When radiographic film is exposed with fluorescent intensifying screens, the spatial x-ray quantum fluctuations are recorded on the film as density fluctuations (“quantum mottle”). From the expression for the Wiener spectrum of the mottle, the standard deviation σDmottle of the density fluctuations has been derived in terms of certain screen—film parameters. It is shown that the Selwyn relation σDd =const does not hold for quantum mottle.
K. ROSSMANN, "Recording of X-Ray Quantum Fluctuations in Radiographs," J. Opt. Soc. Am. 52, 1162-1163 (1962)