Patent GB-761416-A (GB)
Title: Colour-correction systems
IPC: H04N1/60
Filed: 1954-09-14
Abstract: 761,416. Photo-electric colour printing control. RADIO CORPORATION OF AMERICA. Sept. 14, 1954 [Oct. 14, 1953], No. 26591/54. Class 40 (3). [Also in Group XL (c)] The invention relates to colour reproduction printing processes in which the subject is separated photographically into three or more negatives or " separations " representing the red. green and blue, and in which positives are made from the separation negatives and dissected by the half-tone process into negatives composed of dots which are then used to contiol the etching of coated copper plates. Because of the overlapping and incomplete transmission characteristics of the ink colour correction of the negatives has been introduced by known computers which scan the negative and introduce a colour compensation signal into the output of a light-sensitive device, these signals being used to control the production of a colourcorrected positive. The present invention provides for predistorting the signals so that the final printed sheet shall be free of faults introduced by non-linearity in the various printing processes. As shown (Fig. 1) three transparent colour separation positives 10, 12, 14 prepared from negatives exposed through red, green and blue filters are scanned by a flying spot scanner comprising a cathode-ray tube 16. A lens system 18 images the scanning light spot on corresponding areas of each separation. The light passing through each separation is directed to one of associated photo-tubes 20, 22, 24. The signal outputs of these photo-tubes are applied to a colour correction computer 26 which may be as described in U.S.A. Specification 2,434,561 and which calculates the ink-dot sizes of each of the four colours to be printed. One of the computer outputs is selected by a switch 28 and applied to a tone compensator 30 which modifies the signals as previously explained. The signals from the tone compensator modulate the light intensity of a cathode-ray beam exposing tube 34 the time bases of which are operated in synchronism with those of tube 16 and which exposes a colour corrected and tone compensated negative 40. Three such negatives are made each corresponding to one of the separation positives. Colour corrected positives 42 are then prepared and used in the halftone process to etch photo-engraving plates. In Fig. 6 line " A " represents the relation between the ink dots as computed by the colour correction circuit and the ink dots as compensated by circuit 30. If this compensation is applied the final printed sheet will be free of the various dentations introduced by the photographic and printing processes. Tone compensation circuit.-A suitable circuit for modifying the outputs of the colour correction computer in accordance with the curve A is shown in Fig. 5. It comprises four diodes 52, 60, 68 and 82 of which 52 is initially conducting and 60, 68 and 82 are initially cut off and arranged to conduct at different values of the input signal from a pre-amplifier 50 receiving the output of the colour correction computer. For an input voltage range of from 0 to 5 per cent of the ink dot values diode 52 only conducts. The voltage change across load resistor 54 is relatively steep in their range. When the amplifier output becomes 5 per cent diode 52 cuts off while a positive bias on the cathode of diode 60 provided by biasing resistors 56, 58 is overcome. Diode 60 then conducts. A summing resistor 92 is connected to a tap on the anode load of diode 60, so that the voltage charge at the output due to conduction in the diode is less steep. When the 'input voltage corresponds to 20 per cent the voltage at the junction of anode loads 62, 64 becomes sufficiently negative and diode 68 conducts shunting the anode load of diode 60. The voltage change across summing resistor 92 is again reduced in slope. At 95 per cent the positive bias on diode 82 is overcome and it conducts. A part of the voltage change across anode load 84 is applied to summing resistor 94. The individual effects of the conductivities of the diodes are shown in Fig. 6 and the summing resistors 90, 92, 94 are so connected together and to an amplifier 98 that the total effect is to produce a modification of the output of amplifier 50 shown by the curve A. By means of switches 70, 84 and 96 curve B may be produced, which is appropriate to a slightly different printing process.