The following is a program for calculating the coefficients of the dispersion curves for glass. This calculator may be used to find the dispersion curve coefficients from a set of refractive indices. Alternatively, the calculator may be used to convert between the dispersion equation forms. The two forms supported are the Sellmeier (3-term) and the Cauchy (6-term) forms, both of which use six coefficients to calculate, in essence, the square of the refractive index.
In both uses, the method is simple Newton-Raephson iteration against the coefficients over a small set of refractive index values. The user must supply the initial estimate of the coefficients.
The calculator has a small set of glass catalogs that may be used to generate the initial guess. When the search button is hit, these catalogs are scanned for the given glass types. The coefficients found for that glass type are displayed. Internal Schott, Ohara, and Chance catalogs are searched in a prioritized order. I use a dot notation for these catalogs; e.g.: glass N-BK7 would be listed in the catalogs as Schott2K.N-BK7, indicating it is in the Schott2K catalog. If a catalog is prefixed, only that catalog will be searched for the glass; otherwise all of the catalogs are searched. Note that this search includes the equation form, e.g.: a search for BK7 with a Cauchy equation form will end in failure, as BK7 is only included in the internal catalogs with the Sellmeier equation form.
For dispersion equation calculation, I simply put the same glass type and form for both the base and revised values. In the end, I can then compare the two sets of coefficients for a reality check. In this case, the user must provide the full set of refractive index values.
For dispersion equation conversion, if the first form is of a glass already in the internal catalogs, it is easy. Just set up the original glass and the initial estimator glass and go on. If the original glass is not in the inter