Page:Journal of the Optical Society of America, volume 33, number 7.pdf/59

 Table III.—Continued

coordinates (V$X$—V$y$) and (V$z$— V$y$), which are simple functions (2) of X, Y, Z, the corresponding tristimulus specifications. The conversion is easily made with suitable tables (31). The illuminant point serves as origin for the chromaticvalue. Another advantageous feature of chromatic-value is that the constant chroma loci plotted at one value level approximate in diameter those at the other values. Furthermore, it was found feasible to employ a single system of chroma curves for all value levels. This curve equivalence for all value levels vanishes on conversion to the I.C.I. (x, y)-system, but smooth inter-value-level transitions remain. The particular ovoid form finally chosen to define all loci of constant chroma, in Adams’ coordinates, is illustrated in Fig. 11. This shape was the outcome of many trial smoothings on Adams’ charts bearing vector arrows representing the averaged visual estimates previously published. The familiar end-effect of chroma whereby extreme strong samples tend to look stronger than they would elsewhere in the series was estimated to be of the order of half a chroma step and compensated where required. The shape was arrived at independently by two investigators both of whom followed a procedure of progressive approximation in which intra-valuelevel and inter-value smoothing were alternately employed. There was some question about the purple-blue region, but aside from that the shape was felt to fit the data acceptably at every value level, with the aid of a small systematic decentering adjustment. At value 5/ the coordinates of the ovoid origin are (V$X$—V$Y$)=-—0.02; (V$Z$—V$Y$)=-—0.075. The center is shifted by small equal amounts from 5/ down to 1/ which