GB1034246A - Improvements in and relating to an apparatus for producing a master, such as a stencil or an offset plate by electrical perforations - Google Patents

Abstract

1,034,246. Copying by scanning. ZEUTHEN & AAGAARD A/S. Aug. 17, 1964 [Sept. 7, 1963], No. 33565/64. Heading H4F. The invention relates to an apparatus, Fig. 1, in which an original picture 3 mounted on a drum 2 subjected to rotational and axial displacement is viewed by an optical system including a lamp 5 and photo-electric cell 18 (arranged as shown more fully in Fig. 2) to produce an electric signal which is applied to a circuit 12 to control the application of a potential to an electrode 13 whereby a printing blank 4 is perforated, the arrangement being such that circuit 12 is triggered only when the signal exceeds a predetermined value. In accordance with the invention the grey-value of the original picture which results in a signal of the predetermined value is controlled by means of an adjustable dimming member 14 included within the optical system. In one arrangement the member takes the form of a disc constructed as shown in Fig. 3 and adjustable angularly by control 17. The member may be produced photographically or by printing on a translucent plate. An alternative construction, Fig. 4, arranged for translatory motion consists of a perforated opaque plate. Reference is also made to an arrangement in which the transparency varies with the angle of incident light and an arrangement based on angularly adjustable slots. The use of a plurality of light transmitting areas in the light path, as shown in Figs. 3 and 4, allows for irregularity in the distribution of light flux. The variation of light transmission in terms of adjustment of control 17 is chosen to be logarithmic. A further dimming member 20 having a predetermined number, e.g. four, of different transparency values may be provided in the apparatus to permit half-tone reproduction in a manner similar to that described in Specification 954,752. The member is rotated by a motor 22 which is pulsed once for each line 5 carried on original 3 by means of electromagnet 24, 27 which coacts with a ferromagnetic bar 23 carried by drum 2. By this means the four transparency values are introduced in turn, a different one for each line, and cause different grey levels to result in the predetermined signal valve which triggers circuit 12. Reference is made to an arrangement which changes the transparency for every second or third line, and to the use of irregular or random changes. An electric commutator system 29, 30 may be provided for controlling circuit 12 to change the trigger level.