Doug Kerr
Well-known member
As you know, we have adopted the CIE u'v' chromaticity chart (the u'v' plane) as our preferred way to show small chromaticity differences (such as offsets from neutrality, color correction "error", and so forth).
An attraction is that a consistent distance between two points on the chart represents very nearly a consistent perceived chromaticity difference.
In very broad terms, the smallest chromaticity difference that can be noticed on an A-B comparison under consistent observing conditions is on the order of 0.001 u'v' unit.
We use the notation du' and dv' to indicate the numeric difference in the u' and v' coordinates of two chromaticities being compared, and the symbol du'v' to indicate the total distance (total difference) between the two.
Here are two charts to help give an idea of the axes and scale of this chart. The first plots the chromaticity of seven colors expressed in RGB terms (sRGB color space). They are "white" (e.g., 128,128,128); the three primary hues with the corresponding coordinate value of 129 and the accompanying coordinates at 128 each; and the three secondary hues, with the two "key" coordinates at 129 and the other one at 128.
These are of course the smallest chromaticity differences from "white" that we can have in an 8-bit RGB representation at the luminance implied by these coordinate values.
The second chart shows the chromaticity of "white" and eight other colors expressed in L*a*b* terms. They represent all combinations of the values of -1, 0, and +1 for a* and b*, with L*=50. (Remember that in L*a*b* a pair of a* and b* values does not describe a chromaticity unless accompanied by a particular L* value.) The L*a*b* color space here has the same white point as sRGB (D65).
I have indicated on this figure the directions of teh a* and b* axes of the L*a*b* color model as projected onto the u'v' plane.
I hope that these reference charts are helpful.
An attraction is that a consistent distance between two points on the chart represents very nearly a consistent perceived chromaticity difference.
In very broad terms, the smallest chromaticity difference that can be noticed on an A-B comparison under consistent observing conditions is on the order of 0.001 u'v' unit.
We use the notation du' and dv' to indicate the numeric difference in the u' and v' coordinates of two chromaticities being compared, and the symbol du'v' to indicate the total distance (total difference) between the two.
Here are two charts to help give an idea of the axes and scale of this chart. The first plots the chromaticity of seven colors expressed in RGB terms (sRGB color space). They are "white" (e.g., 128,128,128); the three primary hues with the corresponding coordinate value of 129 and the accompanying coordinates at 128 each; and the three secondary hues, with the two "key" coordinates at 129 and the other one at 128.
These are of course the smallest chromaticity differences from "white" that we can have in an 8-bit RGB representation at the luminance implied by these coordinate values.
The second chart shows the chromaticity of "white" and eight other colors expressed in L*a*b* terms. They represent all combinations of the values of -1, 0, and +1 for a* and b*, with L*=50. (Remember that in L*a*b* a pair of a* and b* values does not describe a chromaticity unless accompanied by a particular L* value.) The L*a*b* color space here has the same white point as sRGB (D65).
I have indicated on this figure the directions of teh a* and b* axes of the L*a*b* color model as projected onto the u'v' plane.
I hope that these reference charts are helpful.