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Halftone-Angle Combinations for nColor Separations

(October 2003) posted on Thu Nov 06, 2003


By Mark A. Coudray

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It seems that printers are increasingly moving beyond four-color process into methods of color reproduction that involve printing more than the traditional cyan, magenta, yellow, and black (CMYK) in order to broaden the color range of the printed image. The general term used to describe this type of expanded-gamut printing is nColor or nChannel reproduction, where n represents the number of colors used. This month, I'll briefly review the color theory behind nColor printing, then focus on the mechanics of working with halftone sets that involve more than four colors. Specifically, I'll discuss methods for choosing halftone screen angles to avoid moiré, a difficult process with traditional process color and even more challenging when the halftone screen count increases beyond four. The limits of CMYK The need for more than CMYK has been apparent for years. Many colors simply cannot be reproduced using a traditional four-color ink set. But we can expand the color gamut by adding colors to CMYK. Two common situations drive the need for extra colors. The first is when high-density, pure, saturated color is needed, such as crimson reds, crystal-clear greens and oranges, and rich, deep blues and purples. None of these colors can be easily achieved using CMYK. The second occasion for using extra colors is when the tone range must be lengthened, such as with images that contain very subtle pastel tones like egg shells, linen, baked goods, very soft flesh tones, perfume, ice, and smoke or mist. It is almost impossible to realistically reproduce images of these kinds without using light colorants to extend the tone range. In the past, I have written about expanded-gamut ink sets that include CMYK light cyan (Lc) light magenta (Lm), CMYK red (R) green (G) blue (B), and Pantone's Hexachrome system, which combines CMYK orange (O) green (G). All these are examples of nColor, here involving six and seven colors. The first set is used when the subject matter has great pastel detail, while the other two are selected for images that involve high-density, rich, saturated color. Another expanded-gamut option is to add white as an extra color to help control pastel transitions and to improve contrast within the image. The goal is to apply a very subtle mask of transparent or translucent white to soften the transitions. Effects can be varied by changing the sequence in which the white is printed. Building on CMYK Regardless of which of these expanded ink sets we use, the extra colors can make halftone-angle selection quite tricky. But by applying some of the rules that follow, we can minimize, if not cancel, halftone-to-halftone angle moiré, frequently called cross moiré. The rules are based on fundamental control principles and are merely extensions and variations of procedures used to print standard CMYK images correctly. Let's begin by considering what happens when halftones are combined. When any two repetitive patterns are overlaid, there will be moiré. Some will be offensive and damaging to the image (destructive moiré), and some will be pleasing to the eye and completely acceptable (constructive moiré). The common rosette pattern we see with four-color process is an example of constructive moiré. In process-color printing, we follow some basic rules to achieve acceptable patterns (Figure 1). One of the most simple rules is to keep halftone angles at 30°, 45°, or 60° apart from each other. We are constrained to a maximum of 90° into which we must fit all of these angles. Single-color halftones are most frequently positioned at 45° relative to the vertical and horizontal axes of the image. This is because the vertical and horizontal references are genetic cues for us. Everything about our perception is oriented toward horizontal and vertical alignment. So when we move the halftone angle to 45°, we minimize the influence of these strong reference cues, and the halftone pattern becomes much less noticeable and objectionable to us. However, for screen printers, 45° is a primary moiré angle due to knuckles of the mesh interfering with dot openings. So we must move off this angle by 4-8°--just enough to avoid the mesh knuckles. Two halftones printed together will have the least visible moiré if printed 60° apart from each other. It does not matter what these angles are, as long as the difference is 60°. Common combinations are 15°/75°, 20°/80°, and 22.5°/82.5°. All of these combinations will work for screen printers. However, the safest of them would be the first, since both of the angles are equidistant from 0° and 90°, angles known to cause high levels of interference due to thread eclipsing. If we choose any of the other angle sets, we must keep in mind that the darker the color being printed, the further the angle should fall from 0° (thread), 45° (knuckle), or 90°(thread). Darker colors create greater contrast, so moiré will be visible more easily. For three-color jobs, we separate the screen angles by 30°. Examples might be 20°, 50°, and 80°. This set would be fine for cyan, magenta, and black, but what if we add yellow to make a four-color set? Since yellow has a very low visual contrast, it often is angled 15° from one of the other primary angles. So our four-color set might become C=20°, K=50°, Y=65°, and M=80°. This approach will generate moiré, but since yellow lacks contrast, the moiré seldom will be visible. The exceptions occur when the yellow half-tone combines with other halftone colors to create light oranges, greens, and some browns. Any visible moiré will show up around the 20-30% tone range. More than four When moving beyond four colors, we must keep the basic rules in mind and apply certain variations. Let's begin with the expanded set that includes an extra highlight-white screen. Since white is a noncontrasting color like yellow, we can run the white 15° away from any of the contrasting colors. But this is not my preferred option. The better choice for printing white is to run it at the same angle as the black screen. The logic is that white ink will never print in the exact same area as the black. This means the negative dot on one screen will register in the positive space of the other color. Even if the dots slightly eclipse or trap each other, no moiré will result, especially if the black is printed over the white. Another advantage of this approach is that it results in a smooth surface with no overlapping dots. For UV printers, this will help to cancel cross moiré due to ink piling. Now let's move beyond five colors to six. In the first example, we will choose CMYKLcLm. For this color set, we can apply a variation of the approach used in the previous example, keeping black and yellow at the same positions and focusing only on the four remaining colors. Since light magenta overlays magenta and light cyan overlays cyan, we can simply apply an angle swap to these four colors. For example, suppose that cyan prints at 20° and magenta prints at 80°. We can make light cyan 80° and light magenta 20°. All the resulting angle pairs will be balanced and won't cause moiré when used together:

Colors Angles Difference light magenta / light cyan 20°/80° 60° light magenta/ magenta 20°/80° 60° light cyan/cyan 80°/20° 60° cyan/magenta 20°/80° 60° Now suppose we are using the Hexa-chrome color set (CMYKOG). This gets a bit more challenging because now we can no longer share angles between variants of the same color. However, we can apply similar logic. In this case it helps to visualize a hue-saturation-brightness (HSB) color model that depicts hue as an angle between 0 and 360° on a hue circle (Figure 2). The primary and secondary colors are located at 60° angles relative to one another. A quick observation of the hue circle shows us that mixing two primary colors creates a secondary color (e.g., yellow cyan = green). The circle also reveals that certain colors are polar opposites of each other (180° apart). The pairs are red and cyan, green and magenta, and yellow and blue. These are called complementary colors. When mixed, perfect complements form neutral gray. From this information, we can conclude that it is possible to run any complementary pair at the same angle, since they will not mix to form a secondary color. Where they do mix, they form neutral gray. We can eliminate this gray mix by replacing it with black. To do this, we apply gray component replacement (GCR) through Photoshop at a value higher than 80%. GCR is used to replace chromatic gray elements formed by mixing complementary colors with achromatic (black) halftone patterns that emulate the gray. It's not necessary to know the theory behind GCR, just know that its safe to print complementary colors at the same angle when GCR is applied at higher than 80%. For a Hexachrome ink set, green and magenta are at the same angle, as are cyan and orange. If we were to print a seven-color model with blue, it would be printed at the same angle as either the yellow or the black. My choice would probably be to use the black angle. Moiré develops between black and blue, but is not visible in non-contrasting dark shadows. By using the same angle as the black, you maintain 30° between all of the other contrasting colors. If you happen to be a fine-art printer and need to go beyond seven colors, then consider splitting the tone range and alternating around the hue circle. Using this approach, you can safely print up to 12 colors with no moiré. It is possible to create models for as many as 18 and 24 colors, but you may need to employ other options beyond angle combinations. When you exceed 12 colors, or if you develop a cross moiré with less than 12 colors, the final practical option is to print one or more of the screens as stochastic patterns rather than halftones. This is a randomization of the dot pattern and will not interfere with any of the angled halftone colors. The key is to use a small enough dot, generally between 80-110 microns. This dot represents the equivalent of approximately a 10% tone on halftones between 55-85 lines/in., a resolution level well within the range of any competent screen printer. Stick to the rules Developing a workable approach to nColor reproduction is simply a matter of using basic color theory to expand on the fundamental rules of four-color-process printing. Here are the main points to remember: * Keep all contrasting colors at angles 30°, 45°, or 60° apart. * Complementary colors can share the same angles. * Light and dark variations of a color are either 30° or 60° apart and are set so that light : light and dark : dark are either 30° or 60° apart. * If you print more than 12 colors, or if cross moiré develops with fewer than 12, use stochastic halftone screening with dot size between 80-110 microns for at least one color. These are very solid rules that rarely cause problems. For graphics printers, the biggest challenge is controlling total ink limit and dot piling. Using a separator experienced in nColor work is as critical as applying the fundamentals when you venture down this road.

