Patent GB-737768-A (GB)
Title: Improvements relating to colour printing
Inventor: ALLEN GORDON STANLEY JAMES
IPC: G03F3/08
Filed: 1953-08-17
Abstract: 737,768. Copying in colour by scanning. CROSFIELD, Ltd., J. F. Sept. 1, 1954 [Aug. 17. 1953], No. 22628/53. Class 40 (3). Relates to apparatus and a method for producing corrected colour separation prints for use in multi-colour reproduction in which a photosensitive surface is exposed to an uncorrected print or a colour transparency through a filter. The print or transparency is scanned by a moving spot of light from a source and light abstracted from the beam of light and passing through uncorrected separation prints of each colour, or passing through the transparency and filters of each colour, acts on photo-electric cells to provide signals for colour correction which are fed to a computer controlling the intensity of the light source. According to the invention, the signals fed from the photoelectric cells to the computer are controlled so as to be substantially independent of the intensity of the light source by means of a signal derived either by a further photo-electric cell receiving light directly from the source or from the controlling signals for the source provided by the computer. Fig. 1 illustrates the invention as applied to the production of corrected magenta, yellow and cyan prints together with a black print from uncorrected red, green and blue separation negatives. The prints are produced one at a time and the Figure shows the apparatus set up to produce the magenta print. The scanning spot is produced by a cathode-ray tube 2 the beam of which is controlled to produce a conventional raster 3 and the light from which is projected through a red separation negative 12R on to a sensitized plate on which the corrected cyan print is to be produced as a positive. Located within the light beam are partially silvered mirrors 20, 21, 23 which reflect light to photo-electric cells 22R, 22B, 22G, the light paths to cells 22B and 22G including blue and green separation negatives 12B, 12E, whilst the light to cell 22R passes through the red separation negative 12R. A further partially silvered mirror 24 reflects light to a photo-electric cell 27 for use when producing the black print (see below). The outputs from the photo-electric cells are employed for colour correction and before applying them to the necessary circuits they are, in accordance with the invention, first rendered independent of the intensity of the light source. To this end a further photo-electric cell 32 receives light directly from the scanning raster and provides a signal which is effectively divided into the signals from the remaining cells to cancel the intensity factor. This is effected by first deriving the logarithms of the signals in circuits 31 and 34 and then subtracting the intensity signal from each of the correction signals in mixing circuits 35. The resulting signals are proportional to the logarithms of the transmission factors of the negatives for the three colours, i.e. green, blue and red, but since the inverse densities of the negatives are also proportional to the logarithms of the transmission factors, the signals are directly proportional to the densities of the complementary colours (i.e. magenta, yellow and cyan) to be produced. " Masking " of each signal with respect to the other is effected by means of circuits 36, 37, and the signals are then applied to a circuit which provides a black signal equal to the smallest of the colour signals and also reduces each colour signal by the amount of the black signal. The resulting four signals are applied first to non-linear circuits 39 which reduce intensity over the middle range and accentuate highlights and shadows, and then to further non-linear circuits 40 which correct for the characteristics of the cathode-ray tube. Finally the signals are selected one at a time by switch 41 and applied to the grid 8 of the cathode-ray tube to control the intensity of the scanning spot. Where the apparatus is set up as described with plate 16 exposed through the red separation negative, switch 41 is set to select the output from the cyan channel. The yellow and magenta printers are prepared in a corresponding manner, the blue and green separation negative being placed in turn at position 12R and the other two occupying positions 12B and 12C whilst switch 41 selects the appropriate correction signal. For preparation of the black printer, a black separation negative is placed in position 12R and the three-colour separation negatives placed in positions 12B, 12G and 25. Cell 27 is brought into use and the output of cell 22R disconnected. As an alternative to effecting the intensity cancellation by a signal from cell 32, a signal may be derived from that applied to grid 8 of the cathode-ray tube. Again, instead of being divided logarithmically into the correction signals, the signal may be used to control reciprocally the gains of amplifiers through which the correction signals are fed. In apparatus for producing the corrected prints direct from a colour transparency, Fig. 2 (not shown), the transparency is inserted in the light path before three partiallysilvered mirrors which reflect light to photoelectric cells through red, green and blue filters. Following the mirrors the light projects on to the sensitized plate on which the corrected print is to be produced and an appropriately coloured filter is inserted in the light path as each print is prepared. No filter is required when producing the black print. An arrangement of dichroic mirrors may replace the partially-silvered mirrors and the filters in front of the cells omitted.