. If an optic material is as irregular as the media the radiation summary produced by the camera, firmware, and software could be tremendously suspect. Fortunately, Germanium, a semi-metallic element (atomic number 32, symbol Ge) is highly suitable for midwave and longwave infrared optics even though it is obviously opaque to visible band light. Germanium is expensive, very refractive-that is to say, it bends light aggressively-and temperature changes can result in significant focus shifting.
Germanium lenses are generally coated; such coatings are not necessarily chemically inert nor tremendously durable. Germanium Dioxide-GeO2-is a coating also used to discourage the natural interactions with oxygen in the atmosphere by Germanium.
Germanium trends toward longwave infrared opacity at high temperatures (nearing 100° C); to obviate this problem, always operate your thermal cameras within the ambient specifications provided by the manufacturer.
Be perfectly complaint to manufacturer’s recommendations for chemistry when cleaning a Germanium lens, and never use materials that are physically traumatic to the lens.
This video (2:46) explores Germanium as an optic for Longwave Thermal Infrared cameras.