Re: Questions on optical brighteners, uv and fluorescence
Re: Questions on optical brighteners, uv and fluorescence
- Subject: Re: Questions on optical brighteners, uv and fluorescence
- From: Graeme Gill <email@hidden>
- Date: Mon, 30 Sep 2002 17:35:38 +1000
neil snape wrote:
>
I'll try to re-phrase although it's rather exploration into areas I know
>
nothing about even if it exists.
>
At some point ink can be covering the surface of the paper in a way that can
>
be cumulative or subtractive of the uv and fluorescence issues. Could the
>
inks then filter the qualities of the uv characteristics of the paper as
>
the inks could modify these issues.
OK, assuming that you're not talking about spatial effects (ie. not
talking about edges of dots etc.), then I think in general it's right
to assume that yes, inks will have a filtering effect on the deep blue
and UV wavelengths, the very wavelengths that are absorbed by the whitener
and re-emitted at longer wavelengths (visible blue).
This translates to meaning that the effect of an ink on a paper
with fluorescent whiteners, is not simply modelled by multiplying
the spectral absorbency of the ink by the spectral reflectance of
the paper.
The model I use for compensating for Fluorescent Whitener effects
in profiling certainly takes the view that I need to measure the
absorbency of the ink in the deep blue and UV, to compute how
much stimulation the whitener is getting, and then filter the
resulting re-emitted light by the ink absorbency at the
emission wavelengths. This relatively simple model had a
very pleasing agreement with reality, in the verification checks
I've made.
>
Could applying absolute colorimetry be
>
used in a significant way to modify the behaviour of the base paper to mask
>
uv brighteners?
>
For example uv doesn't pass through glass well at all. Does uv pass through
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the inksets we use or which colors?
I don't see that "masking" the effects of brighteners is at all
desirable. The brighteners are active and present in the real world.
Measuring and modelling the effect of ink on paper as if they weren't
there, isn't going to get results that are useful in the real world
(if this is what you meant by "masking").
The best we can do is more accurately measure and model the effects
of brightners, and compensate for the different measurement
conditions that exist between an instrument, and a viewer in the
real world.
The measurement instruments that are fitted with UV blocking
filters, do seem to suppress the action of the fluorescent paper
whiteners, and therefore measure color in an even more unrealistic
way compared to a human viewer. This also means that it's then not
possible to estimate the quantity of whitener in a paper by examining
the spectral readings from such an instrument.
Graeme Gill.
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