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The Print Sequence Project

(July 2007) posted on Tue Jul 24, 2007

What's the best order to print the four colors in four-color process? A definitive conclusion has eluded even the most technical screen printers. Read on to discover how scientific experimentation has established a better answer.


By Tricia Church

click an image below to view slideshow

Needless to say, we found large differences among the various sequences, and some of the prints were appallingly bad (Figure 2). Dot-on-dot gain is not a pretty thing to see! We formed our own opinions of the various sequences but were then fortunate to find that FESPA Slovakia was holding a meeting at which Bill Appleton was speaking. The experienced Slovakian printers gave their own opinions. There was, of course, no agreement about which sequence was best, though sequence 5 and the two Caza sequences had their supporters. Preferences depended, not surprisingly, on what people were looking for. As we will see, the 15° moiré effect strongly biased many of the judgments. As this effect can be taken into account by a very simple process, we will ignore this aspect of the prints until we come to the section devoted specifically to the effect.

Rather than rely on subjectivity, we decided to create an objective measure we call the Color Fidelity Index, CFI. This captures three independent factors, all of which have to be right:

  • Good grays: Shifts in grays are a good indication of a print generally out of control.

     

  • Good three-color tones: This captures the fact that a lot of the subjective color judgment was based on the more complex tones which often looked far too dark.

     

  • Good shadows: We want to lose as little shadow detail as possible.

We had anticipated using a fourth criterion—the color gamut. But careful L*a*b measurements and plots on the CIE chart showed that there was no significant difference in gamut in any of the sequences, which, in hindsight, is fairly obvious.

The idea was to obtain objective measures for each of these factors, scale each of them from 0-100 then divide the total by three to give us a 0-100 CFI, where 100 is the perfect print. Getting the objective measure for the grays was easy. For a 20%, 50%, and 90% grey, the L*a*b values of the printed and reference grays were measured and the color difference (Delta E) calculated. The Delta E for the three grays were then summed and put on a 0-100% scale with the average value ~50 and defined so that perfect grays (i.e., a Delta sum of 0) gave 100%.


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