The position of the normal line is determined based upon value pairs of the K7 and F2 glass types.¹  This linear relationship of partial dispersion vs. Abbe number is approximated by the following equation:

where the Abbe number, V_d in ZEMAX is given by:

and A_g,F and B_g,F are specific constants for the given relative partial dispersion.

As can be seen from the plot on the previous page, not all glasses are perfectly represented by the normal line, thus the partial dispersion of these glasses is not accurately predicted by the linear equation above.  Instead, the difference, DP_g,F is used to measure the deviation of the partial dispersion along the perpendicular to the normal line.  It is this value which is used to approximate the refractive index of the model glass in ZEMAX, represented by "Dpgf" in the glass solve dialog.

With the three parameters entered into the model glass, ZEMAX uses a proprietary formula to estimate the index at any defined wavelength.  This formula is accurate to roughly 0.0001 (the index is unit-less) for typical glasses in the visible range, and should not be used outside this wavelength range, as the model glass will no longer become an accurate representation.