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FLEXO Magazine : November 2008
TECHNOLOGIES & TECHNIQUES Figure 3 is a test file that can be downloaded from the URL below: http://www.graphicbrain.com/fileadmin/Image_Archive/ Persberichten/2008/deltaE_100Yvs95Y.pdf We urge readers to measure it with your own equipment. How big is the color difference between the left and right? When mea- sured, this will give a very high delta E*ab, although the differ- ence is barely visible. When the test results of VIGC are recalculated with the newer delta E 2000, the figures become much more realistic. The overall average of all devices on the 13 patches is a rather bad 1.56 when delta E*ab is used, but a very good 0.39 when calculated with the more recent delta E 2000. Hagen continued: “The bizarre thing is that some experts don’t want to use delta E 2000 because it is not that good when it comes to rather large color deviations. In those cases, the old delta E*ab performs better. But who is interested in the accuracy of large color deviations? I want accuracy in small color deviations. That is where the battlefield is, where print jobs get re- jected. Not because the colors look very different, but because the delta E formula states that they are different. The printing industry would benefit a lot if the delta E 2000 formula would be the official formula for calculating color differences. But all relevant ISO standards only seem to know delta E*ab. Even the draft for the upcoming update of ISO 13655 on color measure- ment only talks about delta E*ab.” CONCLUSIONS & RECOMMENDATIONS What should we learn from this study? First of all, the measurements from a spectrophotometer—at least the ones used in the graphic arts industry—are not absolute. There can be variations between different devices. Also, the devices need to be calibrated on a regular basis and need to be maintained in a proper state. Periodical cleaning by the vendor may seem expen- sive, but what is the cost of a perfect print job that gets rejected because the spectro- photometer showed the wrong figure? The industry and the standard orga- nizations need to consider using delta E 2000 as the standard to calculate color differences when judging print quality. For small color differences delta E 2000 conforms much better to human vision than delta E*ab. Rejecting jobs because of color differences should be about seeing differences, not just about measuring a certain number. ? ABOUT THE AUTHOR: This article was submitted by VIGC (Vlaams Innocatiecentrum voor Grafische Communicatie/ Flemish Innovation Center for Graphic Communication), a not-for-profit orga- nization that supports graphic arts companies with innovation, with the implementation of new technologies. VIGC offers trend watching services, training and consulting. One of its core competences is color. It is also one of the active members of the Ghent PDF Workgroup, where it performs the compliancy testing of preflight applications. For more information, contact Eddy Hagen, managing director and trend watcher, at firstname.lastname@example.org, + 32 14 40 39 94; or Fons Put, senior consultant at email@example.com, + 32 14 40 39 93.