Infrared (IR) detectors fall into two main categories, thermal and photon. The earliest detectors of IR were thermal in nature, e.g. thermometers. The subsequent developments of these detectors, such as thermopiles, resistance bolometers, Golay cells and pyroelectric detectors, can operate at ambient temperature but have disadvantages of insensitivity and slowness. A wide variety of semiconductor photon detectors have been developed and these possess very high sensitivity, high frequency response but have the disadvantage of needing cryogenic cooling, particularly at longer wavelengths. In...