Patent GB-814347-A (GB)
Title: Method of producing a subtractive colour film copy
IPC: G03F3/08
Filed: 1957-04-18
Abstract: 814,347. Copying in colour by scanning. GRETENER, E. April 18, 1957 [April 18, 1956], No. 12781/57. Class 40 (3). Relates to a method as described in Specifications 791,675 and 791,676 for producing a subtractive colour film recording utilizing a black- and-white recording corresponding to the density of the least of the subtractive colourstogether with three colour recordings whose densities are reduced by the amount of the black-and-white density so that always at least one is equal to zero. The invention is concerned with correcting for errors due to the imperfect absorption characteristics of the colour dyes which, for the particular case considered, may be of the form shown in Fig. 2. The effects of the imperfections are illustrated in the colour triangle shown in Fig. 4, where points 51, 52, 53 correspond to the colours blue (B), green (G), red (R), and the points 34, 35, 36 to the " ideal " subtractive colours " minus blue " (-B), " minus green " (-G), " minus red " (-R), each of infinite density. Reduction of the density shifts the resulting colour point along the dotted line toward the white point 57. The three sectors 58, 59, 60 indicate the colour regions where one of the densities is zero, the arrows adjacent the boundaries of the triangle showing the movement of the colour point with increase of density. Other lines define the boundaries for densities of 0.5, 1.0 and 2.0. These lines correspond to the practical dyes which (with infinite density) occupy positions 54, 55, 56 displaced from the corresponding points for the " ideal " dyes. For example, the minus red dye absorbs slightly in the green region and hence is shifted from the " ideal " point 56 in the direction of the blue point 51. In accordance with the invention corrections are effected to shift the points 54, 55, 56 towards the " ideal " points such that the subtractive colours are each correctly located on the extension of a straight line joining the corresponding complementary colour at the opposite point of the triangle to the white point. The invention is described as applied to a method utilizing electro-optical scanning to derive signals representative of the colour densities, the corrections being effected by adding signals representative of at least one colour to one of the channels for the other colours. Thus in the case of the " minus blue " a signal. is added to the " minus red " channel so as to effect the desired shift towards green, the added signal being reduced to zero as the " minus blue " density reduces to white point 57, and also being reduced to zero as the " minus red " and " minus green " signals increase to a maximum as the colour moves towards the corner points 52 and 53. This variation of the correcting signal is necessary since the need for correction is a maximum at point 54 and reduces to zero at the white point and the corners of the triangle. In the case of " minus green " a similar correction is applied, again to the "minus red " channel since the desired shift is towards the blue point 51, whilst in the case of " minus red " the correction is applied to the " minus blue " channel so as to effect the desired shift towards the green point 52. Suitable apparatus for carrying out the invention is described, Fig. 5 (not shown), utilizing a cathode-ray tube flying- spot scanner and photo-cell assembly to analyse the original film and a synchronously scanned cathode-ray tube to print the reproductions. A black-and-white signal together with colour signals reduced by the amount of the black- and-white signal and representative of the desired subtractive colour densities are derived as described in Specifications 791,675 and 791,676. The channels conveying these colour signals are cross-connected through pairs of amplifiers to introduce the corrections. The first amplifier, which may be non-linear, is controlled to vary in gain directly with the amplitude of the signal in the channel from which the correction is derived so that the magnitude of the correction reduces to zero as the signal reduces to zero at the white point, whilst the second amplifier is controlled in gain inversely in accordance with the amplitude of the signal in each of the other channels so that the correction reduces to zero as the signals increase to a maximum at the corner of the colour triangle.