There are two major reasons for the cooling of IR devices. As pointed out in Chapter 6, all photon detectors must be operated such that their noise is substantially less than the energy levels of detection. Secondly, as pointed out in Chapter 10, there is a need to reduce the flux falling on the detectors from internal sources. In a well-designed IR system, the immediate surroundings of the detectors are cooled so that the background flux from these sources is considerably less than the background arising from external sources (i.e., earth or sky).