Photon Fluctuations, Equivalent Quantum Efficiency, and the Information Capacity of Photographic Images

Abstract

The equivalent quantum efficiency (EQE) enables the signol-to-noise ratio of a detector to be compared with that attainable by an ideal noiseless detector working under the some conditions. It thus provides a scientific basis for the measurement of photographic speed-to-grain ratios. The limitations of an ideal detector are only those imposed by photon fluctuations in the signal. The signal itself may also be measured in terms of these inherent photon fluctuations, and this enables the information capacity of the detector to be expressed in terms only of the EQE and the ratio of the power spectrum of the signal to that of the photon fluctuations. There are several advantages when the information capaCity is expressed in this manner. Not only is the signal specified in a basic farm, but also, since the EQE is the most fundamental of imaging properties, the information capacity of the photographic process can be rapidly compared with that of any other type of detector by substitution of the appropriate value of the EQE. In this way the information capacity can also be compared with the ultimate capacity, which is that of the signal itself and corresponds to an EQE of 100 per cent. Examples are given of the information capacity for a range of values of the EQE and signal size, and its spatial frequency dependence is demonstrated in terms of the transfer function.