The granular nature of the photographic emulsion places a limit on the accuracy with which the density of a small area on the film can be defined. This in turn limits the accuracy with which incident exposures can be calculated when, for example, a spectrograph is employed for radiometric measurements. The quantity of information conveyed by a single density measurement is estimated for a particular emulsion and microdensitometer scanning aperture.
James A. Eyer, "A Limit on the Accuracy of Photographic Radiometry," Appl. Spectrosc. 14, 4-7 (1960)