Method for colour correction by dry dot etching using photographically

Abstract

A method for dot-size correction or modification in half-tone colour separations to be used in the production of printing plates for printing reproductions of coloured originals in which a contact print is overexposed through a photographic mask constituted so as to isolate a selected area in addition to being exposed normally for obtaining an exact copy of an original half-tone separation. The mask is generated electronically using an electrical signal obtained by scanning each separation, digitizing each signal and storing the digital values in a digital storage device. A halftone separation is visualized electronically as an image in a dislay device, e.g., video tube, an area of said image is defined for isolation and correction and the combination of positive and/or negative separations required to serve as a mask effective for isolating the selected area for purposes of dot evaluation is determined via a look-up table. The mask is visualized on the display device, evaluated for correctness, the isolation area is re-defined visually, and a new combination of separations to effectively mask the re-defined area is determined and evaluated. The resultant mask is reproduced photographically and used to identify the needed degree of dot correction. It is then also used to generate a dot-corrected photographic print by subjecting print material for said print to two photographic exposures, one being the normal exposure to reproduce the corresponding halftone separation and the other being an overexposure through the combination of the photographic mask and the corresponding separation for a dose to give the needed change in dot size.