[argyllcms] Re: CMP Digital Target 3 (not 003)

  • From: Gerhard Fuernkranz <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sun, 08 Mar 2009 00:03:54 +0100

Graeme Gill wrote:
> Photo papers use three colorants (CMY), so the resulting spectra are
> always a combination of those three spectra. The real world has many
> different spectra that appear to be the same color to our eyes, and
> such a chart is only exercising one choice of them.
> So as a result it is (theoretically) possible to model a typical RGB
> sensors response to such a chart exactly, even if the sensors
> spectral sensitivities aren't much like that of the human eye.
> Exposed to real world spectra, the profile for such a sensor created
> this way might be rather inaccurate.

On the one hand (when profiling a scanner for scanning photos or film)
it is an advantage that the spectra of photographic media have only 3
degrees of freedom, since this eventually enables a bijective mapping
from XYZ to a single corresponding reflectance/transmittance spectrum on
the medium (for a given illuminant, and for in-gamut colors only of
course), which in turn usually also enables a unique, bijective (though
non-linear) mapping between scanner RGB and XYZ.

But on the other hand the reflectance spectra of photographic media seem
to be pretty metameric wrt. real-world objects, thus a camera profile
generated from an IT8 target (or any other target chemically printed on
photo paper) may result in a color cast when applied to shots of
real-world objects. Eventually it also depends on the used light source
and the camera, of course. Visually judged, the resulting observer
metamerism (camera vs. human vision viewing target vs. real-world
objects) seems to be not a too big problem in conjunction with daylight
(outdoors) or a tungsten light source, but it seems to be rather
problematic e.g. in conjunction with a fluorescent light source.


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