Characterization of overprinted inks is the basis for color management. For multicolor printing beyond CMYK, a reliable characterization of the color of overprints is still a challenge in general. Also, thousands of different spot colors are used in packaging in changing combinations, each asking for another test print.
For the special case of Expanded Color Gamut (ECG), many spot colors can be replaced by a fixed set of 6 to 8 process inks. In that case, the effort of a carefully controlled test print for a full characterization is much more worthwhile, but also indispensable. This has led to an IDEAlliance(r) project to create a standard 7-color CMYKOGV test chart, in which the author participates.
The sampling of ink combinations has been done using incomplete regular grids of up to 4 overprinted inks. A layout has been developed where the first page alone should provide a good characterization, which can be augmented by using pages 2 - 4. The uncertainty from printing fluctuations can be estimated using many duplicate patches. For evaluation, two other charts have been created, one with patch combinations in the grid centers which maximizes interpolation error, and another which covers many overprints of more than 4 inks.
The paper presents the results from the first offset test prints, where all available overprints are predicted from a subset of patches using GMG OpenColor.