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 medium wave infrared and long wave infrared bands.
Germanium is a kind of commonly used infrared material, which can be used in both long wave infrared band and medium wave infrared band.In the LWIR band, it is similar to the corona or positive element in the achromatic lens in the visible band.In the MWIR band, it is similar to the flint or negative element in the achromatic lens in the visible band.The refractive index and temperature coefficient of germanium are two parameters of Chinese traditional medicine.
First of all, the refractive index of germanium is slightly larger than 4, which is favorable for reducing aberration and favorable for design.The dn/dt value of germanium is 0.000396 deg. C., which is too large. Large focal shift may occur in the process of temperature change.It can be said that this material has both advantages and disadvantages.Because polycrystalline germanium has a large problem of non-uniform refractive index, but due to its strong brittleness and easy to become debris, it needs to be treated carefully in the process of optical processing, coating and mounting.Because of the wide range of USES of germanium components, these shortcomings have become solvable compared to its advantages.