Re: To UV or not
Re: To UV or not
- Subject: Re: To UV or not
- From: Roger Breton <email@hidden>
- Date: Sat, 23 Sep 2006 11:27:54 -0400
Troy and Bret,
> I just measured a Monterey gloss sample from Tembec mills. There are
> OBA's in the paper. I can assure you of that.
All right. I'll have to dig out my faithful UV lamp to see this with my very
own eyes on my Monterrey Gloss sample here.
> Not enough to cause the
> this paper to show a spike at 457nm, but enough that you can see with
> a black light.
Here's a related question: in your opinion, other than looking at it with a
UV lamp, how can you tell unequivocally that a given paper sample exhibits
fluorescence? It's just not a question of looking at the b* value being
negative or positive IMO. But that's an important clue. I always believe
that if the spectral reflectance curve is smooth in the 400-450nm range that
the apparent "blueness" in a paper cannot be attributed to FWAs.
> When I perform a quick metamerism check of the paper
> white D50 to F2 using DIN6172 I get a metamerism Delta E diff. of
> 1.44. That is pretty big for the Whtpt of a press stock.
Can you explain this procedure a little more? Briefly. Could the metamerism
index calculation in BabelColor (or Spectrashop) help arrive at similar
conclusions?
> Just cause
> it is dark and dingy compared to a #1 or #2 stock does not mean that
> the papers haven't already been brightened from an even darker state.
OK. But where would draw the line then? When is enough fluorescence to throw
colorimetric measurements off? At what threshold of fluorescence would you
say a visual mismatch will occur?
> A good proofing media will be less than 1 Delta E in the same test.
Meaning that the proof will retain its appearance across various illuminants
by as little as 1 deltaE?
> It sounds like the press stocks are the issue,
Could be.
I wish there exists some kind of rigorous methods for rating the level of
FWA in a given substrate, some way to 'scale' the impact of FWAs in a
colorimetric match.
Incidently, if you take a look at SWOP's recent posting of Fortune Gloss
Coated #3 colorimetric measurements, you'll get the impression that this
substrate is completely neutral (it is shown as CMYK 0,0,0,0 = Lab 93,0,0!)
when a casual glance at an actual sample shows that this grade *is* bluish
in appearance, is being registered as such by instruments (b* -2 to -3), and
probably so because of the presence of OBs.
> now that good proofing
> medias are under control.
Like PressWhite160 ;-)
> Maybe using profiling SW that allows you to
> use your light source Spectral data will improve your matches visually?
The results of this procedure did not convince me. I won't go into the
details of one experiment I took part of at GATF, last summer, but let's
just say I have my doubts.
> Thanks,
> Troy
Regards,
Roger Breton | Laval, Canada | email@hidden
http://pages.infinit.net/graxx
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