The spectral range for IR cameras spans from 1.5 to 14 µm wavelength. The cameras are designed to operate in certain spectral sub-ranges (SWIR: 1.5 µm - 3 µm, MWIR: 3 µm - 5 µm, LWIR: 8 µm - 14 µm). Most IR lenses are made of silicon, germanium or zinc selenide and are optimized just for one of these sub-ranges. Anti-reflection coatings on every lens surface guarantee a high transmission.
The most important feature of a lens is its focal length. The focal length of the lens and the detector size determine the field of view (FOV) of a camera.