The information capacity of a radiation detector is denoted by C and is measured in bits per second. The root-mean-square radiation power incident on the detector is denoted by P and is measured in watts. The interest of this paper is focused on the ratio C/P = I, measured in bits per joule. When I is referred to unit detector area, it is denoted by I* and is measured in bits per (joule/cm). It is shown that for most detectors, the information capacity I has a maximum value for a finite and determinable value of the power P, and that the maximum corresponds to an electrical signal-to-noise ratio in the output of the detector that is of the order of magnitude of unity. It is further shown, subject to a suitable simplifying approximation, that the information capacity I* is equal to one-half of the energy detectivity Δm* of the detector, and that if the modulation of the radiation power is confined to one cps band centered at the frequency f, the information capacity I* is equal to the detectivity D*(f) of the detector.
The analysis of the information capacity is divided into two parts. Section 3 uses a simplifying approximation, and includes numerical results for a number of kinds of detectors, including the dark-adapted human eye, Royal-X photographic film, multiplier phototubes, and cooled infrared detectors. Section 4 contains an exact analysis for two important cases.
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