tau Abbe

In optics and lens design, the Abbe number, also known as the Vd-number or constringence of a transparent material, is an approximate measure of a material's dispersion, with high Vd values indicating low dispersion. It is named after Ernst Abbe (1840–1905), the German physicist who defined it. The term Vd-number should not be confused with the normalized frequency in fibers.