Patent GB-734462-A (GB)
Title: Method of and apparatus for electro-mechanically producing printing formes from lineoriginals
IPC: B41C1/04
Filed: 1953-07-15
Abstract: 734,462. Local copying by scanning. HELL, R. July 15, 1953 [July 15, 1952], No. 19666/53: Class 40 (3). [Also in Group XVI] In a method for the electro-mechanical production of printing formes from line originals an engraving tool controlled by a photo-electric scanning device is, during the scanning of a white area element of the original which is sufficiently far-spaced from a dark area element, introduced more deeply into the material of the blank than when scanning a white area element situated in the immediate vicinity of a dark area element. Each character is scanned by a scanning element 1 and an auxiliary scanner covering an area 1 the arrangement being such that while the element 1 scans a line or dark area no engraving takes place, while the element 1 scans a space, e.g. 11, but a dark area remains on the auxiliary area 2 engraving to a usual depth 6 takes place and while both the element 1 and area 2 scan a space, e.g. 22, deeper engraving takes place to a depth 7 as is indicated in Fig. 6 when the element 1 scans the line 14 (Fig. 2). The auxiliary area may be circular, part circular, or rectangular and may further be given a non- uniform illumination intensity decreasing from the centre, the depth of the etch then becoming progressively variable. As a modification a time delay circuit may be included in the control circuit associated with the auxiliary area 2 so that the depth of cut is gradually increased to the level 7. Fig. 12 shows a suitable control circuit, the original 31 and printing forme 32 to be engraved being rotated by a motor 29. The original is illuminated by light sources 33 and the light intensity on the element 1 focused by lenses 34, 36 and aperture 35 on to a photoelectric cell 37, a rotating disc 38 rendering exposure of the cell intermittent. A similar arrangement 39, 40 . . . 43 is employed for scanning the area 2. Signals from the photoelectric cells 37, 42 are fed from amplifiers 44, 48 and rectifiers 45, 49 to a control device 46 for the engraving tool 47. The signals from rectfier 49 associated with the circuit for the auxiliary area are fed to a control device 50 including a thyratron having a negative bias which is overcome only when the area 2 is free from dark areas. In an alternative arrangement, one cell may be employed with a variable aperture (Figs. 13 and 14, not shown) the output from the cell being fed to a switching device which separates the signals appropriate to each scanning area and also controls the size of the aperture. In a further arrangement light from a single source is split into two beams by a prism and each beam focused through an aperture lens and rotating disc arrangement on to the original (Fig. 15, not shown). A single rotating disc having two sets of holes interrupting the beams at two different frequencies is employed, the light reflected by the original entering photo-electric cells the outputs of which are fed through an amplifier and filters for separating the output into its two frequencies associated with each area. Alternatively periodically primed gas discharge lamps having different priming frequencies are utilized for respectively illuminating the two areas.