[argyllcms] Re: Different Input Profiling Targets, Suggestions?

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 07 Jun 2011 11:38:13 +1000

Derin Korman wrote:
I was wondering if anybody had experience with different targets in
input(camera) characterization.

Particularly interested in Christophe M(won't disclose method of
printing), BasicColor(lightjet on fuji crystal semi-matte) and Gretag
SG (mix dye similar to color checker 24)

Hi,
        I can't give you any specific information, but I can explain the sort 
of things
you should be considering:

Color gamut:

The resulting profile will only be accurate between or near the color values 
contained
on the chart. This means that if the colors you are actually processing with 
your device
go outside the gamut of your test chart, the color values will have been 
extrapolated by
the profile, and are therefore likely to be not very accurate.

Color Resolution:

The profile will be most accurate for colors that are near those contained on 
the chart.
This means that the more closely and evenly spaced within the color gamut the 
value of
the test chart are, the more accurate overall the chart will be. So typically 
the greater
the number of test values, the better.

Dynamic range and White point:

Similar to color gamut, the profile will only be accurate over the range of 
lightness
levels exercised by the chart. At the dark end the ideal black test value will 
be a light
trap. At the white end the ideal white test value would be the perfect 100% 
reflective
diffuser. In practice there is another consideration, which is that by default 
the white
point of the profile is set by the white value of the test chart, and any 
values over
this may be clipped by the profile. So ideally the white patches should 
represent the
white value of the work you will be using the input device for.

Spectral similarity:

One of the fundamental problems with colorimetrically characterizing input 
devices, is
that typically input devices don't have the same spectral sensitivity as a human
observer. This means that it "sees" color differently, and that there is no way 
to
perfectly compensate for this in a device profile. [There will be some spectral 
values
that look the same to the device but appear different to us, and visa-versa.] A
colorimetric profile will best compensate for such differences when the target 
test
colors have the same spectral reflectance characteristics as the the intended 
work. What
this translates to is that you will get best results when the test chart uses a 
similar
printing process to whatever work you will be using the input device for. So if 
you are
intending to scan photographic prints, you should use a photographic based test 
chart. If
you were scanning artworks, then you should use a test chart that has pigments 
that are
similar to paint used on such artworks, etc. When characterizing camera's, an 
additional
source of spectral differences is the illuminating light source used. Once 
again, it will
be best to choose a light source to characterize the camera that is going to be 
most
similar to the light source you will typically shoot photographs under.

Graeme Gill.

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