[ECI-EN] ECI-EN Digest, Vol 36, Issue 4

Dear Ken, there is a lot of confusion in the printing world about these paper matters. First of all, the ISO 12647-2 discribes nowhere a certain Delta-E for paper. It gives L*a*b* values for paper types, saying it must have a tolerance of L*= +-3, a*= +-2 and b*= +-2. If these papers are in that tolerances, it is ok. Most of the papers in the market are somewhere at the edge of the tolerance in the b*-value, so they can be easily out of this tolerance. But, this is not a problem, because the values which are used in the ISO five paper types going back to many years ago, when papers were not that white as they are now. Papers of today have mostly a high content of OBA's (Optical Brightening Agents) which give a brighter visual impression to the paper. On the other hand, if you measure it, the paper gives no higher readings in the white (so the L*-value) but in the blue (the -b* value). Annother (with above related) problem with measuring white paper with a spectral photometer used in the printing industry is, that there is no international calibration standard for the amount of UV of the light source in the spectral photometer. This is one of the reasons, why the paper industry does not measure with D50/2° light, but with D65/10°, because there is an intermnational calibration standard for it, so that we paper makers can measure all the same way. This means in practise, if you measure with your handheld spectrophotometer a piece of paper and I measure the same paper with mine, we could have very different readings, due to the not existing calibration standard of the UV light. This is also the reason, why the paper manufacturers do not give L*a*b* values in D50/2° to their customers, as long as we have not a common standard to measure it. But, we paper manufacturers are working on it. So, I would say, don't worry about the Delta-E's of paper and take it as it is. The ISO standard will also change in the future regarding this. Best regards Peter Peter Hannemann Senior Specialist, Technical Service and Print Technology UPM-Kymmene Sales GmbH Nordlandallee 1 D-26892 Doerpen Tel. +49 4963 401 1607 Fax +49 4963 401 96 1607 Mobil +49 160 90 63 24 90 peter.hannemann(a)upm-kymmene.com UPM-Kymmene Sales GmbH - Esplanade 41, 20354 Hamburg, Deutschland Tel. +49 40 34102-0 - Fax +49 40 34102-114 - www.upm-kymmene.de Sitz der Gesellschaft: Hamburg - Amtsgericht Hamburg, HRB 63207 Geschäftsführer: Dr. Hartmut Wurster