Patent GB-912553-A (GB)
Title: Improvements in or relating to colour reproduction
Inventor: ALLEN GORDON STANLEY JAMES
IPC: G03F3/08
Filed: 1960-02-03
Abstract: 912,553. Photo-electric scanning arrangements for use in colour reproduction. CROSFIELD ELECTRONICS Ltd. Feb. 2, 1961 [Feb. 3, 1960], No. 3882/60. Class 40 (3). In the reproduction of coloured originals utilizing an electro-optical scanner a corrected colour printer or a corrected black printer is produced by : (1) subtracting a signal representing the colour or black-printer to be corrected from a signal for a correcting colour; (2) passing the difference signal through a non-linear circuit which attenuates at least the difference signals of a polarity representing an excess of the colour or black-printer signal over the correcting colour signal; and (3) exposing a light-sensitive layer in accordance with the colour or black printer values to be corrected modified in accordance with both the signal from the non- linear circuit and the signal for the correcting colour. Fig. 1 shows an arrangement for producing a corrected yellow printer in which red, blue and green separation negatives 8R, 8B, 8G are scanned by a cathode-ray tube flying-spot scanner 6 to generate corresponding component colour signals in photo-electric cells 18R, 18B, 18G, the light-sensitive film or plate 10 which is to provide the yellow printer being placed in contact with the blue negative 8B. In accordance with the invention, the blue signal is reversed in inverter 20 and combined with red and green signals in mixers 22 and 24 respectively to obtain the difference signals R-B, G-B. The difference signals are then applied to blocking circuits 30, 32 which are arranged to pass only positive signals, thus eliminating signals corresponding to any excess of B over R and G respectively. The resultant signals pass to a mixer 34 where they combine with the original R and G signals to provide a masking correction signal for application to grid 7 of tube 6. 26 and 28 are shaft controls which serve to adjust the D.C. level of the difference signals so that the blocking circuits, Fig. 2 (not shown), which have fixed bias, may be used to pass only a controlled portion of positive difference signals. 36 and 38 are gain controls. In the case of a magenta printer, apparatus is arranged as shown in Fig. 3 where correction is effected only for the cyan content. The " green "  signal is inverted at 20, subtracted from the " red " signal in mixer 22 and only the positive difference selected by blocking circuit 30. The resultant signal is then combined in mixer 34 with both the " red " and " green " signals to obtain the correction signal. Apparatus for producing a corrected black printer is arranged as shown in Fig. 4. A black printer negative, obtained by exposure to white light through the original or by exposure to the original through red, green and blue filters in succession, is placed in the central position in contact with a light-sensitive film or plate which is to provide the printer, and cyan and yellow separation negatives are placed in the outer positions. Corresponding signals are derived by photo-electric cells 18R, 18BK and 18B. The " black " signal is inverted at 64 and combined in a mixer 66 with the larger of the " red " and " blue " signals as determined by a maximum signal selector 62. The output of the mixer comprising R-BK or B-BK is applied to blocking circuit 70 which passes only the positive difference. The final signal is then combined with the " red " and " blue " signals in mixer 72 to produce the correction signal. A suitable maximum signal selector circuit is described with reference to Fig. 5 (not shown). Reference is made to: (1) an arrangement employing separate scanners for the individual separation negatives; (2) an arrangement for utilizing the original transparency directly, the transmitted light being analysed into its colour components by filters; and (3) an arrangement in which signals representing the colour component to be corrected together with the correction signal are applied to modulate a light source to which is exposed the plate or film to provide the colour printer. The cathode-ray tube modulating signal is preferably applied through a gate circuit controlled by a square wave as described in Specification 835,111 so as to cause the production of alternating signals in the photo-electric cells.