The invention relates to a process and an apparatus for the determination of the halftone percentage values required to obtain a given desired color location using the individual printing colors in autotype multicolor printing.
One of the fundamental problems in reproduction and printing practice is to exactly reproduce a given color with a predetermined color location by printing methods and to determine the surface coverages or halftone percentage values required of the individual printing colors in the halftone films or printing plates upon which the reproduction process is based. Usually, a color atlas is used for this purpose. The atlas contains a plurality of color fields of known graduated halftone percentage values of the individual printing colors. The color to be reproduced is visually compared with the color fields of the color atlas. The color field that in the ideal case is identical with the color to be reproduced or the color field that comes closest to the color to be reproduced, yields the halftone percentage values desired.
A precondition of the reliability of this method is that the color atlas contain a very large number of color fields in as small graduations as possible and that the color atlas be produced under the same reproduction conditions as those provided for the color to be reproduced. In offset printing this includes, among other conditions, the printing inks used, the different halftone forms and layouts, the substrate (paper) to be printed and the printing characteristics of the printing machine used, etc. This requires the production by a reproduction plant of a large number of different and extremely voluminous color atlases, which is very expensive. Furthermore, the use of different and very voluminous color atlases is often considered to be cumbersome and time consuming. Smaller color atlases require a difficult visual interpolation that can be carried out only by experienced personnel in a somewhat reliable manner.
So-called electronic color atlas systems are also known, wherein the halftone percent values and the color coordinates of the color fields of a solid color atlas are stored in tabular form in a computer. The color coordinates of the color to be reproduced are determined by a color measuring device and entered into the computer, whereupon the latter retrieves the halftone percentage numbers fitting the color coordinates entered from the stored table. By means of specially developed interpolation processes and transformation algorithms between the color space and the surface coverage space, intermediate values may also be determined, whereby the problems of visual interpolation are avoided and fewer support locations (color fields) may suffice. However, the aforementioned fundamental problem (i.e., that the color atlas is only valid in a strict sense for the specific printing conditions upon which it is based) is not eliminated by this system.
Another difficulty arises in standard four-color printing, wherein usually a certain proportion of the colors cyan, magenta and yellow is replaced by black ink. Numerous color atlases either do not contain this ink at all or to a limited extent only and it is very difficult for reproduction personnel to decide by visual comparison which is the most suitable or possibly even the only correct proportion of black. The known electronic color atlases again do not provide a satisfactory solution in actual practice for this purpose.