The advantages and limitations of the I.C.I. coordinate system and the trilinear coordinate system yielding uniform chromaticity scales as proposed by Judd are compared. The advantages for engineering applications of a system combining rectangular coordinates with the uniform chromaticity scale (U.C.S.) are pointed out. The U.C.S. triangle is transformed to give such a system. A slight modification of the constants and a simple translation give a system of coordinates in which the first quadrant contains only greens, the second blues, the third purples and the fourth reds and yellows. The resulting diagram is compared with that obtained by MacAdam. Transformation equations for obtaining the rectangular uniform-chromaticity-scale coordinates from the I.C.I. coordinates and for the reverse transformation, the distribution coefficients for an equal-energy spectrum and for the spectrum of the Planckian radiator at 2355°K and at 2842°K, and also the ordinates to be used in computing rectangular uniform-chromaticity-scale coordinates by the Hardy selected ordinate method for a source at 2842°K are included.
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