Patent GB-771249-A (GB)
Title: Apparatus for use in reproducing multi-coloured originals
IPC: H01J17/00
Filed: 1954-06-30
Abstract: 771,249. Copying in colour by scanning. RADIO CORPORATION OF AMERICA. June 30, 1954 [July 30, 1953; July 30, 1953], No. 19184/54. Class 40 (3). An arrangement for providing a signal representing an achromatic component of a subject to'be reproduced in colour from signals representing component colours of an elemental area of the subject, comprises a circuit which produces a signal representative of the brightness of the elemental areas and a circuit producing from the colour-component signals applied thereto a second signal solely when signals representative of all the colour components are present and each attains a predetermined respective value, and a circuit for producing conjointly from said first and second signals the signal representing said achromatic component. The arrangement is employed in making reproductions of an original in colour by the use of three coloured printers and a black printer, the control of which is determined conjointly by the brightness of the area to be reproduced, and by the amounts of the individual colour components required at the area. In the diagrammatic arrangement, Fig. 1, the red, green and blue signals from the scanning of three colour representations 10, 12, 14 of the original are fed to a computer 22 producing signals c (cyan), m (magenta) and y (yellow) applied to an adding circuit 32 and a limiter circuit 34 which passes to a gating circuit 36 a signal corresponding to the excess of the combined signals over a predetermined value. The c, m, y signals are also passed to limiting circuits 38, 40, 42 and to a further summation circuit 44 by which an output signal of predetermined value is applied to the gating circuit 36 solely if signals above a predetermined minimum are applied to each of the limiters 38, 40, 42. If the gate circuit 36 is opened by the output from the device 44, the signal from the limiter 34 is passed to an amplifier 46, the output of which determines the operation of the black printer. This signal output is also fed back to the computer 22 to vary the signals c, m, y which are applied via contacts 48, 50, 52 of a switch 56 to operate a light-modulating device 58 to produce three colour-corrected negatives. The output of the amplifier 46 is passed via contact 54 of the switch 56 and the device 58 to operate the black printer. Detailed circuit arrangement. The outputs at the cathodes of valves 60, 62, 64 fed with the signals c, m, y are applied to adding resistors 66, 68, 70 and to a valve 72, the output of which is applied to a diode limiter 74 adjusted to a voltage corresponding to a predetermined brightness level by a potentiometer 76. The signal above a predetermined value is applied to the grid 78 of a gating valve 80. The signals c, m, y are also applied over resistors 88, 90, 92 to bottom-limiter diodes 82, 84, 86 so that if the signals in all the channels are at or above a predetermined value a corresponding signal is applied by the adding resistors 100, 102, 104 to the valve 106 which applies a pulse to a gating grid 108 of the valve 80 which conducts, and the signal applied to the grid 78 produces a corresponding amplified black signal, n from the anode circuit of the valve 112. In a modification, Fig. 3 (not shown), the signals from the c, m, y channels are applied to three limiters which place predetermined upper limits on the individual signals applied in Fig. 1 to the summation circuit 32 and subsequently to the limiting circuit 34 and to the gate circuit 36. The corresponding detailed circuit arrangement is described in connection with Fig. 4 (not shown). As in the prior arrangement, Figs. 1, 2, the achromatic signal is produced solely if the three colour-component signals are all present and up to or above a predetermined minimum value, and also if the combined signals exceed a predetermined value. In the modified arrangement, however, upper limits are prescribed for the component signals so that the signal for the block printer cannot exceed a predetermined value, and the maximum amount of black ink printed at any area is correspondingly restricted.