GB517265A - Improvements in photo mechanical colour printing processes - Google Patents

Abstract

517,265. Copying in colour by scanning. KODAK, Ltd. April 22, 1938, No. 12068. Convention date, April 22, 1937. [Class 40 (iii)] [Also in Group XX] The black printer used in photo-mechanical two and three colour half-tone or continuoustone printing processes is such that the printing density at each point is proportional to the density at the corresponding point of the original measured by light of whichever primary colour is predominant at that point of the original. It may be produced from a negative in which the density at each point is determined by the density of the corresponding point of the original when measured by light of the primary colour predominant at that point of the original. A half-tone negative made from the original by exposing through a half-tone screen a film comprising three layers sensitive respectively to blue, green and red (or a corresponding tripack) may be used to produce the printer. In an alternative scanning method, suitable for half-tone or continuous-tone work, a white light source 70 illuminates a multicolour original 38 on a transparent cylinder which is rotated on a screw so as to be scanned by an optical system 39, the scanning beam passing through an optical dividing system 40 and three primary colour filters 41, 42, 43 on to three photo-electric cells 44, 45, 46 which control three light valves 50, 51, 52 to regulate the illumination from three light-sources 56, 57, 58 focused by lenses 60, 61, 62 on three lightsensitive layers 65, 66, 67 which are mounted on a rotatable cylinder 68 so that they are scanned by the respective light beams to record the component colour images. This arrangement is described in Specification 488,835. For producing the black printer, three further light-valves 53, 54, 55 successively intercept light from a source 59 focused by optical systems 63, 64 on a sensitive layer 69 similarly mounted on the cylinder 68, and the valves are so controlled from the cells 44, 45, 46 that their openings are inversely proportional to the outputs of the respective cells, so that the amount of light striking the layer 69 is determined by the valve which has an area which is effectively less than that of the other two valves and is independent of the area of the other two. The layer 69 may be processed to a positive and used for printing a negative which is in turn used to print the black printer ; or it may be a light -sensitive resist mounted on a cylinder so that a rotary black printer may be made directly provided that a half-tone screen is introduced into the system. Compensation of the black printer for colour correction of the colour printers may be made by employing, when producing the black printer, in registered superimposition with the original or a real image thereof, a negative mask (or a positive produced therefrom) of the known kind made from the original through a colour filter and processed usually to a low relative density and contrast. A negative mask similar to that usually used for correcting the yellow or magenta printer is suitable. The black printer density may also, as is usual, be reduced by an amount sufficient to give zero printing densities in the high lights.