This invention relates to infrared detectors comprising a housing and a Joule-Thomson cooling element, particularly but not exclusively detectors having fast cool-down characteristics as well as a compact size and light weight and suitable for detecting infrared radiation having a wavelength in the 3 to 5 or 8 to 14 .mu.m (micrometer) wavebands.
Published United Kingdom Patent Application (GB-A) No. 2147739 describes an infrared detector comprising a housing. At least one infrared detector element is mounted at a first part of the housing. A Joule-Thomson cooling element formed around a core is accommodated in a second part of the housing to effect cooling of the detector element during operation of the detector by expansion of coolant into a space adjacent the first part of the housing. Infrared transmissive means at a front of the housing permit transmission of infrared radiation to the detector element. The second part accommodating the cooling element is of low thermal conductivity and capacity and is, in this device, situated behind the first part where the detector element and infrared transmissive window are mounted. Such an arrangement is conventional practice. However, the design of the housing is such that the detector element can be cooled down very rapidly, for example within a few seconds, and the detector is used in situations where the cooled operating state need not be maintained for a very long time, for example at most 5 minutes.