GB945752A - Improvements in and relating to a method and apparatus for producing laminar printing formes - Google Patents

Abstract

945,752. Copying by scanning. ZEUTHEN & AAGAARD A.S. Jan. 18, 1960 [Jan. 23, 1959], No. 1786/60. Heading H4F. In an arrangement for producing a perforated stencil corresponding to an original by controlling a stylus which perforates a stencil blank in accordance with signals derived by scanning the original, the threshold level of the latter signals which determines whether or not a perforation is made is varied in discrete steps during the production of the stencil. Thus, considering the schematic half-tone original shown in Fig. 1 in which the density decreases upwardly from the bottom as indicated by the density of the horizontal lines, the threshold level is varied in discrete steps (e.g. four steps) having values as shown by the height of the vertical lines in Figs. 3 to 6. The levels may be set in the order shown resulting in the production of a stencil according to Fig. 7 but it is preferable to vary such order in the manner shown in Fig. 8. To produce the threshold level variation the value of a resistance which determines the latter is varied in appropriate steps by the arrangement shown schematically in Fig. 9 in which resistances of different values, as. indicated by the different number of lines between a central conductor 7 and the insulated segments a . . . d of a commutatator 6, are inserted between the terminals 2, 3 of a known form of apparatus (Figs. 11 to 13, not shown), when relative motion between the commutator and a brush 8 occurs. In order to produce a shading in the lightest parts of the picture the voltage controlling the stylus is increased during the occurrence of such parts by a further commutator arrangement 9 in which two different value resistances are sequentially inserted between terminals 4, 5 of the known apparatus. Shading in other parts of the picture may be produced by the use of a larger number of segments with appropriate resistances. The four threshold values determined by commutator 6 may be insufficient in the darker parts of the picture and to overcome this some of the steps may be repeated as shown by the diametrically opposite segments a and d of the commutator arrangement of Fig. 10. The two commutators with their associated brushes are provided as an assembly for attachment to known apparatus (Fig. 12, not shown). The latter comprises a pair of rotating cylinders one of which carries the original and is associated with a photo-electric scanning arrangement whilst the other carries the stencil blank (preferably of the homogeneous kind) the perforation of which is effected either by an electrical discharge from the stylus or by a mechanical perforating tool.