Over the past several years, we have made significant progress in setting standards for printing inks. As one result, standard ink sets now can be used to print color separations that are made in different parts of the world.
One of these standards, recently set by the International Organization for Standards, is called ISO 2846-1. It specifies sheetfed and heatset inks, and is based on an American standard called SWOP--Specifications for Web Offset Printing.
In the past year and a half, I have had several inquiries about color separations that were made in Europe and would be printed in the United States. The printers wanted to know what inks to use. I said, "Use SWOP inks--they should work fine." The feedback has been very positive.
ISO 2846-2, which deals with news inks, is still in process. The standard is not far enough along to talk about the numbers. However, our experience with ISO 2846-1 should be a good indication of what is going to happen with the news inks, since the same pigments are used in both ink sets.
ISO 2846-1 deals with both the color and the transparency of the ink set. When we started working on it, we thought that we would have three sets of target numbers: one for the United States, one for Japan and one for Europe. That was because we thought the European standard for cyan was much redder than either SWOP or the Japanese standard (see first graph).
About two years into the process, though, when we actually started measuring proofs and comparing numbers, we found that the colors were all very close. So we were able to select a point for the new ISO standard that was an average of those three and specify a tolerance that would allow them all to meet the new standard.
ISO 2846-1 also specifies how to check inks. A series of prints are made on a given substrate at a series of ink-film thicknesses between 0.7 and 1.1 micrometers for sheetfed inks (0.7 to 1.3 for heatset inks). Then they are measured colorimetrically and compared to the target numbers (see second graph).
Typically, the total color difference (delta-E*) of the first print is greater than the limit. As the film thickness increases, the color difference decreases until we get to a low point. Then, as we continue to increase the film thickness, the color difference starts to get higher again.
As long as any of these points falls within a tolerance box formed by the high and low ink-film thicknesses and the delta-E*, that ink is acceptable. So, in the second graph above, the ink represented by the set of dots to the lower left is acceptable, while the inks represented by the other two sets of dots are not.
The transparency part of ISO 2846-1 is a new test method. A series of prints are made over either a printed black bar or the black bar on a Leneta contrast card. The prints at the various ink-film thicknesses are measured colorimetrically and the color differences between the overprint and the black bar calculated. The slope of the plotted points is used to calculate the transparency value.
In addition to these international standards, the ANSI Committee for Graphic Arts Technology Standards has developed a U.S. standard for sheetfed printing of process color proofs, ANSI/CGATS.6-1995, and a related technical report, ANSI/CGATS TR001-1995. While not a substitute for SWOP, CGATS.6 provides the scientific community with numerical data related to SWOP. The use of this standard should improve the consistency of input materials, films, proofs, plates and inks.
Walt Zawacki is technical manager for color and printing research at Flint Ink Corp. E-mail, email@example.com; phone, (313) 995-3100; fax, (313) 995-5676.
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