Patent GB-761535-A (GB)
Title: Method and apparatus for making colour separation negatives for four colour reproductions
Inventor: HALL VINCENT C
IPC: H04N1/60
Filed: 1952-06-26
Abstract: 761,535. Copying in colour by scanning. TIME, Inc. June 26, 1952, No. 16119/52. Class 40 (3). In a method of making a four-colour reproduction of a coloured original three electrical signals generated by scanning the original and representing respectively the variations of the three primary colour components are electronically modified so that three colour separation negatives respectively exposed in accordance with the three modified signals represent for each area of the original in which a black tone appears, a portion of the black at that area, and the remainder of the black at the area is provided by a black printer. The arrangement may be such that the black printer is controlled according to the minimum value of the colour components with consequent reduction of the other colour components involved. In terms of the electrical signals produced by the scanning, the black printer is controlled by the maximum colour signal and the signals controlling the other colour components are increased, producing darker associated negatives and less colour in the printing process. In the circuit arrangement, Figs. 3, 4, the signals derived from scanning the original are applied over pairs of conductors 11-11, 12-12 and 13-13 to modulators 16, 16a, 16b which may be of the type disclosed in U.S.A. Specification 2,548,901 and providing a linear relationship between output and input. The output of the modulator is applied via a valve 25 to a full-wave rectifier 40 whose output is fed via a biased diode 48 to a final valve 53 controlling the glow-discharge tube 27 which is used to produce the yellow negative 28. The diode 48 is employed to limit the signal to the maximum transmission signal, i.e. the signal corresponding to the absence of a colour component and is biased by a voltage regulator 48<1>. The signals from the conductors 11, 12, 13 are fed by conductors 65, 66, 67 to amplifiers 68, 68a, 68b for the respective colours. For the yellow channel the output of the amplifier 68 is fed to a rectifier system 69 comprising two full-wave rectifiers 79, 82. The three outputs of rectifiers 82, 82a, 82b are fed in parallel to a compressor network 85, which may be of the type described in U.S.A. Specification 2,581,124, whose input at any time is the maximum signal from the three channels and whose output is arranged to be 1 - 1/input. The output is fed via connection 86 to the second grid 90 of the valve 15 of the modulator 16 and corresponding valves of the modulators 16a, 16b of the three channels. The modulator 16 is arranged to have an output proportional to the product of the input to the grid 14 and 1 - input to grid 90, so that the output will, in effect, be inversely proportional to the maximum colour signal and directly proportional to the colour component signal. The maximum colour signal which is available at the cathodes of the rectifiers 79 is fed to the grid of a valve controlling a glow-discharge tube 95 which reproduces the negative for the black printer. In the arrangement as described, the black at any area is wholly provided by the black printer, and the circuit is modified so that the black is partly reproduced by equal amounts of the three subtractive colours and partly by the black printer. The compressor is modified so that its output is 1 - 1/[I(colour max)]<n>, where n is less than 1, and the networks in the control circuits 26, 26a,- 26b limit the glow lamp currents to (Imax.)<n>. Also the signal applied to the glow lamp 95 controlling the black printer 96 is preferably reduced by a compressor network similar to the network 85. Alternatively, the decrease in the density of the black printer may be obtained by underdeveloping the black negative. The circuit modification, Fig. 5 (not shown), corresponds to a simplified circuit arrangement in the special case of n = 0.5 in which the final corrected signal of maximum value can be directly employed to control the black printer to produce the negative of modified density.