On the Relevance of Photon Noise and of Informational Assessment in Scientific Photography

Abstract

In useful and physically meaningful senses, the granularity of a photograph is proportional to the photon noise in the exposing light. Since photon noise is thus a multiplicative factor in the granularity, it is extremely relevant to practical photography. Modern emulsions can have a noise equivalent quantum efficiency as high as 0.01; that is to say, the fundamental noise in the light itself is multiplied some hundred times. This factor is remarkably low in relation to the practical and technological limitations, and shows how highly developed the art of emulsion making has become. The axiomatic basis of informational assessment, and of its meaning as a measure of discriminating power are discussed. An example is given of how the formal analysis can save us from a plausible blunder involving tinkering with the equations of information theory, and an illustration from radar practice is quoted. The definiteness of informational assessment is emphasised, and the available means of calculating and measuring spatial-frequency transfer-functions are briefly referred to. Comparisons are quoted between informational assessment and other abstractions which can be made from the transfer functions.