Limits to Detectors Sensitivity

It must be noted that this radiative power, called the BRN equivalent power, noted here as NEPbr, is frequency-independent and varies with T5/2. Assuming an ideal absorbing medium with e = 1, for A = 1 cm2, T = 300 K and A/ = 1 Hz, NEPbr (cm, 300 K, 1 Hz) is 6 x KT11 W. 

When a thermal detector is at a temperature Tdet different from that of the background, Tback, the total mean square of the radiation power is given by  

When a detector is cooled to 4.2 K with a background temperature of 300 K, it produces a reduction in the room-temperature NEPbr by a factor of l/ /2, while NEPbr (cm, 4.2 K, 1 Hz) deduced from expression (4.3) is 1.4 x 10 15W. This is the reason why, under laboratory conditions, the background radiation (BR) incident on low-temperature thermal detectors is strongly attenuated by filters cooled at the detector temperature, which cut the medium IR background and provide a low value of Tback- An improvement is also observed by reducing the field of view of the incident radiation. 

Expression (4.3) is actually derived by integration from the more general expression 

Currently, a photoconductor does respond to the number of photons that produce an electronic excitation in the detector. When defining qv as the photon quantum efficiency at frequency v and vq as the frequency of interest, it can be shown (see [15] for the derivation) that if the photodetector temperature is much less than the temperature T of the surroundings, the radiation background NEP for a photoconductor is given by  

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