Germanium (Ge) is a relatively hard, high-density infrared transmission material that can block ultraviolet and visible light wavebands, but has a very high transmission of Infrared spectrum in the range of  2-16 µm. Single crystal germanium has a high refractive index (about 4 @ 10.6um) and low optical dispersion, making it ideal for lens design because its high refractive index allows for the construction of other impossible specifications. It is recommended to use AR/AR coating or AR/DLC coating to improve the transmittance. If outdoor use is required, it is generally recommended to deposit Diamond-like carbon (DLC) coating on one side.

At room temperature, the transmittance of uncoated germanium ranges from 2-14μm is about 45%, but it slowly decreases at 100 Celsius degrees, and its transmittance sharply decreases at higher temperatures above 200 Celsius degrees. Therefore, germanium is not suitable for use at high temperature. In addition, if weight is a significant factor and germanium is not very suitable, silicon (Si), zinc selenide (ZnSe), ZnS may be more appropriate.