Two lenses on a sill: the one in sunlight has darkened, the one in shade is still clear.

Coatings and filters

Photochromic

Optical glossaryCoatings and filters

A lens that darkens in ultraviolet light and clears indoors. Most photochromic lenses react poorly behind windshields, which block most UV.

Most windshields block much of the UV that darkens a photochromic lens, so in the car it only part-darkens — it never reaches the shade it has outdoors.

Illustrative · the shape of the principle, not measured data

In more depth

Photochromic lenses contain molecules that change shape when ultraviolet light strikes them, absorbing visible light and darkening the lens. Out of the UV, they relax back and the lens clears. Darkening takes a minute or so; clearing indoors takes longer.

Because the trigger is UV, not brightness, they barely darken behind a car windshield, which blocks most of it. They also respond to temperature, darkening more in the cold and less in heat — so the same lens may be darker on a winter walk than on a summer beach.

Related terms

Spin coatingApplying the photochromic layer onto the lens surface at even thickness, so color depth stays uniform at any prescription. In-mass photochromics carry the chemistry within the material instead — a difference that shows mainly at stronger prescriptions, where thickness varies more across the lens.UV400Full ultraviolet protection to 400 nanometers. Invisible: darkness and UV protection are unrelated properties.PolarizedA lens with a filter that blocks the horizontally-reflected light that reads as glare off water, roads and glass. Permanently tinted.TintA fixed color in or on a lens. Cosmetic or comfort — a tint alone is not UV protection.

Go deeper on this site