When the data is coded linearly, you need 14-bits to display what a human will perceive as a completely smooth gradient. With non-linear (perceptually uniform) coding , e.g. sRGB, this can be achieved with 9 bits. See
Also, for any that doubt the benefit of > 8-bits per channel, consider this link:http://download.microsoft.com/download/5/E/6/5E66B27B-988B-4F50-AF3A-C2FF1E62180F/GRA-T583_WH08.pptxSee slide 48 in this presentation on just-noticeable-differences vs. luminance from WinHec 2008. You see that 8-bits was sufficient for low luminance levels of early displays, but as luminance increases, people are able to discern between 9 and 10 bits. I have looked at the difference between 8, 9, and 10 bit gradients myself many times, and can vouch for its visibility on a good display.
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