spectacle lenses that undergo a chemical reaction when exposed to the ultraviolet (UV) wavelengths in sunlight. The reaction causes molecules within the lens to absorb light which causes them to darken.
Lenses that darken when exposed to intense illumination (e.g. sunlight) and lighten in color when illumination is reduced.
Lenses that remain clear during dim light, but darken during bright light, or exposure to ultra violet light.