In order to compute the dominant wave length and purity of a color stimulus by means of the O. S. A “excitation” data, two values must be obtained by interpolation. The adoption of the osculatory formula for this. interpolation permits the computations to be made with perfect reproducibility. Each of the O. S. A. curves by this method is represented as a series of parabolas of the fifth degree which join at the values specified at every 10 mµ so as to have a common slope and curvature at the junction point. Interpolated values have been computed according to this formula for every millimicron.
DEANE B. JUDD, "INTERPOLATION OF THE O. S. A “EXCITATION” DATA BY THE FIFTH-DIFFERENCE OSCULATORY FORMULA1," J. Opt. Soc. Am. 21, 531-536 (1931)