All infrared detectors are subject to performance limits arising from the statistical nature of the infrared radiation being absorbed. Photon detectors are subject to a limit associated with the statistical rate of arrival of photons from the radiating background to which the detectors are exposed; this is known as the BLIP (background limited infrared photodetector) limit. Thermal detectors are subject to a similar limit; the analysis is in terms of the statistical fluctuations in the incident radiant energy associated with the statistical fluctuations in the rate of arrival of photons from the radiating background.
Thermal detectors are also subject to temperature fluctuation noise, arising from temporal fluctuations in absorbed energy due to conduction or convection of heat from sources other than the radiating background. The detector consists of a sensitive area coupled to a substrate. Construction can be hybrid or monolithic. In either case, heat is exchanged by conduction between the sensitive area and the substrate. This gives rise to random fluctuations in the temperature of the sensitive area; this is known as temperature fluctuation noise. Even if the pixel and the substrate are at exactly the same temperature, temperature fluctuation noise will be present; it does not require heat flow from a warm pixel to a cool substrate. If the detector is not in an evacuated package, temperature fluctuation noise will also result from conduction and convection through the air surrounding the sensitive area. Thus background fluctuation noise can be seen as the ultimate manifestation of temperature fluctuation noise when thermal conduction and convection are negligible compared with radiation exchange between the detector and its surroundings.
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