[argyllcms] Fluorescence - Graeme's article
- From: Roger Breton <graxx@xxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Sun, 28 Aug 2005 19:12:47 -0400
I like Graeme's "Fuorescent Whitener Reflectance Model". It clearly shows
the parameters of the models that need to be evaluate in order to measure
surface colors under FWA paper.
Unfortunately, not all valirables entering the model are defined?
In what Graeme calls the normal spectral reflectance model, there is only
one incident light source, Phi sub I (lambda), which represent the incident
luminous flux upon the surface characteristic of the illuminant.
But in the FWA model, there is a second luminous flux called Phi sub I (mu),
which represent some normalized quantity? But "(mu)" then finds its way into
the Transmittance of the colorant. And then into the "FWA normalized
excitation spectrum".
To me, there is only ONE kind of light source incident upon the surface. But
I agree then that the UV portion of that light has something to do with how
much fluorescence gets excited into the paper.
I am not clear on the "Overall FWA excitation level" and the Quantum Yield
factor.
The "Excitation Spectrum range -- typ. 300-600nm" is not defined either.
If I was to operationalize all the variables in Equation 3, I would not know
what to plug into E, Q and f(lambda).
Before going on with some questions with the rest of the article, I would
like to ask about the following idea which does not see too far fetched for
me. I understand now that I need to evaluate two quantities:
A) how fluorescent is the unprinted paper,
B) how different light sources contribute to this fluorescence.
Question B has some implications for future color matching under different
light sources but Question A poses the fundamental question. Suppose I have
a spectrofluorimeter and I can indeed separate the light reflected off the
paper itself from the light emitted as a result of UV excitation, then can I
assume that the UV excitation will be constant over any colorant level, or
will it vary according to the particular colorant?
If it coud be demonstrated that the UV excitation off the unprinted paper
remains the same for any colorant level (a hard to prove proposition I can
image as 400% of ink is likely to absorb so much of the incident light
source that hardly and UV excitation will ensue) then the "excitation
spectrum" could be subtracted or multiplied spectrally from the raw spectral
measurements of any colors?
Regards,
Roger Breton | Laval, Canada | graxx@xxxxxxxxxxxx
http://pages.infinit.net/graxx
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