Patent GB-538661-A (GB)
Title: Improvements in and relating to photo-mechanical colour printing processes
IPC: G03F3/04
Filed: 1940-02-19
Abstract: 538,661. Copying in colour by scanning. KODAK, Ltd. Feb. 19, 1940, No. 3117. Convention date, Feb. 18, 1939. [Class 40 (iii)] [Also in Group XX] A black printer for use with blue-green, magenta and yellow printers having respective printing transparencies at corresponding points represented by Tbg, Tm and Ty, has a printing transparency at each corresponding point equal to the Nth root of Tbg<SP>N</SP> + Tm<SP>N</SP> + Ty<SP>N</SP>N being greater than unity. Values of N from 1.3 to 10 are referred to. In a modification intended for better reproduction of greys, the black printer has a printing transparency at each corresponding point equal to the Nth root of (Tbg<SP>N</SP>+Tm<SP>N</SP>+Ty<SP>N</SP>) /,P N being greater than unity, and P lying between 1 and 3-preferably N=1.5 and P lies between 1 and 1.5. The transparency values, particularly in the case of' non-transparent printers, may be derived from the relations, density==log. opacity, and transparency= 1/opacity. Corresponding black printer negatives, for use with printers derived from red, green and blue colour-record negatives having densities, Dr, Dg, Db, respectively at corresponding points, have densities at each corresponding point equal respectively to the Nth root of Dr<SP>N</SP>+Dg<SP>N</SP>+Db<SP>N</SP>, and the Nth root of (DrN + Dg<SP>N</SP> + Db<SP>N</SP>) /P. As indicated in Specification 517,265 the colour printers may be reduced in density to compensate wholly or in part for the amount of black printed by the black printer ; and any of the colour printers may be colour corrected by modifying its density in proportion to the density of one or more other colour, printers to overcome the effect upon the areas of pure colour of the deficiencies in light transmission of the inks or colouring matters used in reproduction. Two methods of preparing the black printer are described. In the photographic method, a multicolour original is photographed through red, green and blue, filters yielding three colour record negatives. From these are made high contrast positives, and in this step colour correction may be effected by making at least one colour correcting mask from one of the colour record negatives, and using it in printing from one of the other colour record negatives. The three positives are then added by printing them in succession on to a light sensitive layer which is developed to yield a low-contrast, black printer negative. The contrast of this negative relative to that of the positives should be the reciprocal of the contrast of these positives relative to that of the original. In the electro-optical method illustrated in Fig. 2, an original (not shown) is scanned in known manner, and the light is passed through blue, green and red filters, 20B, 20G, 20R to photo-electric cells 21B, 21G, 21R, the signals from which are amplified by amplifiers 22B, 22G, 22R. At this stage colour correction may if desired, be introduced as described in Specification 488,835 by modifiers 25, 26. The signals then pass through amplifiers 23B, 23G, 23R to operate light valves 24B, 24G, 24R which control the exposure for making colour record negatives or positives or photo-mechanical printers, e.g. as described in Specification 488,835. The black printer is produced by passing part of each signal through an Nth power amplifier, respectively 27B, 27G, 27R, to yield high contrast records of the original, adding these in any suitable device 28, e.g. a simple resistance across which the signals are additively impressed and passing the combined signal first into a Nth root amplifier 29, to convert it into a low contrast signal and then into a linear amplifier 30 to operate a light valve 31 which controls the exposure for making a black printer positive or negative according to its arrangement. Reduction in density of the colour printers 24B, 24G, 24R to compensate for the amount of black printed by the black printer may be effected as described in Specification 488,835 by placing in each colour channel a modifier controlled by the final black signal, e.g. by connecting such modifiers to the leads 32 in the same way as the modifier 25 is connected to the amplifier 22G. Specifications 538,012 and 538,013 describe contrast amplifier systems which may be used in the second method. Specifications 506,581, [Group XX], and 531,262 also are referred to.