The table glass is easy to use, and is a very convenient glass modeling tool available in ZEMAX.  Dispersion data for the table glass is entered via and ASCII text file in a specific file format.

ZEMAX internally uses a cubic spline interpolation of the data entered into the ASCII file.  The advantage of the table glass is that the index at the defined wavelength is exactly the index specified in the table, unlike fitted functions which may deviate from the data being fit at the data points. 

References

ZEMAX Optical Design Program User's Guide, ZEMAX Development Corporation