Method and apparatus for producing half-tone printing forms with rotated

Abstract

In a method and apparatus for producing half-tone printing forms with screens having arbitrary screen angles and screen width, screen dots are exposed on a recording medium by means of a recording element whose exposure beam is switched on and off by a control signal. The recording medium is sub-divided into a plurality of areal elements which are exposed or not exposed by the exposure beam as a function of the control signal. All areal elements exposed in a screen mesh of the rotated screen to be recorded form a respective screen dot. At least two matrices in the form of matrix memories are provided, the individual matrix elements thereof having screen threshold values allocated to them. The screen threshold values forming a screen hill represent the periodic basic structure of a screen mesh and are different for at least one tonal range in the individual matrices. During recording, those matrix elements in the individual matrices whose positions within the respective matrices correspond to the positions of the swept areal elements within a screen mesh are identified. The screen threshold values of the identified matrix elements are called in. Controlled by a random generator, one of the called screen threshold values is randomly selected and compared to an image signal value acquired by means of scanning an original. The control signal for the exposure beam is derived from the comparison, whereby a decision is made on the basis of the signal comparison as to whether the corresponding areal element is to be exposed or not as part of a screen dot.