GB767505A - Improvements relating to the electrical production of printing surfaces - Google Patents

Abstract

767,505. Local copying by scanning. CROSFIELD, Ltd., J. F. July 23, 1954 [Aug. 11, 1953], No. 22141/53. Class 40 (3). A cylindrical or flat printing plate comprises a rigid electrically-conductive backing and a layer of partially electro-conductive material permanently attached to the backing and having its surface scanned by an electric discharge which is controlled during the scanning to remove varying amounts of the surface material in correspondence with the trace of the original to be copied without perforating the layer of partially electro-conductive material. The amplitude or duration of the pulses may be varied and the recesses produced may be of constant area and variable depth. Alternatively, the recesses may be variable both in area and in depth. In the arrangement shown a coloured transparency 7 on a rigid plate is scanned by a flying light spot from a C.R. Tube 4 and the beam passes via a filter 13r to a photo-cell 10r and is split by half-silvered mirrors 11, 12 to impinge on photo-cells 10g, 10b via colour filters 13g, 13b. The cell outputs are fed to amplifier units 15g, 15r, 15b, which may include inverter circuits, logarithmic attenuators and correction circuits, which may be of the type described in Specification 738,118, and then to modulators 16g, 16r, 16b operated to provide the amplitude, or duration, modulated signals to be applied to the discharge electrodes 3g, 3r, 3b. The partially electroconductive layer may be formed by a plastic sheet of polyvinyl chloride impregnated with powdered carbon or metal particles and the electrodes 3g, 3r, 3b are preferably of tungsten wire. The printing surfaces 1 may be flat plates, the electrodes 3g, 3r, 3b being moved rapidly across the plates which are stepped at the end of the rapid line scanning movement.