Method and apparatus for influencing the colour appearance of a colored area in a printing process

Claim 1

In a method for influencing the colour appearance of a coloured area made up of at least three chromatic separation colours in a printing process, on the basis of densitometric measurements in at least one test area in which density value differences between the densitometric actual values and predetermined target values are determined for the separation colours and are used to influence the inking of the separation colours, the improvement comprising inking the chromatic separation colours in at most two chromatic separation colours as a function of the coloured proportion of a sum vector, determining said sum vector starting from the density value differences of the chromatic separation colours and based on a coloured area which is to be influenced as regards its colour appearance in an equidistantly graded colour space, and breaking down the colour shift indicated by said sum vector into a grey proportion which only affects brightness as well as a coloured proportion relating to at most two chromatic separation colours, said sum vector being formed by addition of separation colour vectors associated with the chromatic separation colours, the absolute amount and direction thereof being determined by means of at least one test run so that the sum vector in a plane of equal brightness corresponds substantially to the vector which points from the colour coordinates of the coloured area appertaining to the target values to the colour coordinates of the coloured area appertaining to the actual values, said absolute amount of the separation colour vectors being determined as the product (a) of the percent dot area of the respective separation colour in the coloured area to be influenced and (b) a calibration factor (f) which is determined by approximation as f=80×D-1.1 where D is the mean of the target colour densities of the chromatic separation colours measured in the test areas.