The applications of infrared detectors can be divided into the two main areas of imaging and measurement, the latter including photometry, spectroscopy and range finding. The detector properties such as wavelength range, sensitivity and speed of response can be quite different for each specific requirement and these differences will be covered in detail. The two most important types of infrared detectors are thermal and photon devices. Thermal detectors utilize materials with a strongly temperature dependent property such as electrical conductivity or thermal expansion. The main characteristic of these devices is their broad, flat spectral response, however the major disadvantage is that their response times are relatively slow - typically of the order of milliseconds - which limits their use in certain applications. This situation has been improved recently, with the development of pyroelectric devices, which use insulating materials such as triglycine sulphate which have a permanent internal electrical polarization, the magnitude of which is temperature dependent. With suitable circuitry the response time can be less than 1 μsec. Photon detectors are fabricated from semiconducting materials in which the incident radiation is absorbed causing electronic transitions within the photosensitive material. These sensors have a spectral response with a precise cut-off wavelength, but generally have higher sensitivities and are much faster than thermal devices. Intrinsic photo conductors and photodiodes were first developed using PbS, PbSe, PbTe or InSb; however, these materials are not sensitive beyond about 7 μm. Subsequently extrinsic devices had to be used for longer wave-length detection. The most popular choice was doped germanium, which could be used to beyond 100 μm. The major disadvantage of these devices was the necessity to cool to much lower temperatures than intrinsic detectors. The recent development of the ternary alloy systems, such as cadmium mercury telluride and lead tin telluride, in which the energy gap may be varied by choosing an appropriate composition has now produced high performance intrinsic detectors in the wavelength range 2-20 µm.