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FLEXO Magazine : June 2014
Proper process control procedures would require setting of rational tolerances and not allowing the pressroom or the print buyer to set unrealistically narrow ranges, which will result in frequent “out of control” conditions at the press side. When running to a gray balance condition, then tracking the CIE Lab L* instead of the neutral print density of the 3-color overprints and the CIEDE2000 color differences of the solids and mid tones will provide adequate control to maintain the press at the G7 CMY triplet aim conditions that were established during press characterization. Since NPDC and (100-L*) have similar dependencies on tone value, if one duplicates the K lightness scale with the CMY lightness scale and maintains the ΔCh within specification, then one will obtain the same visually consistent appearance as a neutral based on the log of the luminous reflectance factor (Y). Finally, if the print property of interest has a visual basis—such as contrast, undertone, highlight or shadow levels—then the appear- ance variables will always be more appropriate and more sensitive to process variations than will be density. Print contrast can be measured using the difference in visual contrast. Instead of densitometric con- trast, visual contrast is based on the L* difference between the solid and the three quarter tone normalized by the L* of the solid (ΔL*/L*). The ISO process control standards have introduced the concept of a deviation tolerance, that is, the color difference between any print and the “OK” print, selected during makeready. That difference can be computed from any test target or image area measured after the makeready print was accepted. Even if the image area is not perfectly uniform, spectrocolorimeters provide area averaging so that small inconsistencies in positioning can be averaged out. Pressrun tolerances can be tracked using single side control charts for individual differences (Image 7), or for averages or ranges of multiple readings from sets of targets. This will allow the pressman to follow the conformance to ISO requirements and is a far more accurate met- ric—unlike ISO Status Density without operator ink key correlation. The major criticism of this approach is that, unlike density, there is no guidance to pressmen on how to adjust the press if the color difference is out of tolerance. Thus, it is often useful to track both total color difference (ΔE) and metric chroma difference (ΔC). Since the chart is tracking each ink color individually, it is possible to relate the perceived chroma (ΔC) to the amount of ink, since a thicker ink film will produce a stronger appearance. Always remember: “Densitometry measures what you do not see and colorimetry measures what your customer wants to buy.” n About the Authors: Danny C. Rich, Ph.D is senior color physicist at Sun Chemical Corporation. Ray Cheydleur is OEM technical manager at X-Rite Corporation. Greg Imhoff is a sales and marketing leader in the industrial and capital equipment markets Image 7: Statistical Control Chart example, showing color difference versus impression 44 FLEXO | JUNE 2014