Photochromic lens

In another sort of technology, organic photochromic molecules, when exposed to ultraviolet (UV) rays as in direct sunlight, undergo a chemical process that causes them to change shape and absorb a significant percentage of the visible light, i.e., they darken.

These processes are reversible; once the lens is removed from strong sources of UV rays the photochromic compounds return to their transparent state.

Photochromic lenses were developed by William H. Armistead and Stanley Donald Stookey at the Corning Glass Works Inc. in the 1960s.

A report by the Institute of Ophthalmology at the University College London suggested that at their clearest photochromic lenses can absorb up to 20% of ambient light.

This thermal effect is called "temperature dependency" and prevents these devices from achieving true sunglass darkness in very hot weather.

A number of sunglass manufacturers and suppliers including INVU, BIkershades, Tifosi, Intercast, Oakley, ZEISS, Serengeti Eyewear, and Persol provide tinted lenses that use photochromism to go from a dark to a darker state.