Patent GB-736864-A (GB)
Title: Improvements in black printers and electro-optical methods of making them
IPC: G03F3/08
Filed: 1952-09-08
Abstract: 736,864. Photomechanical black printer. EASTMAN KODAK CO. Sept. 8, 1952 [Sept. 15, 1951], No. 22467/52. Class 98 (2). [Also in Group XL (b)] Relates to the production of a black printer for use in a four-colour photomechanical printing process for the reproduction of multicolour originals. According to the invention a black printer is produced which is characterized in that it gives rise to a black print whose density at each picture point equals the sum of two densities, one of which is a direct function of that of the least dense of the three subtractive colours appertaining to the colour in the original at that picture point, and the other of which is a direct function of the visual tone of the original at that picture point. In carrying out the invention, the original is scanned to derive the primary red, green and blue component signals. After subsequent colour correction, that one of the three signals which differs least from its value for white is selected and then modified so as to increase the difference between it and the signal for white by a factor which is function of the visual tonal density. This is effected by increasing the difference by a factor which is a function of the difference between the original (uncorrected) green signal and the green signal for white and of the difference between the original red signal and the red signal for white and, if desired, of the difference between the original blue signal and the blue signal for white. The final signal is then applied to control the intensity of illumination of a light-sensitive layer which is processed to produce the black printer. Fig. 2 shows a circuit for carrying out the invention. The primary red, green and blue signals are derived in photo-cells 23R-23B and applied to colour correction circuit 28 from which the cyan, magenta and yellow signals appear on leads 31R-31B. The signal which differs least from the corresponding signal for white, i.e. the signal corresponding to the least dense of the subtractive colours, is selected and appears on lead 31. This is the greatest signal where a positive original is being scanned and the least where a negative original is being scanned or where circuit 28 inverts the signals. The signal corresponds to the type of black printer signal disclosed in Specifications 506,581 and 517,265 and may be applied to modulators 35R-35B to effect a conventional black printer correction of the cyan, magenta and yellow signals. In accordance with the invention, the black printer signal is now modified to increase its difference from the signal for white by a function of the visual toned density of the original. This is effected in a modulator circuit 60 (shown in more detail in Fig. 3, not shown) to which the signal on lead 31 is applied together with the original red and green signals and, if desired, also the blue signal, which signals are added first in circuit 55. It is stated that circuit 60 may multiply the two applied signals, although if the circuit is preceded by logarithmic amplifiers, the circuit may simply add the signals and then produce the antilogarithm. The resultant signal is applied to glow lamp 40 exposed on to sensitive film 45. Specification 488,835, (Group XL], also is referred to.