Patent GB-970129-A (GB)
Title: Improvements in or relating to a tristimulus difference computer
IPC: G01J3/50
Filed: 1961-10-19
Abstract: 970,129. Analogue computers. H. R. DAVIDSON. Oct. 19,1961 [June 1,1961], No. 37565/61. Heading G4G. A computer for determining the amounts of colourants in a mixture required to match a colour standard where there is a difference (#K/S i ) (where K is coefficient of absorption and S i    the coefficient of scatter) between the spectrophometric curves of the colour standard and the mixture, includes means for producing colourant voltages corresponding to values of #K/S i  for different wavelengths i which are then converted into voltages representing the difference in tristimulus values #X, #Y, #Z between the tristimulus values X, Y, Z of the colour standard and those of the mixture, manual settings in the computer being adjusted until #X, #Y, #Z are zero which corresponds to the condition of best match, when the amounts of colourants for the mixture can be determined from the settings of the computer. The following equations are given in the Specification for X, Y, Z, where E c  is the spectral distribution of energy in daylight, #x, #y, #z, are the standard tristimulus functions of the eye, R is the reflectance of the colour standard, all of which are functions of the wavelength #, and 1, 2.. n refer to different wavelengths. The following relation is also derived in the Specification where #R i is the difference in reflectance between the colour standard and the colourant mixture at a wavelength i. The networks 10a, 10b, 10c & c. Fig. 1 give outputs corresponding to the K/S values for the individual colourants and various wavelengths and network 11 outputs corresponding to the K/S values for the colour standard at the same wavelengths. For each wavelength the individual colourant K/S output voltages are added and compared with the corresponding K/S colour standard output, the difference voltages at the various wavelengths #K/S i , #K/S 2 , ... #K/S n  being fed to tristimulus difference   circuits 12a, b, c.. n to convert them into voltages representing #X, #Y, #Z (in (3) above), so that when the readings on meters 14a,b, c. .n &c. are zero, the amounts of the various colourants necessary to satisfy this condition can be read from the computer settings. The #K/S i  values are converted into #R i  values in reflectance circuits 15a.. n Fig. 1a, and are then multiplied by (E c #X) i ,(E c #y) i , (E c #Z) i  terms in circuits 16a,.... 16n, 17a.... 17n , ... 18a.. 18n, the sums of the output voltages from said circuits corresponding to #X, #Y, #Z in accordance with (3) above. A circuit diagram is given (Fig. 2, not shown) where each network 10a 10b &c. comprises a potentiometer which can be set to correspond to the amount of a particular colourant in the mixture, resistances being set to the functions of the reflectance for each colourant all at different wavelengths. Potentiometers   set to values of K/S of the colour standard at the   same wavelengths are also provided and the subtraction necessary to derive #K/S i  values is carried out by transformers. These #K/S values are then fed to a circuit, Fig. 3 in which potentiometers 51 1 , 51 2 ... 51 n  are set to values proportional to -2R i <SP>2</SP>/1-R i <SP>2</SP> and the outputs from each of these potentiometers (proportional to #R 1 , in (7) above) is connected across potentiometers 52 1 , 52 2 ... 52 n ; 53 1 , 53 2 .. 53 n  and 54 1 , 54 2 ... 54 n set respectively to values of (E c #x) i  (Ec#Y) i  and (E c #Z) i  the outputs of which are summed in transformers 55 i...n ,56 i...n, 57 i...n and provide outputs corresponding to #X, #Y, #Z, which are reduced to zero by adjustment of the potentiometers (Fig. 2 not shown) determining the amounts of the various colourants. In a modification (Fig. 4, not shown) direct readings of #X, #Y, #Z can be obtained and the match which has been made can be compared for different condition of illumination. Specification 921,290 is referred to.