Abstract: A band of exposed and developed photographic film is transported longitudinally along a predetermined path. The film carries a series of images and each of the images has a pair of edges which are spaced longitudinally of the band and extend transversely of the latter. Transverse, strip-shaped segments of the band are photoelectrically scanned to detect abrupt density changes associated with the edges of the images. In addition, the band is sensed at a predetermined location of its path in order to ascertain the length of the band travelling by the predetermined location. The density and length data are sent to a microprocessor which uses the data to assign a position coordinate to each detected edge and its corresponding image. The position coordinates represent the positions of the images longitudinally of the band.