[argyllcms] Re: Fluorescence

Dear Gerard,

> Reflective objects
> do not have a XYZ color per se, but actually a spectral reflectance.

Yes, which depends on their physical properties, strictly.

> Only if they are illuminated by a particular light source (i.e. if you
> "view them under a particular light source"), they do reflect light,
> which has a particular XYZ color.

Yes, the "particular light source" throws visible energy onto the surface
causing it to absorb certain wavelengths and reflect others. The result
becomes what Danny Rich calls the "Color Stimulus Function". This is the
signal that enter our eyes and strikes the retina. CIE XYZ tristimulus
values can be computed at that point from the relative spectral power
distribution of the light (the source), the spectral reflectance of the
surface and the spectral sensitivity of the chosen observer. OK.

> That's why I attempt not to say simply
> "color of an object", but rather "color of the object UNDER VIEWING
> ILLUMINANT <XXX>",

I appreciate the disctinction, Gerard. I take great pain to do the same in
my wording too.

> because the same object will reflect a different XYZ
> color, if viewed under a different illumination source. But I guess,
> that's clear anyway.

No, it's important to be explicit about it.

> With "(actual) viewing illuminant" I simply mean the illumination source
> you have chosen (say, illuminant A) to view your print (which, as a
> consequence thereof, also becomes the illuminant for which the profile
> will be built).

The actual viewing light source under which I choose to observe the print
becomes the illuminant for which the profile must be built. I agree.

> And with "true absolute XYZ colors (under the actual viewing
> illuminant)" I meant the actual (non-adapted) XYZ colors of the light
> reflected from the printed samples, if the print is illuminanted by the
> chosen viewing illuminant.

"True absolute XYZ colors" mean referenced to the CIE Perfectly Reflecting
Diffuser, which is isotropix and Lambertian, and reflects 100% of incident
light. This corresponds to the XYZ as measured with an instrument.

> I.e. I simply meant to compute the actual XYZ colors of the light
> reflected from the printed samples, if the print is illuminated by the
> chosen viewing illuminant (for which the profile was built), from the
> samples' device values with help of the profile.

OK.

> (Btw, I'm assuming, that the print is always viewed under the viewing
> illuminant which has initially been chosen, and for which the profile
> has been built. I'm not considering viewing the print under different
> light sources.)

Right. Illuminant remains A all the way.

Regards,

Roger Breton  |  Laval, Canada  |  graxx@xxxxxxxxxxxx
http://pages.infinit.net/graxx

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