Fluctuations in streams of thermal radiation

An ultimate limit to the sensitivity of a detector of thermal radiation is set by the inherent fluctuations in the stream of radiation. This paper presents the relevant quantitative relations to enable these fluctuations to be calculated in practical cases. The result is presented as the minimum detectable periodic modulation of the power of a stream for general conditions under which the detector may receive streams of radiation from surroundings at different temperatures and thermal equilibrium is not necessarily established. The modification necessary when the spectral frequency-distributioin differs from that of full black-body radiation is also presented. Limiting factors other than radiation fluctuations are not discussed.