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 σ<i>D</i><sub>mottle</sub> of the density fluctuations has been derived in terms of certain screen—film parameters. It is shown that the Selwyn relation σ<i><sub>D</sub>d</i> =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)