GB893655A - Improvements in or relating to the production of printing formes - Google Patents

Abstract

893,655. Copying-by-scanning. HELL, R. Aug. 13, 1958 [Aug. 14, 1957], No. 26025/58. Class 40(3). The engraving of printing plates in accordance with signals generated by the photo-electric scanning of an original to be copied is controlled in such manner that the contrast in the region of abrupt changes in the tone of the original is increased. This is achieved by simultaneously scanning the original with two light spots, the first being the usual scanning spot I (Figs. 1 and 5) and the second a larger spot 2 (Figs. 1 and 5), the signals produced (when scanning across the demarcation line 3 between a dark portion 4 and a light portion 5 of the original) by the latter spot in photo-electric cell 11 (7, Fig. 2) being subtracted from the signals produced by light spot 1 in photoelectric cell 10 (6, Fig. 2) to derive a correction signal (8, Fig. 3) which is added to (or employed to multiply) the signals from photo-electric cell 10 to produce the signal 9 (Fig. 4) employed to control the engraving tool, The depths of penetration of the tool is thus reduced approaching the demarcation line 3 from the dark side 4 and results in a blacker copy whilst on leaving line 3 the depth of penetration increases for a short distance into the light portion 5 resulting in the production of a still lighter copy. The arrangement for processing the signals to produce the correction signal is shown in Fig. 5 where the output of the respective photo-electric cells 10, 11, rendered suitable for A.C. amplification by known means (not shown) is amplified by amplifiers 12, 13 and after gamma correction in devices 14, 15 and detection in 16, 17 is supplied to a subtraction circuit 18 to produce the signal 8 (Fig. 3). This signal is then supplied through serially arranged amplifiers 19, 20, gain controlled respectively by the detected signals from 16 and 17, to an amplitude limiter 21 and thence via a potentiometer 22 to an amplifier 24 in which it is combined with the signal output from amplifier 12. The combined signal from 24 is then supplied via a further amplitude limiter 25 to control the engraving tool of the plate engraving machine in known manner. The adjustment of the devices 14, 15, 21 and 25 for various contrast requirements is discussed. When the colour separation printing plates are being produced the arrangement described is applied only to the blue and black separations since these, mainly, determine the detail of the copy produced. Specifications 734,462 and 819,244 are referred to.