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| p.2 #1 · How to effectively use a gray card (white balance)? |
That means if on the "grey" patch PS measures 50,100,50 (corresponding to Red/Green/Blue), the software will then multiply ALL Red values in the ENTIRE image by 2x and same with Blue channel, so that particular patch now reads 100,100,100 (Red == Green == Blue). Theoretically this global adjust sets the white balance correctly since Grey should be neutral to R/G/B.
I think it works with gamma adjustments for each channel rather than fixed ratios, so it will increase the red in all pixels but not by the same amount. Otherwise it would ruin the colours of most of the brighter pixels as the red channel is maxed out and the required ratios cannot be represented.
Either that or else it stops adjusting the WB when it gets to the stage that colour channels will be maxed out in other pixels. I have just tried an example where Lr would not turn a yellow patch to grey because it would have blown out the blue channel elsewhere.