When radiographic film is exposed with fluorescent intensifying screens, the spatial x-ray quantum fluctuations are recorded on the film in the form of density fluctuations (“quantum mottle”). The x-ray quantum fluctuations can be regarded as the input and the quantum mottle as the output of the image-forming system. The Wiener spectrum of the quantum mottle has been used to determine the relative modulation transfer function of two radiographic systems.
K. ROSSMANN, "Modulation Transfer Function of Radiographic Systems Using Fluorescent Screens," J. Opt. Soc. Am. 52, 774-775 (1962)