GB1497468A - Method and apparatus for generating screened halftone images by scanning - Google Patents

Abstract

1497468 Facsimile J P BRUCKER 7 June 1975 24527/75 Heading H4F Half tone colour separation negatives for facsimile reproduction, are produced having differing screen pattern angles α to avoid MoirÚ pattern defects. A colour original is mounted on a drum 28, Fig. 1. The separations are produced on films carried by drums 29 to 32 mounted on a common shaft 22 with "original" drum 28. The individual films are exposed by respective dot generators 43 to 46 rigidly attached to a slide 40 driven along a path parallel to shaft 22. Slide 40 carries a scanning detector 42. Dot generators 43 to 46 comprise respective CRTs 47 the displays of which are imaged on to the films by lenses 48. Each CRT 47 has its vertical deflection axis rotated through an angle # from the line direction 52, from which the requisite separation screen pattern angles α (where α = # Œ 45‹) are obtained. A clock signal is recorded on a magnetic strip 55 read by a head 54. Line start signals are obtained from a photodetector 56 registering the passage of a tab 57. Black is screened at α = 45‹ (# = 90‹), yellow at α=0‹ (#=45‹), magenta at α = + 15‹ (# = + 60‹) and cyan at α = - 15‹ (# = -60‹). Fig. 17 shows the magenta screen pattern. A to F represent consecutive scan lines. Lines 94 indicate the vertical deflection axis of the relevant dot generator and line 144 shows the screen pattern axis. As line A is scanned in direction 52 successive dots 0, 3, 6, 9, 1,...are exposed which are incremented along lines 94 by an amount yielding the + 15 degree screen angle of the magenta separation. The other separation's screen patterns are formed in a similar manner described with reference to Figs. 15, 16, 18 (not shown). Logic circuits for controlling the dot generators are described with reference to Figs. 2 to 14 (not shown). The separations may be enlargements. The recording medium may be lithographic plates, gravure plates, deformable plastic sheets, metal plates and paper sheets. The record medium and original may be mounted on flat surfaces scanned by a reciprocating scanner. Dot generators may be modulated lasers, vibrating styli, modulated arc light or electrostatic ink dot accelerator.