For a given set of dispersion data, ZEMAX can fit the data to any one of four dispersion formulas:  the Schott, Herzberger, Conrady, or Sellmeier 1. For a complete definition of each of these formulas, please refer to “The glass dispersion formulas” section of the chapter in the ZEMAX User’s Guide entitled "Using Glass Catalogs".

Basically, ZEMAX will calculate the coefficients of the chosen formula which best fits your data set. These coefficients are then stored into the glass catalog for your new glass, and are used to calculate the index of refraction for any given wavelength.

How is this done? Using the Glass Fitting Tool, which can be accessed by selecting Tools > Catalogs > Glass Fitting from the main menu:



Selecting this feature will activate the Glass Fitting Tool dialog box:



Data given in terms of wavelength-index pairs are provided to the Glass Fitting tool via an ASCII text file, which must have the following format:

TEMPERATURE T₀
PRESSURE P₀
l₁  n₁
l₁  n₂
.
.
.

where T₀ is the temperature (in degrees C) and P₀ is the pressure (in atmospheres) at which the data were measured, and l is in microns. The name of the text file containing these data must have a .IND extension, and this file must be located in the \ZEMAX\Glasscat\ directory. More information about the file format may be found in the chapter of the ZEMAX User’s Guide entitled "Tools Menu". While there is no upper limit to the number of data points that may be provided in the file, at least three points are required to get a fit to the Conrady dispersion formula, and six or more (and preferably twelve to fifteen) points are required to get a fit to the Schott, Herzberger, or Sellmeier 1 dispersion formulas.