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FLEXO Magazine : August 2009
TECHNOLOgIEs & TECHNIQuEs If you ask people which spot color is preferred, they cannot rank them without a reference. When asked to rank these spot colors (Figure 6), most people will ask a question such as “compared the what?” However, if you show them a reference color, they can rank them pretty consistently and their ranking will correlate closely with the ∆E of that color compared to the reference. order to hue. People do not evaluate hue in terms of “more” or “less.” There is a logical order, but one hue is not logically greater or lesser. But there’s a little more to the story than simply expanding chroma. While it is true that people consistently prefer higher chroma, the degree to which they want to expand chroma depends upon the subject matter. In Figure 8, it is clear that while the proportional expansion of chroma improves the highly saturated colors like the sweaters, it actually degrades the more neutral colors like the fl esh tones (center photo). A new technology called “selective gamut expansion” enables the gamut to be selectively expanded based on the image. In the example below, the artist has chosen to expand the chroma of the fl esh tone to a smaller degree than the sweaters. The result is a signifi cantly better image than either the original CMYK image or the overall gamut expansion image. figUre 6. People cannot rank colors without a reference. However, when you show them four versions of the same image, people don’t ask for a reference or select their preference based on a minimum ∆E. However, they do rank them the same! Why? People consistently rate or rank images according to chroma. They also select an image based on their own personal references (and preferences) about what an image should look like—in this case, fl owers (Figure 7). figUre 8. A standard CMYK image (left), the same image with expanded gamut (center), and again with select gamut expansion (right). The user controls for selectively expanding the image are extremely simple. The “diffi cult” part is the math underneath. When you add a color, like orange, other colors must be subtracted in exact amounts in order to keep the lightness and hue the same. And while this math might be diffi cult and time consuming for a human, it’s easy for a computer. This “re-separation” is a basic color management transformation using color information from the color profi les (Figure 9). Look at the two ‘Freeze ‘n Thaw’ Flower packages again. In Figure 10, we’ve gone a step further than simply expanding the image. We’re treating the entire package like a single process figUre 7. People tend to rank images according to what they think the picture ought to look like. While the technology exists to expand chroma (C), lightness (L) and hue (H) in all three dimensions, the research suggests that one would logically desire to expand in only the L and C directions (not the H). Additionally, while research shows that it is undesirable to change hue, it is also does not make sense to “expand” hue. Unlike L and C, there is not greater or lesser 28 F LEXO figUre 9. Expanding chroma while keeping hue and lightness the same is easy to do with the right software, but diffi cult to do by eye. Augus t 20 0 9 www. f l e x o g r a p h y. o r g