USC Diagrams; Uniform Chromaticity Scales; Yu′v′
Both the CIE 1960 UCS diagram and the 1976 CIE u′v′ chromaticity diagram are associated with the 1931 Y tristimulus value to provide a complete trichromatic specification since V = V′ = Y.
The 1931 CIE system of colorimetry  allowed color stimuli to be defined in terms of tristimulus values XYZ but does not provide a particularly uniform representation of color stimuli in visual terms. If lines are drawn on the CIE 1931 chromaticity diagram that represent equal perceptual steps, then the disparity in the lengths of the lines is as great as 20 times, with the lines in the green region, for example, being much longer than those in the blue region .
Properties and Current Status
All chromaticity diagrams, whether xy, uv, or u′v′, have the property that additive mixtures of colors are represented by points lying on the straight line joining the points representing the constituent colors . However, the CIE 1960 UCS diagram and the 1976 CIE u′v′ diagram represent substantial improvements over the 1931 CIE xy chromaticity diagram in terms of the visual uniformity of the spaces. Whereas lines that represent equal perceptual steps drawn on the CIE 1931 chromaticity diagram differ in length by as much as 20 times, the same lines drawn on the CIE 1976 u′v′ diagram differ in length by about four times (and over much of the diagram, the difference is not greater than two to one) .
CIE 1976 UCS is useful for showing the relationships between colors whenever interest lies in their discriminability. However, it is mainly used for the representation of self-emissive colors on display devices or those produced directly from light sources since the diagram assumes that colors are of equal luminance (this condition is rarely met in practice with reflective surface colors). Note, however, that CIELUV, the first approximately uniform three-dimensional space, is a transformation of the CIE 1976 UCS chromaticity coordinates u′, v′, and Y.