
Model glass formulary
The model glass is specified by its refractive index nd at d wavelength and by its Abbe V-value, defined by

where the three reference wavelengths are F (486.13nm), d (587.56nm), and C (656.27nm).
If the specified refractive index and Abbe V-value are positive, LensForge will calculate the refractive index according to a simple two-parameter model, the simplest Sellmeier formula:

The values of A and B are determined by fitting this functional form at the three wavelengths F, d, and C to reproduce the specified values of nd and Vd.
If the refractive index or Abbe V-value are negative, LensForge assumes that the refractive index is linear in wavelength. The linear functional form is as follows:

To use the Sweatt model of kinoforms [Smith2005] set the reference refractive index to a large value such as nd=10000, and set the Abbe V-value to Vd=-3.4534.