Patent GB-761342-A (GB)
Title: Improvements in electro-optical colour reproduction
IPC: G03F3/08
Filed: 1952-06-26
Abstract: 761,342. Copying in colour by scanning. EASTMAN KODAK CO. June 26, 1952 [June 27, 1951], No. 16052/52. Class 40 (3). In an electro-optical colour reproduction process in which a first primary colour signal is colour corrected by a correcting signal derived from a second primary colour signal, the correction factor, which determines the value of the correcting signal, varies between limits Kl, K2 and is arranged to increase with increasing signal value of the first-mentioned primary colour and to decrease continuously with increasing signal value the other primary colour. In such an arrangement the correction factor is variable within the prescribed limits and produces a correcting signal which varies according to the relative amounts of the colours occurring at the area to be reproduced. The limits K1, K2 pertaining to the correction of one primary colour by another are obtained by trial and error adjustments at the appropriate stages of the circuits producing the correcting signals for the colours concerned, and by obtaining photoelectric responses through appropriate filters from patches of the colours with the dyes or inks on the paper to be used for reproduction. In the case, for example, of the correction of the green signal by the red signal white and black patches are employed in addition to patches of "full red " derived by yellow and magenta and " full green " obtained by cyan and yellow. The correcting signal is adjusted at the stage of the application of the correcting factor so that the green signal from green is the same as the green signal from white, defining the limit K2 for the correction factor. The apparatus is then adjusted so that the green signal from the full red is equal to the green signal from the full black to determine the value of the factor K1. The correction factor may vary between the limits K1 Œ “ (K2-K1) and K2 Œ “ (K2-K1) instead of the limits K1, K2. The corrections of one colour by another are rendered necessary by a number of factors, in. cluding the imperfections of the inks as regards their reflections and absorptions, the nature of the paper on which the inks are laid down and various effects resulting from the successive applications of the inks of different colour and producing deviations from the additive law of colour densities. In the generally known arrangement, Fig. 1, a multi-coloured original on a transparent drum 10 is scanned by a chopped beam from a source 12 focused on a spot 15 and projected via lenses 17, 20 on to mirrors 25 associated with colour filters 26 and photo-cells 27 feeding an electronic circuit E. Unsharp masking signals may simultaneously be obtained by a half-silvered mirror 31 in conjunction with a relatively large aperture in a diaphragm 32 and mirrors 35 associated with colour filters 36 and cells 37 also connected to the electronic control circuit E. The colour corrected signals are applied to glow lamps 40 producing three colour negatives, and a fourth lamp 40 may be employed to produce a blackprinter negative with corresponding modification of the signals generated to produce the three colour negatives. The correcting signal may, in lieu of producing a modified electric signal, control a shutter 46 to produce the modified or corrected light-beam incident on the associated colour negative. In the diagrammatic arrangement, Fig. 2, green and red signals occurring on leads 50, 51 are fed partly over circuits 52, 53 to a circuit 54 producing division so that the output over lead 55 is effectively G/R, and this is arranged to act as a multiplying factor in the circuit 56 which is arranged to modify the red signal exponentially. The amplifier 58 is therefore fed with the green signal and a correcting signal having a factor which increases with the green component and decreases with increasing red component. The electrical circuit is adjusted preferably at some point in the circuit 54 so that the limits K1, K2 are achieved for the conditions specified. In a modification, Fig. 3 (not shown), the signal output of the modifying circuit; such as 56 of Fig. 2, is applied to a control shutter operating in conjunction with a glow lamp operated by the amplified green signals. A further modification, Fig. 4 (not shown) produces a more complicated correcting signal involving G/R and also a factor including the differences of the logarithms of the numerical values of the G and R signals. In the circuit arrangement, Fig. 5, corresponding to the diagrammatic system of Fig. 2, the red signal on lead 51 is amplified in the pentode 101, rectified in tube 102 and is applied via potentiometer 103 with such polarity to the first grid of a pentode 100 that this grid is more negative with increasing red signal. The green signal is applied over conductor 52 and potentiometer 104 to a further grid of the valve 100 so that its output is proportional to G/R. The red signal is amplified by a valve 105 of which the output is fed via a rectifier 106 to a non-linear network which provides an exponential function of the input and comprises thyrite resistors 107, 108 in series with a potentiometer 109 and potentiometers 110, 111 in series with a thyrite resistor 112 and a variable resistor 114. The modified red signal, derived from the resistor 116, is applied inverted to the first control grid of the valve 117, to the third grid of which the amplified green signal is applied over conductor 50. The settings of the potentiometers 109, 110, 111 are determined by the output of the valve 100 controlling a motor 115 and the output of the network may be proportional to G R raised to the power -. The potentiometers R 103, 104 are initially set by trial and error to determine the constants K2, K1 experimentally. A further circuit, Fig. 6 (not shown), is generally similar to Fig. 5 and comprises the modification in which the correcting signal is applied to a light valve, as in the diagrammatic modification, Fig. 3 (not shown). Figs. 7, 8 (not shown) comprise modifications in which the correction signal is a function of G-R and also to an alternative circuit for obtaining a correcting signal involving the red signal R raised to the power of the correcting factor G/R. Additional circuits, Figs. 10, 11 (not shown), produce corrective signals involving the differences of log G and log R and R raised to the power of G/R. Specifications 638,012, 552,367, 561,844, [all in Group XL], 712,499 and U.S.A. Specification 2,517,586 are referred to.