Thermal camera lenses are made out of substances like germanium, or other materials with low absorption in the infrared spectrum. But why is it necessary to use these specialized components instead of a more common substance like glass?
Infrared cameras don’t work the same way as regular, visible light cameras. Regular cameras function more or less the same way as a human eye, receiving radiation in the visible light spectrum and turning it into an image. Infrared cameras, on the other hand, make images from heat, aka infrared or thermal radiation, instead of visible light.
Infrared radiation behaves very differently from visible light. Because of this, thermal camera lenses need to be made from different materials than regular cameras. The properties for a given material in the visible world might have no bearing on its properties in the infrared world—glass, for instance, is extremely transparent to radiation in the visible spectrum, but in infrared, glass is completely opaque.