Patent GB-934108-A (GB)
Title: Improvements in or relating to colour reproduction
Inventor: ALLEN GORDON STANLEY JAMES
IPC: H04N1/60
Filed: 1960-02-03
Abstract: 934,108. Photo-electric scanning apparatus for use in colour reproduction. CROSFIELD ELECTRONICS Ltd. Feb. 2, 1961 [Feb. 3, 1960], No. 38861/60. Class 40 (3). Relates to the reproduction of coloured originals utilizing an electro-optical scanner, and is concerned with the production of corrected colour printers and corrected black printers. Fig. 1 shows a first arrangement in which red, blue and green separation negatives 8R, 8B, 8G are scanned by a cathode-ray tube flying-spot scanner 6 and corresponding component colour signals are derived in photoelectric cells 18R, 18B, 18G. As illustrated, the arrangement is set up to produce a yellow printer by exposing a light-sensitive film or plate 10 to negative 8B. The " blue " signal is inverted in stage 20 and combined with the " red " and " green " signals in mixer 22, 24 so as to develop the difference signals R-B, G-B. In accordance with the invention, maximum selectors 30, 32 derive respectively the larger of R and G-B, and G and R-B. The resulting signals are combined in mixer 38 and applied to the modulating grid 7 of scanner 6 to effect masking correction. Gain controls 26, 28, 34, 36 permit predetermined proportions of the signals to be selected. In a modification, mixer 22, gain control 26 and maximum selector 32 may be dispensed with and the " green " signal supplied directly to mixer 38. The same effect may be obtained by setting gain control 26 to the " zero " setting. The apparatus may be used for producing corrected cyan and magenta printers by using the appropriate negative to be corrected in the central (printing) position and the remaining two negatives in the outer positions. A maximum signal selector may replace mixer 38 or a single maximum signal selector may replace maximum selectors 30, 32 and mixer 38. For producing a corrected black printer, an uncorrected black printer negative, produced, for example, by successively exposing the original through red, green and blue filters to white light, is used in the central (printing position) and two colour separation negatives, usually yellow and cyan, are placed in the outer positions. Fig. 3 shows a simplified arrangement in which a single maximum signal selector derives the larger of the red and green signal, i.e. the signal whose amplitude is closest to the amplitude for " black," for application over lead 52 to effect correction masking of the yellow printer. To produce a black printer a black printer negative is used in the central position and yellow and cyan negatives in the outer positions. If desired, a fourth position may be provided for the magenta negative and the maximum signal selector arranged for selection of the largest of the three colour component signals. Reference is made to the use of separate sources for scanning the individual negatives, the use of the original signal representing the colour to be corrected together with the correction signal to modulate a separate light source for exposing the plate as film on which the converted printer is to be produced, and the use of the arrangement described in Specification 835,111 where the cathode-ray tube is modulated by a pulse source 50 to cause the production of alternating signals by the photoelectric devices. A suitable maximum signal selector circuit is described with reference to Fig. 2 (not shown).