Patent GB-921290-A (GB)
Title: Improvements in or relating to color mixture computer
IPC: G06G7/75
Filed: 1960-02-29
Abstract: 921,290. Electric analogue calculating. H. R. DAVIDSON. Feb. 29, 1960, No.7078/60. Class 37. For a colour reflectance or transmittance computer it is postulated that, for a colour to be matched: (1) F(R)=aF(R A )+bF(R B )+cF(R c ) +dF(R D ), where R A , R B , R c , R D  represent the reflectances or transmittances of four component colorants at a specified wavelength of light on a given medium or substrate, R represents the reflectance or transmittance of a mixture of colorants in proportions a, b, c, d, and F is a function which is additive in the colour   mixture, given generally by (2) F(R)=log 1/R     for light transmission and (3) F(R)=(1-R)<SP>2</SP>/2R for light reflection. Other functional laws are admissible. In Fig. 2 similar colorant circuits 10, 11, 12, 13 comprise potentiometers 15, 16, 17, 18 energized from constant voltage sources to develop analogue voltages of coefficients a, b, c, d to supply respective parallel resistance networks 19 1  to 19 10 , 20 1  to 20 10 , 21 1  to 21 10  and 22 1  to 22 10 , each corresponding to a given wavelength of light and having a law corresponding to function F(R) for each individual colorant, and variably tapped to derive voltages representing the respective components aF(R A ) &c. of equation (1) which are series summed over ganged selector switches 23 to 26 for each predetermined wavelength of light. A further circuit 14 energized from a constant source comprises variablytapped parallel resistances 28 to 28 10  of predetermined law and produces voltages representing F(R) for a colour to be matched; the tap settings being determined by equations (2) or (3) from measurements of the colour reflectance or transmittance at the several wavelengths of light and selecting the appropriate voltage by switch 29 ganged to switches 23 to 26 for connection in series opposition with the summed voltages of circuits 10 to 13 to the vertical plates of a cathode-ray tube whose horizontal plates are incrementally biased from a constant voltage excited potentiometer 36 linearly tapped by switch 37 ganged with switches 23 to 29 so that for each wavelength the requisite difference may be displayed as a spot deflected from zero in appropriate series (Figs. 3, 4, not shown). Manual adjustment of the several potentiometers of circuits 10 to 13 to restore each spot in turn to zero enable the proportions and reflectances or transmittances of the component colours to be determined for accurate colour matching. In a modification (Fig. 6) for comparing the transmittance or reflectance of a batch of mixed colorants against the colour to be matched a circuit 38 similar to 14 inserted by opening switch 39 may be adjusted to supply a voltage representing F(R) for the mixture computed as for circuit 14, selected for each wavelength of light by switch 41 ganged to the other switches and applied in opposition to the voltage from circuit 14; the colorant circuits 10 to 13 being first zeroized and subsequently adjusted to restore the C.R.T. deflecting to zero with polarity reversal as necessary by switches 42, 43, 44, 45 in the colorant circuit inputs; the proportions of each individual colorant to be added or subtracted for correct matching being thus determined from the settings of circuits 10 to 13. Electronic switching for different wavelengths may replace mechanical switching. U.S.A. Specification 2,631,609 is referred to.