What’s unique and advantageous about the table glass is that the index at the defined points is exactly the index specified in the table.  This is unlike the fitted function, which may deviate from the data being fit at the data points.  To prove this, open the text of the Dispersion Diagram and compare the wavelength vs. index listing to that of the original defined ASCII table glass file. 

In this demonstration, change the “Min Wave” and “Max Wave” to 3 and 4 respectively.  Note that at each wavelength defined in the TABLETEST_2.ZTG file, the index for that wavelength is identical to what is reported in the text listing of the Dispersion Diagram.  A few points for comparison are shown below: