Photochromic lens

The glass version of these lenses achieves their photochromic properties through the embedding of microcrystalline silver halides (usually silver chloride) in a glass substrate. Plastic photochromic lenses use organic photochromic molecules (for example oxazines and naphthopyrans) to achieve the reversible darkening effect. These lenses darken when exposed to ultraviolet light of the intensity present in sunlight, but not in artificial light. In the presence of UV-A light (wavelengths of 320–400 nm), electrons from the glass combine with the colourless silver cations to form elemental silver. Because elemental silver is visible, the lenses appear darker.

${\displaystyle {\ce {AgCl + e^- <=> Ag + Cl^-}}}$

Back in the shade, this reaction is reversed. The silver returns to its original ionic state, and the lenses become clear.

${\displaystyle {\ce {Ag - e^- -> Ag^+}}}$

${\displaystyle {\ce {Ag^+ + Cl^- -> AgCl}}}$

With the photochromic material dispersed in the glass substrate, the degree of darkening depends on the thickness of glass, which poses problems with variable-thickness lenses in prescription glasses. With plastic lenses, the material is typically embedded into the surface layer of the plastic in a uniform thickness of up to 150 µm.

Typically, photochromic lenses darken substantially in response to UV light in less than one minute, and continue to darken a little more over the next fifteen minutes.[2] The lenses begin to clear in the absence of UV light, and will be noticeably lighter within two minutes, mostly clear within five minutes, and fully back to their non-exposed state in about fifteen minutes. 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.[3]

Because photochromic compounds fade back to their clear state by a thermal process, the higher the temperature, the less dark photochromic lenses will be. This thermal effect is called "temperature dependency" and prevents these devices from achieving true sunglass darkness in very hot weather. Conversely, photochromic lenses will get very dark in cold weather conditions. Once inside, away from the triggering UV light, the cold lenses take longer to regain their transparency than warm lenses.

Photochromic lenses filter 100% UVA as well as UVB.[4] UVB light is more energetic and causes sunburn as well as skin damage including cancers, UVA light causes skincancers but not usually sunburn. UVB is blocked by all glass, UVA light is not blocked by ordinary windows or lenses glass.[5]

A number of sunglass manufacturers and suppliers including INVU, BIkershades, Tifosi, Intercast, Oakley, Serengeti Eyewear, and Persol provide tinted lenses that use photochromism to go from a dark to a darker state. They are typically used for outdoor sunglasses rather than as general-purpose lenses.