Optimization Algorithm that Generates the Lowest ΔEab Values to a Reference Standard Based on Spectral Measurements of Solid Inks in Offset Lithography


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Søren Tapdrup Jensen


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ISO 12647-2 specifies CIELAB values for primary and secondary colors, but only tolerances for the primary solid colors. Press operators in lithography still favor density measurements for process control to assure quality and reproducibility during a production run. Since there is no direct relationship between density and CIELAB measurements, there is a gap between what the standard specifies and what the industry is actually doing.

This research investigates the possibility of using the tolerances specified for the primary colors to achieve a better conformance of the secondary colors. Combining Beer's Law and Hamilton's Trapping Equation into an algorithm provides a method to predict the outcome of resulting secondary colors if the ink layer thicknesses of one or more of the primaries are altered. The research goal was to provide the press operator with a set of rules and applications to help him or her make decisions on how to reach the primary and secondary aim points for an OK sheet in as few steps as possible.

The algorithm was tested against 197 different measurements for all six colors (C, M, Y, R, G, B), simulating 197 different printing conditions. The results show that the algorithm has high degree of accuracy in predicting the ink layer thickness that conforms to ISO 12647-2 aim point, but errors in the prediction occur when the measured sum of the secondary colors have a low ?Eab to the standard.

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