What is "Foveon Blue"?

Mark Scott Abeln wrote:

xpatUSA wrote:

xpatUSA wrote:

My feeling is that if the WB were set for achromatic light, the lamp source color itself would be rendered correctly. The nearest WB on my cam to achromatic light (i.e. 5455K, vide illuminant 'E') is the 'sunlight' setting which is 5400K - and a small adjustment to the so-called tint (green/magenta) would bring the WB quite close to achromatic.

So I shot the sky (only) again today, using 'sunlight' WB. The cam was aimed up between my trees and the angle was about 45 degs from the sun. According to WikiPedia the sky should be pretty blue at that angle, although there was a light haze up there cutting back the saturation some.

Opened the image in SPP 3.5 with the histogram selected to 'All'. The blue was well over to the right as expected but the red and green were apart, with red quite a bit to the left of green. Then I got on the color circle adjustment thingy and increased in the magenta direction while watching until the red and green came together. It only took +4M to do that.

Not done yet. I don't know how 'pure blue' a sky should be and whether green or red should be significantly scattered compared to blue at that sun angle.

Rayleigh scattering is the process where light is diffracted around the molecules of oxygen and nitrogen in the air, which causes the blue sky. This scattering is proportional to the fourth power of the frequency of light, and so blue light will be scattered much more than green, which in turn will be scatter more than red. And so, we find that the RGB color of the blue sky will have B > G > R.

If you take a photo of a really deep, blue sky, with a daylight white balance, and plot the fourth root of the linear sRGB values against the approximate spectral frequencies of the sRGB primary colors, you’ll get a roughly straight line through all three points — but this undoubtably is influenced by the metamerism of your particular model of camera. Note that water vapor and dust in the air will scatter all the frequencies equally, making the line flatter and more horizontal.

Pardon me, but I'm getting slightly grumpy. I'm shot down in flames on sky shots, lamp shots and no doubt the circling sharks are still thinking about watch hands and lamp spectra (a great silence there, though), while all I seek is the Great Universal Truth. What is the correct white balance for light sources?

Anybody?