Photochromism
In simple terms, photochromism is a mechanism by which ophthalmic glass darkens outdoors, when exposed to ultraviolet (UV) rays, and fades back to its initial transparent state indoors or behind a UV-filtered window.
The result is added vision comfort and better protection of the retina.
Since its inception and first commercial release by Corning in 1964, photochromic glass has become more sophisticated and diversified. Yesterday limited to glass, photochromism is today available on most ophthalmic materials, such as nylon and polycarbonate. Complementing the “in-mass” technology, which consists in casting a material that already incorporates the photochromic molecules, the “coating” technology involves a surface treatment of the lens (see SunSensors® HPC). Thanks to its most recent developments, eyeglass wearers can thus benefit from better performance (deeper darkening, faster lens reaction) on higher-refractive index lenses. This explains why photochromism is an increasingly successful option when buying prescription lenses.
In more technical terms, photochromic lenses have the capacity to vary their absorption characteristics in response to radiation wavelengths (notably ultraviolet A, or UVA, radiation). This modification is reversible.
Their special property is thus to undergo a change in their range of transmission, according to changing ambient-light levels. Every photochromic lens has its own separate transmission curves in the clear state and the darkened state.