Historically the most common material utilised for IR imaging optics has been Germanium. The reasons for this are simple. Germanium is a unique material with exceptional optical properties arising from its high refractive index and excellent transparency in the LWIR (8-14um) spectral waveband. In recent years however, Germanium has become scarce and expensive primarily due to Chinese export restrictions, and fast-growing demand in both optical and electronics applications, stressing supply chains. Chalcogenides glasses are quickly becoming the alternative IR optical material of choice.
Read tech note: https://www.resolveoptics.com/combatting-the-global-shortage-of-germanium-for-ir-lenses/
