GCR in Action
A few years ago I was the production manager of a mediumsize print shop in the US. One of our clients regularly sent us very expensive high-quality color printing work—just the sort of thing that we normally loved to do. However, these jobs used to cause us a lot of problems, and for a long time we could not work out why.
The printers hated them. As soon as the jobs got onto the press, everything seemed to go wrong. The colors changed constantly, so the ink levels had to be continually adjusted to compensate. The result was a lot of manual quality control afterward in the bindery. We typically had to order more paper for these jobs so that we would have enough good copies to supply the quantity ordered. Also, given the additional time we spent getting the job done, we had to add around 15% to the price.
The problems eventually turned out to have been caused by the designers who created the digital files. They were using tint books to select very rich, dense colors that used only C, M, and Y. None of them contained any black at all. These color specifications were then applied to large solid areas in the job, as backgrounds to everything else.
As already mentioned, C, M, and Y are designed to be transparent, whereas black is designed to be opaque. When you are printing a transparent color onto white paper, the white of the paper becomes part of the color you see. When you print black, the paper does not show through, and so it is not involved with the color you see.
If, for example, you printed a 70% tint of magenta with a perfectly even flow of ink, all the sheets you printed would look the same. If the ink flow became lighter, so would the tint, simply because the ink is transparent. If there is not so much of it there, more of the paper is able to show through. Similarly, if the ink flow increases, the tint will get darker.
Running a press is a bit like driving a car. You cannot just set the steering wheel once and hope that you will keep following the road. You have to make constant adjustments—a little to the left, a little to the right—to get where you want to go. On the press, the operator has to increase the ink a little bit, then decrease, then increase—and so on—in order to maintain a balance between the ink flow coming down onto the plate and the amount that the paper takes away in the process of getting properly printed. So minor fluctuations are happening all the time.
When you have three transparent colors combining to create a tint, all present at fairly high percentages, then even minor fluctuations will mean that your tint changes color all the time. This means that the operator will spend all day running up and down the press trying to make adjustments to each unit in turn to compensate. Not surprisingly, it is not much fun.
6.1 The tint shown on the left, 90% C + 70% M + 60% Y, can be simplified—roughly—by removing all three colors at the 60% level and replacing them with a 60% black tint together with the remaining 30% C and 10% M, as shown on the right.
As an example, let us take a tint made up of 90% cyan, 70% magenta, and 60% yellow. Not only would it be quite difficult to print, it would also get through a lot of ink. This is definitely something to consider, because the colored inks are much more expensive than black.
However, most of the color can actually be removed from this mix and replaced with black.
Imagine drawing a horizontal line through the tint percentages at the highest possible point at which they are all present (in this case that would be 60%) and removing everything below it, leaving behind 30% cyan, 10% magenta, and no yellow at all. If the ink that has been removed (60% of C, M, and Y) is replaced by a 60% tint of black, the resulting color is close to being the same (fig. 6.1). That’s the theory. In practice, the color would not only be slightly too light, it would also be too warm, as cyan is not as strong as yellow and magenta. If the black and cyan are increased slightly, the color will be perfect.
What we did, of course, was to apply the theory behind GCR to remove most of the transparent colored inks from the tint. As a result, printing will be much easier because the bulk of the color is now carried by black—which, being opaque, is more stable even when there is some fluctuation in the ink flow. It is also much cheaper.
It is easy to try this for yourself in Photoshop. Create a foreground CMYK color using just C, M, and Y, and then remove equal percentages of magenta and yellow and a slightly higher percentage of cyan. Then add the same percentage of black as you removed of magenta and yellow. You will have to juggle things further to get them to be the same, but it can always be done. The amazing thing is that Photoshop can do this automatically in images on a pixel-by-pixel basis.
The result of all this research was that we were able to substitute our own tints for those of the designers, and the printer was able to spend the day walking rather than running. The bindery was also happy because there was no more manual sheet-by-sheet quality control right through the job. But happiest of all was the client, when told that in future the price of his work would be 15% lower.