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FLEXO Magazine : February 2009
if the same instrument is used for the measurements and that it is not possible to measure, for example, on a monitor or read the L*a*b* values from Photoshop and use these as specifica- tion. Unfortunately many printers, especially print sellers, do not know enough about this subject to protect themselves from the malpractices of the print buyers who also don't know but extract a section from a standard or from general purchasing terms from a competitor. WHY NOT L\E2000? Human perception of color differences is so subjective and.ilE depends on so many very different factors that there is always a good reason to find for any .ilE calculation method. Take .ilE94, used mainly in the textile industry. This is probably the best for the spectrophotometer manufacturer specifications as all the instruments are specified with .ilE94 tolerances. These numbers look good, but .ilE76 would give numbers 3 to 10 times larger! If the .ilE2000 average is 0.39 as stated compared to the I.S6 for .ilE76, this is realistic. It would mean that the printer would not have to start any correction until this 0.39 becomes about 1.0. If the difference would be linear, this was 4 .ilE76. This is again ex- actly the range according to the standard so there is nothing dif- ferent except for the psychological border of 1 .ilE as visible differ- ence, which is, for a printer, not relevant as his viewing conditions are far from ideal. .ilE2000 works fine, better than .ilE76 for light colors/tints of one single color. For mixed colors, which is FC printing per defi- nition, or darker colors the difference disappears and .ilE76 be- comes better. So why confuse the printer with another calculation which does not allow him to control his job. It may be better to discuss using Lch, where the h, or delta h, gives the color angle in the color circle. As every printer knows, a small difference in dark- ness of a color is hardly visible and seldom annoying; a difference in color angle is immediately visible and often very annoying. So it is not only about large or small color deviations but also about the color in which the deviation appears. CONCLUSIONS In the tests executed by IGT and during the preparation for ISO 12647 audits, we found more problems with cleaning, wrong settings, wrong .ilE calculation, calibration procedures etc. than with the actual differences between the different instruments. A .ilE of 3 or 4 is no problem if the source is known. So, training and education throughout the organization is getting more and more important. The author is right stating that we should not judge only by the numbers, rejection should be because of visual differences. So the discussion should be about the viewing conditions on the press console, perfect Dso color matching tubes with an acrylate plate (which is a perfect UV shield) collecting all spray powder from the press environment, etc. Sincerely, Wilco de Groot IGT Testing Systems. AST-100 AST-l00TSY viscosity sensor integrates into existing press-floor-control systems AST-300SY touch screen controller provides an easy to use graphical interface Simple Clean-In-Place design with no moving parts Economically priced Tel: 800.628.8139 or 508.946.6200 www.inkviscosity.com Flexibility for ink viscosity control! I -+S '! ifO* Flexol!Jraphlc Products and Services http://members.flexography.org
Sustainable Winter 2009