Patent GB-819244-A (GB)
Title: Process of and apparatus for the electrical production of rastered printing plates
IPC: G03F3/08
Filed: 1955-11-10
Abstract: 819,244. Local copying by scanning. HELL, R. Nov. 10, 1955 [Nov. 10, 1954], No. 32187/55. Class 40 (3). In an arrangement for producing a printing plate by an engraving tool operated in response to the photoelectric scanning of the elemental areas of the original which is to be reproduced, an area larger than and surrounding the elemental areas is also scanned and the average brightness of the larger areas is employed to modify the operation of the engraving tool over the entire tone value range or over solely one or more parts thereof. As shown in Fig. 1, light beams reflected from the original mounted on a drum 3 are focused by lenses 6, 12 on small and large apertures in diaphragms 7, 13 after having been chopped at different frequencies by discs 10, 16 rotated by motors 11, 17. The beam passing through the diaphragm 7 is focused on to a photo-cell 9 connected via amplifier and corrector circuits 18, 19 and a modulator circuit 20 to a circuit 21 in which the carrier is removed. The output is passed to a further circuit 22 and via amplifier 23 to the engraving system 24 operating the cutting tool 25 which engages the plate on the drum 4. To generate the dot or screen frequency the output from a tone generator 26 on the shaft 2 of the driving motor 1 is-applied to a squaring device 27 and a circuit 28 producing a sawtooth waveform which is amplified at 29 and fed to the circuit 22. The beam passing through the larger aperture is focused on to a photo-cell 15 whose output, after amplification, correction and removal of the carrier frequency at the circuits 30, 31, 32 is passed via selectively closed switches 33, 37, 41 to amplitude discriminators 34, 38, 42, and to inverter circuits 36, 40, 44 which may be short-circuited by switches 35, 39, 43 respectively. The circuit 34 is arranged to suppress all signals having an amplitude above a predetermined level and the circuit 42 suppresses signal voltages below a predetermined value. The circuit 34 preferably includes a valve which is modulated up to the grid' current region and the amplitude threshold circuit 42 comprises a valve which is biased negatively and conducts solely in response to relatively high input voltages. The circuit 38 operates so that picture voltages below a certain value and above a predetermined higher value are suppressed. The outputs from one or more of the channels 33 ... 36, 37 ... 40, 41 ... 44 are fed to the element 20 to modify the signals representative of the scanning of the elemental areas. In a modification, Fig. 2 (not shown), large and small areas are scanned in alternating succession by diaphragms moved to provide large and small apertures and operated under control of an electronic switch which diverts the output of a photo-cell amplifier alternately to the correction channel 30, 31, 32 ... and to the picture channel 18, 19, 20 .... In a further modification, Fig. 3 (not shown), a beam from a single source is split by prisms and directed through small and large apertures and through differently spaced apertures in a chopper disc. The beams chopped at different frequencies are reflected from the original picture to separate photocells which simultaneously feed an amplifier whose output is applied to a frequency selective network so that the signals representative of the elemental picture areas and of the larger areas are separated and fed to the channel 18, 19, 20 . . . or 30, 31, 32 .... The elemental scanning areas may be circular, square or rectangular and the larger areas may be circular or square.