ECI-EN Digest, Vol 36, Issue 4
by Peter.Hannemann＠upm-kymmene.com
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
