Abbe-Number

Also known as Abbe constant, it inversely quantifies the amount of dispersion in an optical medium. It is given by a function of the refractive index of a material at the f (486.1nm), d (587.6nm), and c (656.3nm) wavelengths. A material with a high Abbe number means that the different wavelengths will have nearly the same index of refraction in that medium, yielding less separation between wavelengths of light.