[argyllcms] Re: Lighting IT8.7/2 targets for camera profiling

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 26 Jul 2011 15:14:00 +1000

Stephen T wrote:
> I am intending to profile my camera. The calibration file for my IT8.7/2 
> target is calculated
> with "light D50, viewing angle 2". So, the correct light to photograph this 
> target is D50?

Ideally yes. But don't confuse the standard light used to define the CIE values 
with
what you can use the target for. The idea is that a profile will let you 
transform from
one colorspace to another via the PCS (Profile Connection Space). D50 is the
standard white reference and spectrum of the PCS. So even if you create a 
device profile
using a different light source, the profile will still do a matrix transform
from your actual white point to the standard D50 white point of the PCS.

This is less ideal than measuring under the same spectrum that the test chart
was measured under, or measuring the reference values of the chart under the 
same
spectrum of light that you will be shooting under, but it approximates it
using colorimetry, and conforms to the ICC interchange standard.
(Using an IT8 chart is an approximation in itself, since the dyes used in it
 probably don't represent the spectral characteristics of what you are 
shooting.)

> Light sources I can think of are: early morning sunlight, mid-morning 
> sunlight, noon daylight
> (too much UV? the target is printed on paper with OBA), simulated daylight 
> (Solux 4700K halogen
> lamps, I have four but the total light is still rather dim compared to the 
> mighty sun),
> electronic flash (is the spectral power distribution adequate?). Which light 
> source is
> recommended?

The light source closest to the one(s) you will be shooting under. If you shoot 
under
a variety of light sources, then you may want to make several profiles.

> Another question: 288 patches is not many for a CLUT profile and should I use 
> a shaper-matrix
> profile instead?

Probably. You can always try both and see which one you prefer.

Graeme Gill.

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