In a multicolor offset press the process inks (kcmy) will be printed consecutively on the substrate from one printing unit to the other. The printing dots and elements in different process color will be printed either isolated, partly or completely overprinted depending on the halftoning. In a multicolor press the inks will be printed wet on wet. That means that in the area where process inks overlap each other one or more inks will be printed on another ink which is not dry enough. The adhesive power between the wet inks is different and less compared to the adhesive power between one ink printed on the top of a less wet ink or even a completely dry ink. The adhesive power between the substrate and printed ink is also different from the adhesive power of one ink on the top of another one. Depending on this adhesive power and the inks' inner cohesion power the thickness of the second printed ink varies. The thickness or amount of the second printed ink on the first one can be determined; its value is called trapping. The amount of the trapping will also be changed due to different parameters such as ink temperature, dampening, printing speed etc. An important outcome is that the gray balance and the color appearance (secondary and tertiary colors) of the printed product also depend very much on the trapping's amount. That shows how important it is to have an explicit value for the trapping. The amount of the second printed inks will be determined by trapping formulas. This value should be useful for the printer at the press. Unfortunately, the conventional trapping formulas are only useful for the "relative" comparison of trapping between two print products. All the conventional formulas for trapping only deliver the amount of the second printed ink on the top of the first one in percent. This value for its own (e.g., 63%) is not really useful and meaningful for print machine operators. There are three different formulas for determining trapping. These are Preucil, Ritz, and Brunner. All of them are based on density only. The results of these formulas are different to each other, especially the Brunner formula differs to the other ones. Here a method will be introduced which is based on spectrometry and will complement the conventional formulas.
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