Materials used in infrared optical systems generally have the following characteristics:
(1) infrared materials are not only limited in variety, but also expensive (usually in the thousands to tens of thousands of yuan per kilogram).
(2) the refractive index temperature coefficient (dn/dt) of some materials is large, resulting in a larger focal distance with the temperature drift.If the operating temperature range is wide, appropriate selection of infrared optical materials or necessary measures must be taken to compensate. 
(3) some optical materials are fragile and have poor chemical stability, which makes the processing and installation difficult and the yield is not high. 
(4) many optical materials are opaque and show different colors according to the material and wave band.
(5) when infrared optical materials are heated, self-radiation will occur, leading to the formation of stray light.