Jeffrey Stevensen wrote:
If you meter the sunlight, what happens to your accuracy as more or less of these lights mix in different parts of the scene? It's my experience that color temperature will vary over as little as a couple feet or even the angle of the metering instrument to the light sources, or the angle of the target to the main light (the sun) and the mix of other reflections, or the color of a wall or pavement or greenery and trees.
Right, but this doesn't really matter - in the end it's what the human observer would use as their white point that counts. The illuminant color temperature is just a starting point to guess/estimate what that is.
It seems to me that the colorimetric accuracy would come down to only a very specific white balance in one tiny part of a scene.
Colorimetric accuracy is independent of white point - ie. XYZ is absolute, not white point relative. XYZ is the light levels integrated with certain spectral sensitivities - the ones typical of a human observer. The way these three levels are balanced (gain adjusted) in the eye and nervous system is what sets the observer white point. White point is something of interest after you've captured the colorimetery, when you want to re-interpret a colorimetric image for a media/on a device which will cause the human observer to be adapted to a different white point than they would be in the original scene.
One would have to completely control and dominate the lighting with controlled lighting in order to have any predicted knowledge of the accuracy of a rendered color.
Not so if you have a colorimetric capture device.
And as Andrew has pointed out you would have to measure every color everywhere so as to have two "patches" to actually compare, an impossibility.
Not so, if you use a camera which is colorimetrically accurate. Using spot measurements of real world objects under real world illumination is just a way of confirming that your colorimetric camera is operating accurately. There are two complementary ways in which a colorimetrically accurate camera can be approached: 1) change it's spectral sensitivities to better match the human observer 2) Come up with ways of compensating (ie. calibrating) for the interaction of illuminant, object reflectance spectra and the non-colorimetric camera sensitivities. The latter can never be a perfect way of repairing the first defect, and has various degrees of practical difficulty. Graeme Gill.