[argyllcms] Re: Fluorescence
- From: Gerhard Fuernkranz <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Sat, 27 Aug 2005 22:47:42 +0200
Roger Breton schrieb:
they aren't illuminant independent, but they contain enough information,
in order that *both*, a) the "true" absolute XYZ colors (under the
actual viewing illuminant)
Actual viewing illuminant? You mean "adapted" from their D50-based original
values?
Roger,
maybe I haven't expressed clearly. Let's start here: Reflective objects
do not have a XYZ color per se, but actually a spectral reflectance.
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. That's why I attempt not to say simply
"color of an object", but rather "color of the object UNDER VIEWING
ILLUMINANT <XXX>", because the same object will reflect a different XYZ
color, if viewed under a different illumination source. But I guess,
that's clear anyway.
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).
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.
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.
(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.)
and b) an illuminant relative transformation
from the viewing illuminant to D50 (that's how ICC-absolute intent is
actually interpreted by the ICC!)
Viewing illuminant? That's starting to sound chinese to me :((
See above what I mean with "viewing illuminant".
Then, all D50-based measurements of my spectro in reflectance mode are
considered "made for a D50 viewing illuminant", right?
I would say so - or maybe "computed/calculated for D50 viewing
illuminant". Notice, the spectrometer actually measures the spectral
reflectancies (or one should better say "radiance factors", as I've
learned in sci.engr.color) - but it does not directly measure any color.
The XYZ colors of the measured samples under a particular illuminant are
eventually computed, from the measured radiance factors, the illuminant
spectrum (e.g. D50), and a given set of observer CMFs (e.g. 1931 2°).
If fluorescense comes into the play, the situation becomes even more
complicataed, since then the spectral radiance factors of the object are
no longer constant, but also change as a function of the illumination
spectrum. That's where FWA compensation comes into the play. Given the
spectral radiance factors that have been measured under a particular
instrument light source, the FWA compensation attempts to predict the
radiance factors under a different given light spectrum.
E.g. in order to obain XYZ colors under D50, the following steps are
required: 1) You make measurements with a spectromenter (which has a
tungsten light source) and get the spectral radiance factors. 2) The
measured radiance factors are FWA corrected to predict the radiance
factors as they would have been measured if the instrument had a D50
light source. 3) The corrected radiance factors and the well-known D50
spectrum ared used to compute the colors under D50. Steps (2) and (3)
are implicitly performed by the profile command, if you specify the
options "profile -f -i... -o... ...". Without "-f", profile performs
only step (3).
Regards,
Gerhard
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