Patent GB-674025-A (GB)
Title: Improvements in or relating to colour printing
IPC: H04N1/60
Filed: 1947-08-05
Abstract: 674,025. Electric analogue calculating systems. INTERCHEMICAL CORPORATION. Aug. 5, 1947 [July 8, 1944], No. 21196/47. Class 37. [Also in Group XL (b)] In a method of making corrected colourseparation images for reproduction of a coloured original by superimposed prints from dotstructured plates, the electrical signals representing the different spectral components of the coloured original are modified by one another, and the effect of each of the overprint combinations of the reproduction inks is taken into account in controlling the extent of such modification. In other words the amounts of the different colour components which are utilized to reproduce the elemental areas of the original are modified to take account of the variation introduced by the amounts of superposed colours at the elemental areas. The method is stated to depend on the solution of a number of equations proposed by Neugebauer (Zeitschrift fur Tech. Physik, 36, 22-89, 1937) in which the colour components of an elemental area are expressed algebraically as the sum of component terms which are functions of the proportional areas of the component colours and multiplied by the spectral colour components of the individual colours and of the various overprint combinations. In a threecolour process with three unknown variables the equations are theoretically capable of solution, but in the case of a four-colour process, i.e. three colours and a black printerthe equations are theoretically incapable of solution. With the employment of a black printer one of the component colour areas may be taken to be zero since the black area can be regarded as the superimposition of the three colours at that area. In other words, the area of superpositioning of the three colours is replaced by the black area, and the area component value of one of the colours is, in that case, assumed to be zero. Normally it will be the colour component of smallest areal value. The equations are solved by electrical computing apparatus in which the spectral components of the component colours, of the overprint combinations of the paper on which printing is to take place, and of the black ink are set into the apparatus and the operation is such that after the component values of the elemental area have been fed into the apparatus, it operates to adjust automatically the values of the variables so that at least one defining the requisite area of one of the colour components is reduced substantially to zero. Before the apparatus is used for deriving the signals to produce the separate plates, the correct spectral compounds of the colours and overprint combinaations have to be derived and this is effected by passing light from three sources through colour separation positives made by three filters from a chart comprising separate sections representing the paper, the colours of the individual inks, of the overprint combinations and black and feeding in turn the appropriate signals for each of the sections to the calculator, Fig. 3A, so that for the white section the knobs Xw, Yw, Zw are adjusted so that with X=X1, Y=Y1, Z=Z1, c=m=y=o. The colour chart is then moved so that light from the three sources passes through the section Pc to the photo-cells and the knobs Xc, Yc, Zc are adjusted so that with X=X1, Y=Yl, Z=Z1, c=l, m=y=0, and so on for the various sections of the colour chart. The computing apparatus is shown diagrammatically in Fig. 3 in which the tristimulus values X, Y, Z of the component colour are fed into subtractors which are fed with values X1, Y1, Z1, respectively from three computing channels, the outputs X=Xl, Y=Y1, Z=Z1 being fed through amplifiers HGA, the outputs from the amplifiers being used as initial values of c,m,y. The values of c,m,y are also fed to a selector from which the least value is fed as output to a fourth amplifier which passes this signal as the value of n, the adjusting process being effected automatically until one of the values c,m,y is substantially zero. The values are then fed by a selector switch to recording printers embodying means for engraving separately the dot-structured plates corresponding to the component colours and the black plate. The computing channels are shown diagrammatically in greater detail in Fig. 4 in which the circuit connections to generate the nine component terms of each of the expressions X1, Y1, Z1 are indicated. The variables y,c,m,n and their inversesl-y, 1-c, 1-m, 1-n are converted to appropriate square wave form and are appropriately applied to multiplying apparatus to produce the products indicated in the squares, these products being multiplied by the appropriate spectral component values Xw, Xc... before being added together to form the components X1, Y1, Z1. Electrical multiplying circuits: The multiplying arrangement is based on the assumption that if an event a occurs for a per cent of a time period, and if an event b occurs for b per cent of the time, the probability of a,b occurring simultaneously is represented by the product ab. In employing such an arrangement the values to be multiplied are applied as voltage values which determine the relative oscillating and non-oscillating periods of a frequency generating circuit in a fixed time interval, and the trains of waves generated by different voltage values are applied to a circuit including a valve which is triggered only when the two waves occur simultaneously, the integrated current time output of the valve being a measure of the product of the quantities. The frequencies used must be irrationally related and the output circuit of the valve includes a filter which eliminates the individual frequencies and the beat frequencies. This subject-matter is included in Specification 674,116. Specifications 506,258, [Group XX], 506,259, [Group XL], and U.S.A. Specifications 1,631,963, 2,136,340 and 2,185,139 also are referred to.