The accessibility of high-speed computing machinery makes practicable the use of a routine for the least-squares fitting of a three-term Sellmeier equation to a set of experimentally determined values of index of refraction. The constants of a two-term Sellmeier equation are evaluated by a method described previously [O. N. Stavroudis and L. E. Sutton, J. Opt. Soc. Am. 51, 368 (1961)]. These are then used in a preliminary fitting of another term. The rough fit is then improved by an iterative process which includes an acceleration technique to speed convergence to the final result. In a typical example the average residual of index is only about 2×10-5 for 46 wavelengths from 0.2652µ to 10.346µ.
LOYD E. SUTTON and ORESTES N. STAVROUDIS, "Fitting Refractive Index Data by Least Squares," J. Opt. Soc. Am. 51, 901-905 (1961)