An upper limit is obtained for the sensitivity of any radiation detector which satisfies Lambert’s law and whose effective area is independent of the wave-length of the radiation. Expressions for the minimum detectable power are given by Eqs. (3.8) and (5.4).
The treatment given is based on thermodynamics, and makes no use of statistical mechanics. In the special case in which the fluctuations in the output of the detector are due entirely to fluctuations in the radiation, the results obtained agree with those obtained from a statistical mechanical treatment of the fluctuations in the radiation.
The treatment of the first five sections is confined to detectors, called “temperature detectors”, which operate by means of the change in their temperature brought about by the radiation. Such detectors include the thermocouple, the bolometer, and the radiometer. In Sections VI and VIII it is shown that the results hold for any detector satisfying the conditions in the first paragraph, provided that the emissivities are interpreted as quantum efficiencies. Section VII indicates that if the condition regarding Lambert’s law is given up, then there exists no lower limit on the minimum detectable power. Section IX is devoted to numerical examples and a comparison with the reported sensitivities of bolometers.
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