Patent GB-791676-A (GB)
Title: Method for processing a subtractive motion picture film
IPC: H04N1/60
Filed: 1955-09-27
Abstract: 791,676. Local copying involving scanning. GRETENER, E. Sept. 27, 1955 [Sept. 29. 1954], No. 27547/55. Drawings to Specification. Class 40 (3). Relates to the processing of a subtractive colour film of the type described in Specification 791,675, in which a black-and-white layer is provided in addition to the customary three coloured layers. In such a film the density of the black-and-white layer is made to correspond at each image point to the density of the smallest of the three colour densities in a normal threelayer film and the densities of the colour recordings are reduced by the amount of this black- and-white density so that at any point only two colour recordings, the so-called colour residues, are produced. In accordance with the invention, the black-and-white recording is produced by the simultaneous printing of two images. One of these, termed the basic image, represents the sum of the three-colour separation components of the original and is produced by a direct photographic process so as to be of high definition, and the other one of which, termed the correction image, is produced by a process involving an electro-optical point-by-point scanning of the original colour separation components. By appropriate selection and manipulation of the electrical signals the correction image is formed with a density determined by the ratio of the greatest of the separation components to their sum (corresponding to the ratio of the least predominant to the sum in terms of subtractive colours) whereby the act of printing through the basic and correcting images in superposition produces a black-andwhite recording in which the density corresponds to the smallest of the subtractive colours. In one way of carrying out the invention, red and green colour separation prints of the original are produced together with a print representing the combined effect of the three colours. The basic image is produced from this third print by a direct photographic process, and the correction image is produced by the simultaneous pointby-point scanning of all three prints, the scanning being effected by a conventional cathoderay tube flying-spot scanner and photo-electric cells. The cell output signals are applied to a circuit in which a blue component signal is first derived by subtraction of the red and green signals from the third print signal, and then the greatest of the red, green and blue signals is selected and its ratio with the sum of the signals determined. The resulting signal then controls a synchronously scanned cathode-ray tube printing the correction image. In a second way of carrying out the invention, the process starts with red, green and blue separation prints, the basic image being formed by successive printing from these prints, and the correction print is formed as before. In a third way the process starts from a normal colour film of the original. The basic image is obtained by printing from this film with three suitable lights of different colour, and the correction image is obtained by scanning the film with a flying-spot scanner whilst viewing it with three photo-electric cells equipped with colour component separation filters. Reference is made to a method of determining the density of the correction image directly from the position that the colour of an image point occupies in a colour co-ordinate system. The photo-electric cell signals are first normalized and then applied to deflect a cathode-ray tube in such manner that the spot occupies a position indicating the colour in a co-ordinate system. A mask having a distribution of densities related to the co-ordinate system and of magnitudes determining the correction densities is then placed in front of the tube screen and a photo-electric cell employed to derive from the transmitted light a signal for printing the correction image. Specification 737,212 also is referred to.