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FLEXO Magazine : February 2008
a new model. With only a few small changes to Hunter's origi- nal math, this new map became the recommended and interna- tionally sanctioned method for reporting colorimetric values. The asterisk designation is used to identify the CIE Lab as different from Hunter and has for the most part displaced Hunter Lab as the preferred color space for most printers today. Concurrent with the release of the L*a*b* (pronounced "L star," "a star," "b star"), or CIE Lab (pronounced "See Lab"), the stan- dards group also adopted a variant, L*c*h°, which uses polar notation to plot colors. Both of the new metrics are essentially the same, except L*a*b* plots using Cartesian (rectangular) co- ordinates, and L*c*h° plots using polar coordinates. The value c* (chroma) refers to the distance of the color from the neutral axis. The higher this value, the more saturated the color is---flexo pressman often refer to colors with a high chroma value as being clean. The h° value, or hue angle, defines the hue of the color. A pure red color has a hue angle of 0 (this would equate to +a in an L*a*b* world). A pure yellow color would have a hue angle of 90°, green is 180°, and blue is 270°. Of course this hue angle can be any value between these. Back to that lime green for the Widget account. They prefer to use L*c*h° values for specifying standards. In this case, they have asked us to match a color that has an L* value of 80.58, a c* value of 41.20, and an h° (hue angle) of 134.29. They could have specified the color using L*a*b* coordinates. If they had done so, the values would have been L* = 80.58, a* = -28.77, and b* = 29.49. This is the same color, and plots at the same loca- tion. It is just a different method to plot or locate the color in our three-dimensional color space. Figure 6 shows our lime green color and the relationship between the L*a*b* and L*c*h° plotting systems. The impor- tant thing to understand is that these two models plot onto the same map. Regardless of your preference for L*a*b* or L*c*h° data, the use of modern color measurement instruments and software al- low us to define color standards numerically, communicate this data electronically, compare printed materials to the defined standards, get to color quickly on pressruns cut makeready costs, monitor press production, and reduce waste. ABOUT THE AUTHOR: Bob Binder has been at X-Rite for eight years in a number of positions, including applications special- ist, training specialist, customer success manager, and now applica- tions support manager. Prior to X-Rite, he worked for eight years as the graphics manager at Jefferson Smurfit Corp. in Carol Stream, IL---a folding carton printer. Prior to that, he spent five years as the graphics manager at Koch Label Co. in Evansville, IN. Binder holds a Bachelor's degree in Printing Management from Western Michigan University. FIGURE 6. A color plotted on a CIE Lab map. FIGURE 5. Hunter's method of plotting color. TECHNOLOGIES & TECHNIQUES