Re: On the effect of fluorescence
Re: On the effect of fluorescence
- Subject: Re: On the effect of fluorescence
- From: Robin Myers <email@hidden>
- Date: Mon, 28 Feb 2005 10:33:26 -0800
From: email@hidden
Subject: Re: On the effect of fluorescence
Date: 28 February, 2005 10:32:06 PST
To: email@hidden
On 28 Feb, 2005, at 07:37, Roger Breton wrote:
I'm comparing two sets of measurements made from the same litho press sheet.
Up until now, I was under the impression that the UV-cut filter found on the
DTP41UV and, possibly, the Spectroscan (and other UV-cut fitted instruments)
would mostly affect colors with L* > 70, because colors in that range become
progressively affected visually by the hue of the paper which is the vector
of fluorescence excitation.
But, no, it does not seem that way in the face of some new evidence which
never occurred to me. Based on the two sets of data I'm working on, it's
plain to see that fluorecence seems to have practically no effect on solid
cyan (100%C with L*=56), surprisingly little effect on solid yellow (100%Y
L*=88), barrely detectable effect on a solid black, but what a difference on
on solid magenta (100%M L=46)! Almost 2 DeltaE, in fact. That's not just
random instrument variation.
And say that I thought that the net effect of fluorescence would generally
be to fool the instrument, and the profiler, to see more blue into colors
than there 'perceivably' is. Hence, the typical compensation of profilers to
throw more yellow into the mix to compensate for all that extra blue. But I
realize, now, I was wrong.
The UV Cut filters on the DTP-41 and the Spectrolino remove the UV component of the illumination. When the UV is removed, then the fluorescent whitening agents (FWA) of the paper are inactive. The FWA have an influence on any halftoned color where the paper is not completely covered by ink. The inks themselves can act as UV blocking filters, making the FWA inactive for color where the paper is entirely covered with ink.
As a side note, using a UV Cut filter also makes readings more manageable with printers using fluorescing inks. There was, at one time, a printer with a fluorescent magenta ink that made color management very variable. The only way to get reliable readings was to use an instrument with a UV Cut filter.
That could explain why uncorrected gray balance appears objectionably on the
green side?
Uncorrected gray balance appearing green could be caused by many things. You need to be more specific here for an answer.
Any ideas? Tales from the trenches?
I test the printer paper before making any target measurements. If an FWA is present, I use a UV Cut filtered instrument. Although it is possible for software to guess that an FWA is present and remove some of the effects from the measurements, it is also possible for the software to misdiagnose certain colors (e.g. blue-grays) and apply correction where none is needed, producing a wrong result. Using a UV Cut filter removes these ambiguous situations from the measurements, making the software work better.
Incidently, would anyone know the difference between X-Rite and
GretagMacbeth UV-cut filtration? Is there an ASTM standard on the
specifications of UV filtration, by any chance, or is it to the good
judgement of an instrument manufacturer?
I do not believe there is any ASTM standard on this since the type and amount of UV blocking filtration would depend on the instrument's light source (e.g. tungsten-halogen, xenon flash tube).
Robin Myers
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