illuminants
illuminants
- Subject: illuminants
- From: email@hidden
- Date: Wed, 12 Nov 2003 22:24:50 EST
In a message dated 11/12/2003 6:33:44 PM Pacific Standard Time,
email@hidden writes:
>
Why two profiles in ACR at different color temperatures?
>
>
If we have one measured profile at one color temperature, why can not the
>
other
>
one be mathematically predicted since the spectrum distribution of the light
>
source can be calculated at a different color temperature.
Bertho,
One method is to perfom a spectral camera characterization to get the camera
spectral responses (a major undertaking).Then for each image, estimate the SPD
of the white point for which the image was taken, then calculate the profile
for that white point. Remember, you can only calculate an illuminant's
spectral power distribution (SPD) from the color temperature under very narrow
conditions such as the D-series or blackbody where the relationship is known. In
general, many different spectral power distributions can have the same
correlated color temperature (CCT) or tristimulus values (color). This is related to
illuminant metamerism. So the ACR idea is sensible: Take two very different
illuminants with different CCTs and color that span typical usage conditions.
Determine profiles for both. Interpolate between them as intelligently as
possible depending on the camera's estimated scene white point for each image
Eric Walowit
Tahoe
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