Background thermal noise often becomes the limiting factor
while detecting low signals of photon flux from a distant thermal source. This
cannot be eliminated by simply cooling the photodetector. A general method is
developed here to calculate the background thermal noise and the noise
equivalent power (NEP) for background source temperature T, with any
given frequency-dependent quantum efficiency function η(ν) for the
photodetector. Applications of our analysis to some specific model forms of η(ν),
with finite bandwidths, show that earlier calculations are highly inadequate
for peak efficiency frequencies νo < kBT/h,
where kB is the Boltzmann constant and h is the Planck
constant. The NEP, which determines the limit on the lowest signal power that
can be detected by any given photodetector, is much higher in these frequency
regions compared to earlier approximate estimates.
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