Patent GB-739397-A (GB)
Title: Method of and apparatus for making a black printer for use in the reproduction of multicoloured original
IPC: H04N1/60
Filed: 1953-08-26
Abstract: 739,397. Copying in colour by scanning. EASTMAN KODAK CO. Aug. 26, 1953 [Aug. 27, 1952], No. 23511/53. Class 40 (3). In an electro-optical scanning system, a black printer for use in reproducing a multicoloured original is made from a signal corresponding to the ortholuminous value of the original (i.e. the tone apparent to the human eye) after it has been modified by at least two (cyan and magenta) of the colour component signals. The modification is effected by dividing the ortholuminous signal by the colour signals whereby the resulting black printer is effectively reduced or masked by the amount of tone which will be printed by the colour printers. The ortholuminous signal is derived by scanning the original through a very light yellow alter or by scanning a colour separation of the original made through a yellow filter. Alternatively the signal is derived by simply adding the three primary colour signals suitably weighted to correspond to the visibility curve of the human eye. In one embodiment of the invention, Fig. 9, photo-electric cells 25 receive light from the red, green and blue components of the original and supply A.C. signals to a circuit 26 from which colour corrected magenta, yellow and cyan signals are applied via rectifiers and limiters 70 to glow lamps 36 to print the colour printers. " Under-colour " correction is effected by means of masking circuits 42 to which the least predominant subtractive colour signal is applied from selector circuit 41. An ortholuminous signal is derived on wire 33 by a circuit 56 which adds selected proportions of the red, green and blue signals from photo-electric cells 25. Suitable circuits for 56 are illustrated in Figs. 5, 6 and 7 (not shown). Part of the magenta signal is fed through resistor 71 to a compressor circuit 74 and the output applied to a modulator tube 75 so as to divide the ortholuminous signal. In a similar manner a portion of the cyan signal is applied through resistor 81 and compressor 84 to divide the ortholuminous signal in a further modulator tube 85. The resultant signal is then applied through compressor 86 and rectifier limiter 87 to energize glow lamp 35 to print the black printer. The characteristics of compressors 74 and 84 are selected to provide desired masking factors. A square root law is referred to where a 50 per cent masking factor is desired with a linear ortholuminous signal. In general, it is stated that the per cent masking by the magenta signal should be such that the amount of black (usually none) is produced in a saturated red area, and that the per cent masking by the cyan signal should be such that the correct amount of black (usually a small amount) is produced in saturated blue areas. A small amount of yellow may be added to the magenta and cyan signals through resistors 72 and 82, the amount being adjusted so that substantially no black is recorded in pure yellow areas. The yellow signal may be prevented from adversely affecting other colours by the use of non-linear resistors. A portion of the third signal may also be added in each case as indicated by dotted connections 73 and 83. Compressor 86 is employed to introduce any desired distortion in the black printer. As an alternative to providing under-colour correction by the least predominant subtractive colour, the ortholuminous signal may be employed as it appears on wire 33 or at any subsequent stage. In a second, generally similar embodiment, Fig. 8 (not shown), a computer circuit receives the corrected colour signals from circuit 26 and derives a signal representative of the ortholuminous value of a print which would be made from the signals, the signal being then divided into the ortholuminous signal from circuit 56. Specifications 488,835 and 517,265, and U.S.A. Specification 2,581,124 are referred to.