1 Introduction

Optical glasses are characterized and designated by their refractive index and dispersion. The most common measure is the refractive index at the wavelength of the He d line (587.6 nm) or the Na D line (589.3 nm). The difference in the refractive index at the hydrogen F (486.1 nm) and C (656.3 nm) lines, nF-nc is called the average or principal dispersion. The ratio (nF - nc)/(nd - 1) is called the relative dispersion; the reciprocal of this quality is the Abbe number vd. i.e.,

Vd = (nd-1)/(nF - nc).