When parallel light rays are refracted by a prism, a rainbow-hued spectrum comes out. This phenomenon is called "dispersion". In case of photographic lenses, the dispersion causes color fringes at the edge of subjects, which is called axial chromatic aberration, and as a result, deterioration of image quality of photography. There is a limit to the correction of chromatic aberration, using regular optical glass lens elements only. Some aberrations not corrected by optical glass are called secondary spectrum or residual chromatic aberration or secondary chromatic aberration.
The artificial crystal fluorite lens element, featuring very low optical dispersion index, was developed by Canon to eliminate secondary spectrum. Canon succeeded artificially crystallizing calcium fluoride (CaF2) into fluorite at the end of 1960s. Canon EF lenses are the only interchangeable lenses for 35mm SLR at the time that employ fluorite lens elements. In the late 1970s, Canon also developed special optical glass lens elements with very low dispersion index called Ultra-low Dispersion (UD) glass lens elements, and in 1990s an upgrade version of UD glass, called Super UD glass, was developed. Fluorite, UD and Super UD glass lens elements are widely used in EF lens series, for super-telephoto L series lenses, as well as in telephoto zoom and wide angle lenses.