Patent GB-827865-A (GB)
Title: Improvements in or relating to electro-optical systems for four colour reproduction
IPC: H04N1/60
Filed: 1956-06-04
Abstract: 827,865. Copying in colour by scanning. TIME Inc. June 4, 1956 [June 15, 1955], No. 17222/56. Class 40 (3). In an arrangement for making a coloured reproduction of an original subject by employing light beams to derive from elemental areas of the original three colour component signals which are subjected to colour correction and utilized to generate a black electrical signal corresponding to the colour-corrected signal of relatively greatest intensity, at least two of the colourcorrected signals are utilized to develop an additional electrical signal which is changed in amplitude by a factor dependent on the polarity of the relative difference in amplitude of the said two signals whereby the additional signal varies in amplitude as a function of the dominant wavelength of the colour of the area and which is utilized to modify the black signal so that its amplitude is partially a function of said dominant wavelength. The variation of the black signal is effective to change the density of black ink deposited in an ink print in relation to the hue of the region, for example a decrease as the dominant wavelength changes from the blue or bluish purple hue to that of the yellow or yellowish-red region. Diagrammatic arrangement, Fig. 2.- The blue, green and red colour component signals derived from a scanning arrangement 50 are fed to circuits 51, 51<1>, 51<11> whose outputs are fed to mask modulators 52, 52<1>, 52" and also to rectifiers 53, 53<1> and 53<11>, the outputs of which are fed back to their associated circuits 51, 511, 51<11> and over leads 54, 55, 56 to a black correction circuit 80. The output of the rectifier 53<1> controls the modulator 52, the rectifiers 53, 53<11> via circuit 57 control the mask modulator 52<1>, and the maximum signal of the rectifiers 53, 531, 53<11> is selected by circuit 58 to control the red mask modulator 52<11>. A black signal generator 70 selects from the outputs of the rectifiers 61, 62<1>, 61<11> fed over conductors 64, 65, 66 the maximum signal which is applied over conductor 71 to modulators 60, 601, 60<11> to vary the signals passing over rectifiers 75 ... and amplifiers 76 ... to yellow, magenta and cyan glow lamps 77, 771, 77<11> producing the negatives for the corresponding printing plates. The signal from the generator 70 is also passed over conductor 79 to a black correction circuit which also receives over conductors 54, 55, 56 uncorrected colour signals from the rectifiers 53, 531, 53<11> and over conductors 81, 82 colour corrected signals from rectifiers 61, 61<11> of the blue and red mask modulators 52, 52<11>. The corrected black signal from the circuit 80 is amplified at 85 and controls the black glow lamp 86 producing the negative for the plate of the black printer. Detailed circuit arrangement, Fig. 3.-The maximum signal over leads 64, 65, 66 is selected by three cathode-coupled valves 90, 90<1>, 90<11> and passed from resistor 91 and lead 79 to the cathode 156 of a valve 153 in the black-correction circuit 80. The uncorrected signals over conductors 54, 55, 56 are fed over resistors 114, 115, 116 of different values such that the signal at the junction 117 corresponds to 5%, 75% and 20% of the blue, green and red signals, and is termed the ortholuminous signal representing the integrated visual brightness of the colour. This signal derived at junction 121 from the cathode of valve 120 is combined at junction 100 with a percentage of the linear black signal derived from the valve 153. In the highlight region the ortholuminous signal, curve II, Fig. 4, will drop more rapidly than the linear black signal indicated by curve I so that the corrected black curve indicated at III will drop more rapidly in this region with a reduction of the density of the negative, and the deposition of more black ink with improved detail in this region. In the shadow region the amplitude of the ortholuminous signal is much reduced and a valve 130 conducts so that a percentage of the increased anode voltage is applied via network 140, resistor 145 and switch 146 in the closed position to junction point 102 so that the resultant black signal, curve III, has a higher rate of change in the shadow region than that provided by the linear black signal, curve I. As a result the amount of black ink laid down is relatively greater and the undesired colour toning in the shadow areas is avoided. The black signal applied over conductor 84 to the amplifier 85 is limited by a biased diode 103. For the correction of the linear black signal in respect of the dominant wavelength and purity, the corrected blue signal over conductor 64 is fed over lead 81 and resistor 150 to the grid 152 of the valve 153. The corrected red signal over conductor 66 is applied to the grid 152 via lead 82, rectifier 154 and resistor 155, and the arrangement is such that a signal is applied to the grid 152 only when the red signal, weighted by resistor 155, is greater than the blue signal weighted by resistor 150. If, for example, the colour scanned is yellow, the signal on blue lead 64 is small, whilst those on leads 65, 66 (red, green) are high. The voltage on the cathode 156 will be high, and since the red signal is high and the blue signal is low the voltage on the grid 152 will be lowered and the valve 153 cut off so that the voltage at the plate 157 is high and a positive voltage will be passed via resistor 159 to junction point 98 to add an increment to the linear black signal. Similarly in the case of a red signal, an increment is added to the linear black signal. If the original colour is blue the signal on 64 is high and those on 65, 66 are low. The voltage on the grid 152 will be the same as that on the cathode, the valve 153 will conduct and no increment is added to the linear black signal. In the case of a purple colour on the original, the blue and red signals will be high, the grid voltage will be greater than that of the cathode of the valve 153 which conducts and no increment is added to the linear black signal. If the original signal is neutral, i.e. black, grey or white, so that the entire colour content is achromatic, and the purity is 0%, the signals on the leads 64, 65, 66 are all equal. The triode conducts and the linear black signal is not modified. The increase of the black signal in the yellow or yellowish red areas results in a decrease, as compared with blue and bluish-purple areas, in the amount of ink laid down when these areas are reproduced. In response to the change, however, of the purity from a high value to a low value, the signal from the triode 153 decreases, with reduced effect on the amount of black ink laid down in the print, so that at 0% purity, i.e. no chromatic content, the black signal is unmodified. Specifications 761,535 and 765,443 are referred to.