Patent GB-920070-A (GB)
Title: Colorimetric computer
IPC: G03B27/72
Filed: 1959-10-12
Abstract: 920,070. Electric analogue calculating. HAZELTINE CORPORATION. Oct. 12, 1959 [Nov. 19, 1958], No. 34501/59. Class 37. A computer solving plural simultaneous linear equations determining nominal density values of colour compensating filters for use with a subtractive light source illuminating a photochemical process so as to simulate an additive light source, and relating the total densities of a given combination of colour compensating filters used with such additive light source to the nominal densities of said filters used with a subtractive light source, with the inclusion of a correction term in each equation corresponding to a variable density neutral filter, and the addition of an equation relating the required density of such neutral filter to a total neutral density and the number of filters employed in the subtractive light source; comprises a plurality of computing networks each solving one such equation, within each network plural variable impedance branches having values proportional to selected terms of the equations comprising the product of a constant and one of said total densities, within each network an impedance branch corresponding to the correction term; the networks being energized from predetermined electral sources so that the required solutions are given by the combined outputs of said several impedance branches; and control means provided for the variable impedances of all the network branches corresponding to said selected terms of the simultaneous equations to obtain solutions thereof corresponding to the nominal density values of a neutral filter and plural colour compensating filters, all of which are to be used with a subtractive-light source simulating a given additive light source. Specification 841,125 describes apparatus used in printing colour films wherein a colour negative 11 (Fig. 4) is scanned by a cathode-ray spot, the emergent light is separated by dichroic mirrors into red, green, and blue components which are applied to respectively sensitive photo-cells, and the output voltages thereof are amplified at 15, 17, 19 to voltages proportional to the respective exposures of the positive printing film to energize potentiometers 21, 23, 25 which excite networks 27, 29, 31 simulating the dye density transfer characteristics of the positive film emulsion to derive voltages proportional to the colour due densities produced by the exposed emulsions after processing. Such voltages are cross-coupled in a network 33 simulating the density contribution of each dye to the total density of the print presented to the red, green and blue components of white light incident thereon, and the crosscoupled density representing voltages are applied to apparatus 35 comprising non-linear exponential amplifiers converting the density voltages into component colour light intensity voltages operating a tricolour cathode-ray tube presenting an electrically simulated image of the positive print. Controls 21a, 23a, 25a of potentiometers 21, 23, 25 are adjusted for the desired intensity and colour balance of the electronic image, and the attenuations of the red, green and blue components of an additive light source for producing positive prints from the negative image (Fig. 5, not shown) are correspondingly adjusted in logarithmic increments. Where, however, the printing light source is subtractive, utilizing white light from lamp 39 passing serially through adjustable cyan, magenta and yellow filters C, M, Y attenuating red, green and blue, and through adjustable neutral filter L (Fig. 6) the filters also attenuate all primary colour components of white light to varying degrees (Figs. 1, 2, 3, not shown) so that compensation of the setting of potentiometers 21, 23, 25 is necessary before setting the filters to corresponding values of density. The effective densities Dcb, Dcg, Dcr; Dmb, Dmy, Dmr; and Dyb, Dyg, Dyr of the respective C, M, and Y filters for the blue, green and red light components to which the positive material is sensitive are determined from published data (Figs. 1, 2, 3, not shown) and it is shown that the total attenuations Dr, Dg, Db presented by the filters of Fig. 6 to the red, green and blue components of white light to which the respectively sensitive film emulsions respond are given by where Dn is total neutral density given by Dn=0.04N+1.00Dl; Y, M, C are the nominal densities of the yellow, magenta, and cyan filters; N is the number of filters employed; Dl is the neutral filter density; and the numerical coefficients are chosen for a particular emulsion and range of filter from which it is shown that so that by substitution of the required settings of potentiometers 21, 23, 25 for Dr, Dg, Db in the equations the values of C, M, Y, Dn necessary in a subtractive printer to obtain the same positive film exposures as those given by an additive printer using filters of densities given by the potentiometer settings, may be computed by the apparatus shown in Fig. 7. Equal sources of direct voltage are centre-tapped to ground and connected across computing networks 51, 53, 55, 57 comprising for equation (4) an arm 511 having a variable resistance 1/1.13 Dr in series with parallel arms 512, 513 having variable resistances and 1/0.04Dg and 1/1.09 Dn; for equation (5) an arm 531 having a variable resistance 1/1.19 Dg, in series with parallel arms 532, 533, 534 having variable resistances 1/0.24 Dr, 1/0.06 Db and 1/0.89 Dn; for equation (6) an 1 arm 551 having a variable resistance in 1.10Db in series with parallel arms 552, 553, 554 having variable resistances 1/0.17 Dg ,1/0.07 Dr and 1/0.86 Dn ; and for equation (7) series arms 571, 572 having variable resistances 1/Dn and 1/0.04 N . Ammeters I C , I M , I Y , I L  are connected from the respective arm positions to ground, and are calibrated to indicate the required values of cyan, magenta, yellow and neutral filter densities in Fig. 6. After insertion of the requisite values of Dr, Dg, Db, arms 513, 534, 554, 571 may be varied together to zeroize I C , so that the required filter combination is reduced to magenta, yellow and neutral filters of densities given by ammeters I M , I Y  and I L ; the factor N being known for adjustment of arm 572. The arms 511, 532, 553 dependent on Dr, arms 512, 531, 552 dependent on Dg, and arms 533, 537, dependent on Db may be ganged with the controls 21a, 23a, 25a of potentiometers 21, 23, 25 (Fig. 4) for corresponding variation; the potentiometers and variable resistance arms being logarithmically stepped and controlled by selector switches (Fig. 8, not shown).