The main difference between the infrared optical system and the visible optical system is that only a limited number of materials can be effectively applied to the mid-wave infrared and long-wave infrared bands, and even fewer materials can be applied to the two bands at the same time.
The materials used in the infrared optical system generally have the following five 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.
The transmittance of surface loss of commonly used infrared materials: in the actual application process, the efficient anti-reflection film coated can achieve quite high transmittance (95-98%), without containing the transmittance curve of sulfur glass (601228SeAsGe and 651520SeAsGe).With a thickness of 1mm, the average transmittance between 8 and 11 pressures m is more than 65%, the fluctuation is less than 3%, and the loss is less than 2.4%/cm.