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FLEXO Magazine : February 2009
INDUSTRY INDICATORS D:: o I- - a III III :z: I- o I- D:: III l- I- III .... - DEAR EDITOR: I am writing in reference to the article "How Accurate are Your Spectrophotometers?" in FLEXO Nov. 2008. The article is, in itself, very inter- esting and gives a good view on prob- lems of color measurement in the graph- ic arts sector. Looking, however, to the measurements, the conclusions and the recommendations, I see a lot of inconsis- tency and would like to place some side- notes or change the spotlight from some items to prevent further confusion. It is surprising that VIGC is so surprised about finding varia- tions of up to 4 DE; this is a long-known fact and the reason that the tolerances in the ISO standards 2846 and 12647 are as large as they are right now. Due to the variation in geometries, light sources and other variables that are not well specified in the ISO 13655 it is still a surprisingly small variation. E.g. in Figure 1, seven devices were compared, with significant but very specific differences. Unfortunately, there are no serial numbers of software revisions specified. These where only seven, my firm did similar research on more than 40 instruments in early 2008, of which 21 were of the same "type" as VIGC's seven. With these 21 and the recorded serial numbers and revisions, we can easily see that three of the seven instruments are of a differ- ent revision and sold in packages for a different target group! I am not here to defend the spectrophotometer industry, but it is not fair to say that these instruments are "not top class." It is like in high school, one can be top of the class each year but only in the last year you are really the top off all classes. One cannot expect the top of the class of $1,000 instruments to have the same performance as the class of $5,000 or $15,000. It would also be a miracle if the average of the $1,000 class would match the average of the other classes-although they may, under cir- cumstances, give identical results. TEST SETUP In the past there have been a number of published and un- published comparative tests of spectrophotometers. Some exam- ined new instruments; others looked at used instruments under conditioned circumstances, and more still as done by VIGC. All methods have their specific advantages and disadvantages; all also had a specific purpose. The method chosen byVIGC com- TECHNOLOGIES & TECHNIQUES FIGURE 1. Deviations from the absolute value for seven devices of the same brand. How Accurate Are Your Spectrophotometers? VIGC Study Reveals Instrument Consistency Can be a Nightmare Q uality assessment in the graphic arts industry depends mostly on the use of spectrophotometers. Printers use them to check their production processes. Customers use them to evaluate the print job for acceptance. Quality demands are getting stricter every year; the spectrophotometer decides whether a job is accepted or nol But when VIGC, the Flemish Innovation Center for Graphic Communications, did a study on the accuracy of those devices, they found deviations up to a delta E of nearly 4! "Color quality is the biggest challenge in the printing industry," said Eddy Hagen, m....ging director and trend watcber ofVIGC. "Graphic arts companies will try everything to get the colors as desired by the customer. Those customers will use it as the most important criterion to accept, or reject, a print job. This makes the devices to measure that color quality quite essential So you would expect that the quality of those devices is top class. But it isn't." After having experienced some issues with different devices, VIGC started its first tests to compare multiple devices in the summer of 2007. e saw some deviations between the different spectrophotometers that we use ourselves," explained Fons Put, senior consultant with VIGC. "So we set up a procedure to check and compare different devices. As the reference we used the GretagMacBeth NetProfiler test chart. This is a test chart which comes with a certificate stating the L *a*b*-values of the different patches, measured with three spectrophotometers under ideal conditions. The certificate is valid for l.2. months only, so it needs to be renewed every yeiU'. And then we measured the 13 patches on the test chart with different spectrophotometers. For two patches we also measured the repeatability of the devices, mean- ing 10 measurements in a row." DEVIATIONS DETAILED Over the past year, VIGC has tested over 20 different devices. Most were currently in use by printing companies working in cooperation with VIGC. This is in contrast with similiU', smaller studies that have been done in the past. Other studies used de- vices acquired directly from the vendors. VIGC tested devices that are out in the field, that are used on a daily basis by companies in the industry. This gives them a very interesting overview of the capabilities of spectrophotometers in daily life. Those capabilities are not what people think. When a customer demands a maximum delta E of 2., which is often the case for .. FLEXO NOVEMBER 2008 www.flexography.org prises all from purchasing policy up to maintenance and train- ing. This is expected to give the largest variation. As mentioned before, this was considered when setting the tolerances in the different ISO standards for color conformance. Personally I don't know how to interpret an average .ðE of more than 13 totally different colors. If, however the maximum is 3.77 and the average (arithmetical mean of 13 .ðEs) is only 1.56, there must be also at least 50 percent of the measurements below 1.56. This is not so bad as the rest of the article suggests. The test target used was the N etprofùer chart. To call the color data received with this chart "the exact value" is a contradiction to the fact that the values are measured with 3 spectrophotometers and averaged! That the values of the VIGC meters are better in correspondence with this chart than the others is quite logical as theirs are calibrated with this chart and presumably the other devices were not. That newer types of devices perform better is also not a very big surprise. We found in our, and in a German test done in 2007, that there is almost no difference in result be- tween older and newer devices of the same type. Could it be that this brand giving particular differences in the red and orange is giving the right values and the other brand(s) make a structural mistake here? As long as there is no absolute reference, it is diffi- cult to say who is right. Simply using the (much) larger number of instruments in the test should not be the conclusive factor. If the customer expects much smaller tolerances than speci- fied in, for example, ISO 12647 and the printer accepts the job, he should know about the differences in measuring criteria and the background of the tolerances in the standards. In that case, he has to explain to the customer that this is only possible FLEXO 2009 www.flexography.org FEBRUARY
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