Digital BW, The Print

Tony Sleep wrote:

> I missed the original, but the idea that OBA's cause metamerism is not 
> right. If you use carbon inks on OBA paper, you do not suddenly get 
> metamerism. Bromides containing OBA's did not display metamerism.
> 
> Metamerism is due to some ink pigments themselves reacting to UV with a 
> degree of flourescence, causing a shift in the intensity and/or frequency 
> of reflected light, and the image colours to change in an unbalanced 
> manner. Such fluorescence can make it hard or impossible to adequately 
> profile inks that have this behaviour, as the colour of the illuminant and 
> amount of UV changes the image colour balance.
> 
> If you use metameric inks on papers containing OBA's you are going to make 
> matters a little worse, but the real problem is the inks susceptibility to 
> flourescence, not the OBA. They'll substantially do it anyway, even on 
> papers without OBA's.
> 
> That is *the* issue with printing B&W using CMY pigments, and why 
> metamerism is a problem with such inks to a greater or lesser degree. I 
> have fairly recently gone from Generations G4 pigments to HP Vivera 
> pigments, and the newer HP inks are vastly less metameric, OBA in the 
> paper or not.

Metamerism (as we define it here) is caused by changing 
light spectra in general. UV light and UV light + 
fluorescence is just a part of changed light conditions. It 
can happen between two light sources both without UV light.

You have gone from a CMYK or a CcMmYK(k)  inkset to an 
inkset that has 3 grey inks replacing composite grey mixes 
with neutral grey inks, with the Z3100 that's extended to 4 
grey inks and extra hues to substitute CM and MY  mixes etc. 
That's the main cause for less metamerism with Vivera inks. 
4 and 6 inksets never delivered consistent neutral B&W 
prints, metamerism (including metamerism as a result of UV 
light and UV light and fluorescence).  "The" issue isn't 
UV>fluorescence>metamerism but how one can reduce color 
mixing in the first place by substituting color mixes by a 
single pigment. That single pigment should then show a low 
metamerism on itself, carbon does that, the neutral grey 
inks of HP Vivera (carbon + a neutraliser colorant ) does 
that too.
The Lyson Small Gamut dye ink didn't even while its CMY 
saturation was brought to a minimum.

I have used Generations G4 and am using the Vivera Z3100 inkset.

> 
> If you want to check the effects of OBA's, stick a sheet of 
> perspex/plexiglass over the print. It blocks UV and you'll see the paper 
> and inks unaffected by flourescence of either OBA's or pigments.

PMMA doesn't block UV. It actually lets more UV through than 
plain glass. For both there are filtered and less filtered 
versions. A plain sheet of Perspex shows more fluorescence 
in the print with UV light on it than glass will do. You 
better try outdoor qualities of PolyCarbonate then as they 
have UV filters applied in co-extrusion to protect the 
material itself against UV light. That makes the material 
more grey or slightly yellow however. It scratches more 
easily than Perspex too. PMMA is very UV light resistant and 
one of the best transmitters of light in total.

http://en.wikipedia.org/wiki/Acrylic_glass

If you would use a UV filter frame glass (like plain window 
glass is in a way) you would also loose the OBA effect to a 
degree. In that case less OBA in the paper, coating, is a 
wiser step for framed and unframed prints. Not to mention 
the more complex profile creation for framed or unframed 
prints on OBA containing or non-OBA papers with a 
Spectrometer that has a UV filter on the sensor or on the 
light source or no filter at all. My choice is (if my 
customers allow it) to use media with less OBA  and profile 
with a Spectrometer sans UV filter. But I can do both.



-- 
Met vriendelijke groeten, Ernst


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