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Germanium is a relatively common and stable infrared material with a refractive index greater than 4, so its reflection loss will exceed 50%. After coating with an antireflection coating, the transmittance of germanium can be significantly improved, reaching more than 98%.
Germanium can be used in both long-wave infrared and middle-wave infrared. As far as the optical properties of germanium are concerned, two parameters are important, one is the refractive index, and the other is the dn/dt coefficient. The refractive index of germanium is slightly greater than 4, which means that a lens surface with a small sag (large radius) is more reasonable, and a higher refractive index is beneficial to reduce aberrations, which is beneficial to the design. The dn/dt of germanium is 0.000396/°C, which is a large value compared to glass lenses used in the visible light band (dn/dt is about 0.000036/°C). This results in large focal plane shifts with temperature changes in infrared optical systems, and the amount of defocusing is often large, So these systems must also be used in conjunction with some form of athermalization technology.
Germanium is a relatively common and stable infrared material with a refractive index greater than 4, so its reflection loss will exceed 50%. After coating with an antireflection coating, the transmittance of germanium can be significantly improved, reaching more than 98%.
Germanium can be used in both long-wave infrared and middle-wave infrared. As far as the optical properties of germanium are concerned, two parameters are important, one is the refractive index, and the other is the dn/dt coefficient. The refractive index of germanium is slightly greater than 4, which means that a lens surface with a small sag (large radius) is more reasonable, and a higher refractive index is beneficial to reduce aberrations, which is beneficial to the design. The dn/dt of germanium is 0.000396/°C, which is a large value compared to glass lenses used in the visible light band (dn/dt is about 0.000036/°C). This results in large focal plane shifts with temperature changes in infrared optical systems, and the amount of defocusing is often large, So these systems must also be used in conjunction with some form of athermalization technology.
