ZEMAX computes the index of a model glass by idealizing the dispersion of a glass using the index of d-light, N_d (at 0.5875618 mm), the Abbe number (V_d), and a term which describes the deviation of the partial dispersion (DP_g,F) from what is known as the Normal Line.

The last term, DP_g,F, is very important to the definition of a model glass in ZEMAX.  Generally, the refractive index and Abbe number alone are insufficient characterizations of optical glass for high quality optical systems, and thus the addition of the relative partial dispersion term provides a more accurate description of its properties.¹ 

The general relative partial dispersion, P_x,y, for the wavelengths x and y is described by the equation:¹

For all glasses in ZEMAX, including the model glass, ZEMAX uses the reference wavelengths g (blue Mercury line at 0.4358343 mm) and F (blue Hyrdrogen line at 0.4861327 mm) to define the relative partial dispersion.  Therefore, the relative partial dispersion becomes:

The majority of glasses (often referred to as the "normal glasses"), when plotted on a graph of P_x,y vs. V_d, follow a linear relationship.  It is this line which is referred to as the normal line, and it is essential in calculating the deviation of partial dispersion for the model glass in ZEMAX.  The diagram below (courtesy of Schott, see reference [1]) plots P_g,F vs. Abbe number for Schott's optical glass assortment, with the addition of the normal line.