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FLEXO Magazine : August 2008
TECHNOLOGIES & TECHNIQUES maticity in two dimensions. Lightness is not accounted for in this approach. Color gamuts can be portrayed by plotting mul- tiple points to create an irregular closed shape. This represents the boundary of the particular imaging system in terms of its color gamut and achievable colors. Colors that fall outside of the plot are not repro- ducible by the system while colors that fall inside the plot can be reproduced. Figure 2 shows the gamut boundary FIGURE 2. THE GAMUT BOUNDARY OF AN INKJET PRINTER VERSUS A TYPICAL WEB OFFSET PRINTING PRESS. Opaltone is another system, which uti- lizes red, green, and blue. In addition to extending the gamut for purposes of colorfulness, both systems aim at helping to reproduce spot colors. This becomes valuable when printers are faced with many individual colors in a job. Instead of printing each color in its own station, the extended gamut approach allows printers to use the six or seven colors in some sort of combination to minimize the number of required print stations and subsequent manufacturing costs. Whatever the approach, understanding the impact on the color gamut is impor- tant if one wants to characterize and im- prove it. Even for traditional, non-gamut expanding approaches, it is valuable to understand the color gamut for process control and continuous improvement. Many factors impact the color gamut, such as: inks/colorants, substrate, other process consumables, color sequence, the type of printing process itself, and others. LEGACY APPROACHES Traditionally, describing and charac- terizing color gamuts have been done one of two ways. Both are based on CIE (Commission Internationale de l’Eclairage) colorimetry. While this is not the place for a thorough review of color theory and color spaces, it should be understood that describing color is three-dimensional with the three ordinates being lightness, chro- maticity (or saturation), and hue. One plotting technique utilizes the CIE Chromaticity Diagram (Figure 1). Here, color is plotted in terms of hue and chro- 5 0 F LEXO of an inkjet printer (the larger plot) along with a gamut boundary of a typical web offset printing press. On one hand, it does show a signifi cant difference between the two devices. However, since color is three- dimensional and this plotting technique only plots in two dimensions (lightness is missing), it is not possible to ascertain which specifi c colors fall inside or outside the gamut. Other shortcomings include the fact that equal distances on the dia- gram do not correspond to equal visual differences, the diagram was originally designed to plot the color of light sources as opposed to the color of objects, and it is conceptually awkward to explain in terms of normal color descriptors1. And fi nally, there is no way to quantify the gamut. A second approach involves plotting color in the CIELAB color space (Figure 3). Here, colors can be described numerically in three dimensions with L* representing lightness, and a* and b* used to locate a color in the color space, so that hue and chromaticity can be calculated and as- sessed. L* serves as the vertical axis while a* and b* serve as the x-y axes. Often, L* is plotted separately from a* and b*, which are plot- ted two-dimensionally. Further, C* (chroma) and ho (hue angle) are calcula- tions based on a* and b* to derive colorimetric values for chromaticity and hue. To display the gamut, a hexago- nal plot is made utilizing the three subtractive primary colorants (cyan, magenta, and yellow) along with the overprints of the three (red, green, and blue). This approach also suffers from the fact that it is only two-dimensional. However, with the L*a*b* coordinates AUGUS T 20 0 8 www. f l e x o g r a p h y. o r g known, calculations can be made of the area of the hexagonal plot and can be used to compare various conditions, to some level. This approach does not discriminate where the six endpoints fall along the lightness axis. Hence, it is not a valuable calculation. NEWER APPROACHES With the advent of color management and ICC profi les, a large sampling of a device’s color output can be used to math- ematically describe the three-dimensional nature of the device’s color output. The steps include: 1 Output of a profi ling target (CMYK . for hard-copy, RGB for monitors). 2 Color measurement of the target . utilizing spectrophotometry. The result is a data set of L*a*b* values for each patch in the profi ling target. 3 Generation of the ICC profile. . The profi le represents how that device renders color given a particular input (CMYK or RGB values). Once it has been created, the profi le is primarily used to convert image data either from one color space to another (RGB to CMYK or CMYK to RGB) or within a color space (CMYK to CMYK or RGB to RGB). This is how various devices simulate the color output of other devices. For example, computer displays can act as “soft proofi ng” devices, simulating the color output of a printing press. Another application is to color man- age a digital hard-copy proof to a printing press. Utilizing ICC color management FIGURE 3. COLOR PLOTTED IN THE CIELAB COLOR SPACE.
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