What does „ABBE number“ mean?

The ABBE number is a dimensionless parameter named after Ernst Abbe. It is a measure of the material dispersion and shows how much the refractive index changes with the wavelength of the light. The higher the dispersion, the smaller the ABBE number. The number is given by a function of the refractive index of the material at 486.1 nm, 587.6 nm and 656.3 nm.

The ABBE number is mainly used to characterize and monitor the quality of glasses and polymers used in lens production.