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FLEXO Magazine : November 2008
TECHNOLOGIES & TECHNIQUES quality print jobs in Belgium, he wants a device that measures the color as accurately as possible. However, the VIGC study re- vealed deviations up to 3.77 delta E for specific colors. On average, the deviation per instrument of all 13 patches was 1.56. SAME, BUT DIFFERENT In the study, VIGC encoun- tered multiple devices of the same type or the same brand. Is there a relation between the type, the brand and the ac- curacy? “That’s an interesting question,” said Put. “There was one general rule: the newer types of devices perform better. With devices that were a few years old, sometimes we got good results with the first one and bad results with the second one. Our own main spectrophotometer, which is calibrated regularly on that NetProfiler chart, was the best of them all. Another de- vice, the same brand, the same type, more or less the same age, performed really badly.” When the measurements of all 13 patches were averaged per device, VIGC found deviations from the exact value rang- ing between delta E 0.45 for the best device and 2.74 for the worst one. This means that several devices showed— on average—higher deviations than the margins that cus- tomers expect from their printers for high quality print jobs. The highest deviation for indi- vidual patches was a delta E of 3.77. “Also interesting was that one brand had quite strong deviations in the red and orange. We found this on multiple devices of that specific brand.” Even within a certain type of device, VIGC found very big differ- ences. Figure 1 shows the deviations from the absolute value for seven devices of the same brand, the same type. DEVIATIONS & CAUSES With the older devices, one major reason can be mainte- nance. “We know that some devices performed poorly because 30 F LEXO FIGURE 2. The difference in composition of the light source used in two different spectrophotometers. the optics or the calibration tile were dirty,” explained Put. Spectrophotometers need regu- lar calibration and also periodic cleaning. Another reason can be the light source used. Put continued: “No light source has a perfect ‘spectral power distribution.’ And if you don’t have much power in certain wavelengths, not that much color can be reflected in that region, which limits the accuracy of de- tecting small variations in that color region. An LED light source has a completely different spec- tral power distribution from a gas filled tungsten bulb. And both are used in spectrophotometers.” Figure 2 shows the difference in composition of the light source used in two different spectrophotometers. WHY NOT DELTA E 2000? The big differences that were found can cause trouble: custom- FIGURE 3. When measured, the left and right side will give a very different delta E*ab measurements, although the difference is barely visible. ers demand a delta E of 2, but their measurement device might be of a delta E of 3. A simple – and valid – solution for the indus- try would be to accept delta E 2000 as the formula to calculate color differences. “When people talk about delta E, they usually refer to delta E*ab, also known as delta E 1976. This is also the formula that is men- tioned in the relevant ISO standards. But this formula is very inaccurate when it comes to small color differences,” insisted Hagen. “I can show you a pair of colors with color difference of delta E 5 which is barely noticeable. Take a 100 percent and a 95 percent pro- cess yellow from ISO coated. The deviation is just noticeable, but if you calculate it with delta E*ab, you get a difference of 5. Delta E* ab doesn’t really conform to the human perception of color differences. The newer delta E 2000 does. Take the same yellow color pair and you will get a delta E of approximately 1. Which conforms to the initial idea of delta E: a delta E of 1 is the small- est noticeable color difference.” NOVEMB E R 20 0 8 www. f l e x o g r a p h y. o r g