Ink Trapping Ratio Estimator

Ink trapping here means wet-on-wet transfer: how readily a second ink film accepts onto an already-printed first ink film rather than onto bare paper. It is entirely distinct from prepress trapping, the chokes and spreads applied to artwork to hide misregistration, and the two share a name only by accident. The measurement is densitometric. A reflection densitometer reads the first-down solid, the second-down solid, and the two-colour overprint through the appropriate status filter, and the Preucil relation gives a percentage: overprint density minus first-down density, divided by second-down density. One hundred percent would mean the second ink transferred onto the first exactly as well as it would onto paper, which never quite happens on a wet multi-colour press. Realistic expectations depend on substrate and process. Coated stock on a sheetfed offset press commonly returns roughly 70 to 90 percent, uncoated stock lower at around 60 to 80 percent, and newsprint lower still, because absorbent surfaces change how the first film sets before the second unit strikes. Poor trap shows as weak, desaturated overprints - reds and greens that go muddy while the primaries look correct. The formulas have known limits: Preucil assumes densities are additive and drifts at very high or low readings, the Brunner apparent-trap variant uses the higher single density as its base and normally returns a lower number from the same data, and none separates trap from dot gain, so the line-screen gain figure shown alongside is a rough companion, not a correction. Any specification should state which formula and filter status it assumes.

Multi-colour presses created the problem: once several units printed in sequence on a single pass, ink had to be laid onto ink that had not yet set, and colour reproduction became dependent on transfer efficiency that nobody could measure directly. The tool that made measurement possible was the reflection densitometer, which spread through commercial printing after the Second World War and turned press control from a visual craft into a numeric one. The standard calculation is credited to Frank Preucil, who worked at the Lithographic Technical Foundation in the United States and published on colour measurement for lithography in the 1950s; the same research produced the GATF colour circle and colour hexagon used to plot hue error and greyness, after the Foundation became the Graphic Arts Technical Foundation. Preucil's trapping equation was adopted because it needed only three densitometer readings a press operator could take from a control strip on the sheet. A second widely used variant is associated with the Swiss printing consultant Felix Brunner, whose System Brunner control strips and quality-control methods became standard across European offset work from the 1970s. Both approaches were folded into the international standardisation of offset printing, notably the ISO 12647 series for process control in the 1990s and its revisions, which specify measurement conditions, densitometer response and tolerances so that a trap figure quoted by one plant means the same thing at another.