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FLEXO Magazine : February 2009
TECHNOLOGIES & TECHNIQUES L* a* b* - FIGURE 1. ISO 12647-6 color requirements. C 54 -36 -50 M 50 71 -2 Must match to within 68% of production y 88 -9 88 8 L\E must be within K 26 a 2.5 L\E Proof must OK sheet match the numbers Proof Production Color Measurement on a Flexo Press with an RGB Camera By John Seymour A number of companies are offering CIELAB or CIELAB- like measurements with their RGB camera web inspec- tion systems. Unfortunately, the capabilities and limita- tions of these systems are not always clearly identified and un- derstood. This paper is an attempt to set reasonable expectations for CIELAB measurements derived from an RGB camera. Regardless of claims or implications otherwise, CIELAB mea- surements derived from an RGB camera will not agree well with a spectrophotometer. Because of this, an RGB camera should not be used to decide at makeready whether the color is in tolerance. However, the conclusion of this paper is that after color OK, colori- metric measurements derived from an RGB camera can be used to maintain color to a target, and to reliably report color differences. ONLINE ACCURACY NEEDS The ISO 12647 family of documents gives requirements for the print process. Part two covers commercial web offset, part three covers coldset presses, part four covers gravure, part five covers screen printing, and finally, part six covers flexo.There are three requirements for color in 12647-6:2006. These are illustrated in Figure 1. - The first requirement is that the measurements of the proof must match a set of numbers specified in the standard. There are required CIELAB values for solid and overprint patches on uncoated paper, coated paper, corrugated board, and for film and foil. The second requirement is that the color OK sheet must match the proof-and not the specified numbers-to within 8 E. This spec refers only to the solid patches. If no proof is supplied, then the numbers in ISO 12647-6 are the target. Finally, the third requirement is that 68 percent of the sheets from the production run must match the OK sheet to within 2.sLlE. Note that there are stacked up tolerances here. While the tolerances between production sheets are fairly tight, a produc- tion sheet may be up to 1o.sLlE away from the numbers in ISO 12647-6 and still be considered acceptable. One shortcoming of the standard is that it does not say how close the proof has to come to the published numbers, so this first requirement does not help us decide how accurately we need to measure. The other two tolerance windows, however, can be used to establish how accurate a color measurement de- vice must be. FEBRUARY 2009 www.flexography.org FLEXO
Sustainable Winter 2009