Computer Calibration of the Wavelength Drive of a Prism Spectrometer

The Sellmeier equation has been slightly modified to describe the drum position as a function of frequency for a Littrow type prism spectrometer. A widely available IBM Share program in Fortran II can be quickly and easily adapted to fit calibration data to this equation. The program provides a least-squares estimate of the constants in the equation when two of the constants, the curve asymptotes, are moderately well estimated. When this method was applied to a thermostated Perkin-Elmer model 99 double-pass monochromator, the resulting standard deviation sigma between data points and computed points was only slightly larger than reading repeatability. Prisms used were SiO(2), CaF(2), NaCl, KBr, and CsBr. The best fit was obtained for NaCl with sigma = 0.25 drum numbers between 1.4 micro and 13.7 micro. The maximum sigma obtained was 0.72 drum numbers for KBr in the 10-22 micro range, which corresponds to a standard deviation in wavelength of 0.01 micro. Using the calculated dispersion constan, a second program prints a table of drum numbers vs wavelength and wavenumber at intervals as small as the error in the fit will allow. This table is an accurate and convenient substitute for the wavelength calibration graph.