Method and device for generating a digital lookup table for printing inks in image reproduction equipment

A method for generating a digital look-up table for three printing inks in image reproduction equipment from color scan values obtained by photoelectric scanning of originals, comprising the steps of:

• measuring full colors of the three printing inks and white by scanning corresponding originals;
• determining model colors that have an approximately linear relationship to the printing inks by use of a position of a chromaticity range of a print scale having scale colors, three primary stimuli of the model colors in a chromaticity diagram each lying on an extension of trajectories of the scale colors through a white point, a triangle set up by lines running between said three primary stimuli enclosing the chromaticity range of the print scale;
• adjusting a scale division of the primary stimuli with a cube root to a linear gradation of the printing inks;
• determining a number of fields to be scanned by using space diagonals of the printing inks as a function of said model colors;
• further processing scan values of said fields to be scanned by a cubic spline interpolation with a higher number of support points for generating a look-up table;
• storing values of the model colors as a function of printing ink values;
• making the color stimuli for cyan, magenta and yellow for the model colors depend on the primary colors according to the following relationships:
• ##EQU9## wherein the stimuli for cyan, magenta and yellow are referenced C.sub.t, M.sub.t, Y.sub.t and the primary stimuli red, green and blue are referenced "R, G, B";
• measuring the space diagonals at several scanning points and using the space diagonals for determining scan values;
• and
• a cubic power of said scan values determining a number of fields to be scanned.