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Boosting Process-Color Ink Gamut with Fluorescents

(December 2004) posted on Wed Dec 15, 2004

Coudray examines ways in which fluorescent colors can add vibrancy and depth to dull process-color images.


By Mark A. Coudray

The color gamut of process-color (CMYK) inks has always been a problem for screen printers. Frequently, printers boost troublesome process colors by adding fluorescent pigments, which can have a dramatic effect on greens, oranges, blues, and purples. Fluorescent pigments are mixed with process colors in the Pantone Hexachrome system, which is why prints produced with this system look so intense. Adding fluorescent pigments to the process colors you use may sound like a great idea--it definitely can give printed pieces some punch. But doing so also can lead to really bad side effects. This month, we'll look at ways in which you can overcome the limitations of the CMYK color gamut by correctly using fluorescents and other methods.

The problems with pigments

Poor purity of the cyan and magenta pigments is usually to blame for the shortcomings in the CMYK color gamut. Cyan is typically contaminated with equal amounts of yellow and magenta. This has the effect of graying the color and giving it a dull, muddy appearance. The amount of contamination ranges between 18-26%, depending on the brand of ink and the density at which it is printed.

The magenta is primarily contaminated with large percentages of yellow and usually less than 10% cyan. The amount of contamination in magenta depends on which pigment is in the majority. The two most common pigments used for magenta are rhodamine red and rubine red. The rhodamine is much more yellow; the rubine is too blue. It is common practice to mix these two pigments to obtain a more neutral, ideal, magenta hue. While the actual magenta hue may improve, the resulting mix also will be dull and muddy. In either case, the color is off from the ideal value and you must compensate accordingly in the separations.

A reliable reflection densitometer will tell you the percent of hue error and grayness in the ink. These values are useful for the initial comparison of different brands of process-color ink. The common perception among printers is that SWOP specifications for process color define what the actual color values of the process inks are. They do not. When the specifications were developed, the committee was careful to not specify a specific pigment or recipe. They did this for three reasons. They did not want to economically limit the choice of pigments, thereby dictating the price of a specific color. They were concerned with the availability of pigments. If a formula for pigment was specifically chosen, and this choice became unavailable in the future, the specification would not be worth much. Finally, they didn't want to limit the choices because new pigment-technology developments might deliver a better gamut or color range than pigments available at the time the specifications were established.

The gray areas of the color gamut

It is important to understand what it takes to increase the size of the ink gamut. You also need to know about the effects of adding fluorescent colors on press and what types of compensation for these additions are needed in the seps. Let's start our journey with the proof.

Whether you use an inkjet or traditional analog proof like Matchprint, if you add fluorescent colors, you will not match the proof. Printers usually add boosters because the printed image looks dull, dark, and muddy. The addition of boosters will not improve dot gain, but it will definitely brighten the final print. That's because fluorescent colors absorb visible light and fluoresce at a different wavelength. The frequency is much cleaner and free of the contaminating influences of the native, non-fluorescent pigments. In addition, the reflected fluorescent color is much brighter than a reflected traditional pigment. Where a normal yellow may have a brightness value of 95-98, a fluorescent yellow can be as high as 108. This brightness gives fluorescents their intensity and purity.

Screen-printed images don't match the colors in a proof because the pigments in screen inks do not match the pigments in the proof. If the colors don't match, the final print won't match. As basic as this sounds, most printers still do not get it.

Problems with adding fluorescents occur when the subject matter contains very strong, pure color--the intense, deep red of a Ferrari, for example. In an effort to obtain a closer match to the proof, printers add fluorescent boosters to brighten the image and increase the apparent purity of the color. This can definitely be helpful and, if used carefully, can work quite successfully.

However, problems crop up in the neutral colors and gray balance when fluorescent colors are added. The fluorescents can completely destroy both of these key areas. In the case of neutral colors, what do you think would happen to the flesh tone of the beautiful model standing next to our Ferrari? Her tones would be superheated to the magenta side, and she'd look as if she'd been in the sun far too long.

Indeed, any neutral will be seriously cast in the primary color direction of the fluorescent booster. This phenomenon is uncontrollable once you've made the separations. The only way to successfully use fluorescent boosters in an existing set of seps is to make an additional spot plate. This defeats the purpose of adding the fluorescent pigments in the first place; however, it will allow you to boost specific color ranges.

For instance, the addition of fluorescent magenta will boost oranges, reds, and violets. But such an addition also will have a negative effect on purples and blues, tending to cast them toward the violets. Since most flesh tones are primarily reds and oranges, making an additional spot plate will prevent the undesirable shifts in these color areas.

Another key problem area is the gray balance of the separation. The gray balance is the skeleton of the sep. If the skeleton is deformed--not neutral gray--the separation will never balance on press. There will always be at least one color dramatically out of balance when everything else is right. That's because the fluorescent pigment boosts the primary color away from the known values the separator used when he produced the original separation.

Even if you manage to obtain a reasonable grayscale, you will notice it is localized--only part of the tone range will represent the correct gray value. Other areas of the grayscale will show a strong color cast. When this happens, you simply will not be able to deliver an acceptable final print. This doesn't mean the game is over. A smart press operator will go back to prepress and let them know what he wants to do. If he knows it's important to add a booster in order to make the primary subject area really pop, he simply needs to communicate this fact to the person making the seps.

Using chromatic gray balance is the preferred approach for making color seps because it delivers life-like natural color. It uses all three of the primary process colors (CMY) to create the neutrals and grays in the image, as well as the gray balance of the image. However, when fluorescent colors are added, the chromatic gray balance gets destroyed during separation. To compensate, the best approach is for the separator to apply very large amounts of gray component replacement (GCR). This effectively removes the need to balance all three process colors to obtain the neutrals and grays in the image. Instead, any color can now be made up of two process colors plus black. The neutrality comes from the black. We have effectively replaced chromatic gray with achromatic gray (gray made with black). So far so good.

But the plot thickens. With high GCR, we are now able to deliver those unobtainable, intensely pure, bright, hot colors. We can still maintain our neutrals and grays (mostly). But the hitch lies in the effect of adding black to color. Black sucks the life out of any color it mixes with. So we find ourselves on the blade edge of a very thin, sharp sword. We can clearly boost the intense colors with fluorescent pigments while maintaining reasonable grays and neutrals. But the addition of the black in the neutral areas kills the subtle color in those areas. In the case of our flesh tones, the effect is the addition of gray. The danger is that flesh tones can take on the "dead" look of a corpse.

The addition of black to certain colors can be devastating. For instance, when you add black to yellow, you don't create a darker yellow. Rather, you end up with a dull, olive-drab green. This is most noticeable when you have yellow in shadow areas. In the case of an automobile, the yellow in the sunny areas will be very bright and clean, because of the fluorescent addition, while the shaded areas of the car will be green. I have seen examples where the car appears to be two toned. Very unnatural.

The colors most affected by the addition of black are yellows, oranges, and light greens. To evaluate this effect, take any Pantone book and look at the darker formulas--the dark values below the middle pure color on the page. You will clearly see which colors are adversely affected by the addition of black.

As with most technical issues in color reproduction, we can attain acceptable results if we compromise. Acceptable means not perfect or optimal, but better than if we take no action at all. In order to use fluorescent pigments, we definitely need to move the neutrals and grays onto the black screen. GCR is the way to go, but don't apply it at a 100% level. Pure GCR would allow maximum gain from the fluorescent colors, but there would be too much destructive color mixing. Therefore, back the GCR values down, concentrating in the 85-90% range. This will replace most of the gray with black, but it will still provide chromatic gray balance in the pastel areas. If there is any area where the fluorescent colors do not overpower, it is in the pastels. You'll be able to maintain reasonable neutrals and grays, even with the addition of boosters, when you use this approach. The neutrals may appear slightly more colorful or chromatic, but this is what you want the boosters to do.

This technique can help intensify your prints, allowing them to deliver strong, punchy color while maintaining acceptable neutrals. It's most useful when you have tricky, out-of-gamut colors falling into the green, orange, pink, violet, magenta, and red areas. However, it does not work as well for purples, blues, and cyans because fluorescent blue is the weakest of all the fluorescent colors.

Before you head down this road, be sure to meet with your sales and production personnel to discuss the direction you'd like to take. When used properly, this technique can greatly intensify the sharpness and brilliance of the reproduced image. With judicious use of fluorescent pigments--just enough so that the print doesn't slap you in the face--the final image will appear noticeably superior to a conventional process-color print, and even a trained observer will have difficulty figuring out why.

©2004 Mark Coudray. Republication of this material in whole or in part, electronically or in print, without the permission of the author is forbidden.


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