A system of measuring the densities of images on photographic films, comprising a conventional projection unit, in which a measuring device (7) is mounted slidably between the film (6) and the lens (9), capable of longitudinal movement in relation to the film and equipped with a plurality of light-transducing elements (74) arrayed in mutual alignment transversely to the image (I) and positioned to receive respective quantities of light from discrete portions (ZP) of a transverse reading zone (Z), which are admitted through a slot (72), reflected by a mirror (71) and focused by a lens (75). With the measuring device (7) translating longitudinally, such that the slot (72) can cover the image (I) progressively and in its entirety, the single values sensed by the light-transducing elements (74) are fed into an electronic processor (C) and processed together with other data in such a way as to determine the filtration and illumination parameters governing reproduction of the image (I).