Method and apparatus for manufacturing screened printing forms

Claim 1

Process for the production of half-tone printing blocks having screens of optional screen angles and screen line spacings from originals by means of a recording element moved relatively over a recording medium, wherein a screen consists of a plurality of periodically repeated screen grid elements, wherein screen dots of different size are recorded as a function of the tonal values of the original, in which: a) the original is scanned opto-electrically dot by dot and line by line to obtain image signal values which are a measure for the tonal values of the original, b) a matrix is preset with a number of matrix elements whose locations within the matrix are determined by co-ordinate values (x'; y'), and each matrix element has allocated to it a screen threshold value (R) representing a tonal value, the three-dimensional function R = f (x'; y') forming a screen hill (40, Fig. 2) representing the fundamental structure of a screen grid element, the base area of which is formed by matrix elements and whose cross- sectional areas lying parallel to the base area define the shape and size of the screen dots which are to be recorded as well as their position within a screen grid element as a function of the tonal values, c) the recording medium is subdivided into a plurality of adjacent areal elements aligned in the recording direction of the recording element the screen dots being built up within each screen grid element from areal elements recorded by the recording element, d) the particular matrix element whose location within the matrix corresponds to the position of the areal element within the screen grid element in question is determined for each areal element of the recording medium, and the screen threshold value (R) allocated to the matrix element determined is called up, and in which e) a recording signal controlling the recording element is obtained by comparison of the screen threshold value (R) called up with at least one image signal value, the signal comparison leading to the decision whether the areal element is or is not recorded as part of a screen dot within the screen grid element in question, characterized in that for the purpose of complementary reduction of Moire patterns during printing block production, f) at least one other matrix is preset and its matrix elements also have screen threshold values (R) allocated to them, which differ, however, for at least one preset tonal value range from the corresponding screen threshold values (R) of the first matrix, thereby forming at least one other screen hill (52, Fig. 2) having a shape altered, with respect to that of the first screen hill (40, Fig. 2) within the tonal value range, g) the positionally corresponding matrix elements within the matrices are determined for each areal element, and, h) one of the screen threshold values (R) allocated to the matrix elements determined is selected at random for comparison to the image signal value.