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As indicated in the previous section, the 1931 CIE x,y Chromaticity Diagram (or xyY diagram) was inadequate because the two-dimensional diagram failed to give a uniformly-spaced visual representation of what is actually a three-dimensional color space. You can seen this problem clearly in the following illustration of the xyY chromaticity diagram:
Each line in the diagram represents a color difference of equal proportion. The distance between the end points of each line segment are perceptually the same according to the 1931 CIE 2° standard observer. As you can see, the lines vary in length, sometimes greatly, depending on what part of the diagram they're in. This disparity in line length indicates the amount of distortion between parts of the diagram.
To correct this, a number of uniform chromaticity scale (UCS) diagrams were proposed. These UCS diagrams used a mathematical formula to transform the XYZ values or x,y coordinates to a new set of values (u,v) that presented a visually more accurate two-dimensional model.
In 1960, CIE adopted one of these as the 1960 CIE u,v Chromaticity Diagram:
Compare this to the 1931 diagram in the preceding section. The effect was to elongate the blue-red portions of the diagram and relocate the illuminant (or white point) to decrease the visual disparity with the green portion.
However, this was still found unsatisfactory and in 1975, CIE proposed modifying the u,v diagram and supplying new (u',v') values. This was done by multiplying the v values by 1.5. Thus in the new diagram u' = u and v' = 1.5v. The resulting diagram was adopted as the 1976 CIE u',v' Chromaticity Diagram:
While the representation is not perfect (nor can it ever be), the u',v' diagram offers a much better visual uniformity. This can be seen by comparing the following illustration of the u',v' diagram with the x,y diagram at the top of this section:
The lines in theu',v' diagram represent the same as in the x,y illustration, only here we can see the lines are more nearly uniform throughout the diagram.
One other point to make about the CIELUV model is the replacement of the Y lightness scale with a new scale called L*. The Y scale is a uniform scale of lightness with equal steps between each value. However, this kind of scale is not adequate to represent differences in lightness that are visually equivalent. For example, a difference between values of 10 and 15 on the Y lightness scale differ by the same magnitude as values of 70 and 75. We do not see the values as being the same, however. We have much less ability to differentiate between degrees of lower values than we do of middle and higher values.
Using a mathematical formula, the Y values were translated to other values that are approximately uniformly spaced, but more indicative of the actual visual differences. The resulting scale, L*, closely models the Munsell system's scale of Value. The major difference is that L* uses a scale of 0-100, while Munsell's Value uses a scale of 0-10.
The L* lightness scale is used for CIELAB as well as CIELUV.
The value of CIELUV lies in the fact that, like CIEXYZ and xyY, it is device-independent and therefore not restrained by gamut. It is an improvement over CIEXYZ and xyY in that it better represents uniform color spaces.
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