Photochromic lenses: How they work

Photochromic lenses are spectacle lenses that are clear (or nearly clear) indoors and darken automatically when exposed to sunlight. Other terms sometimes used for photochromic lenses include “light-adaptive lenses,” “light intelligent” and “variable tint lenses.”

The most popular brand of photochromic lenses sold worldwide is Transitions Lenses. Because of this, some people, including some Eye Care Practitioners refer to photochromic lenses as “transition lenses.” But there are other brands of photochromic lenses available as well. Contact lenses are also available with photochromic technology. Johnson &

Johnson

How photochromic lenses work

Tiny molecules of silver halide and chloride are embedded within a photochromic lens, which is invisible and clear until exposed to sunlight/UV rays. A chemical process takes place when exposed to sunlight/UV and the molecules effectively move, change shape and absorb the light. This process changes the molecules and they darken to a sunglasses tint.

The photochromic lens will remain in this ‘darkened’ sunglasses state whilst exposed to sunlight/UV and the molecules will reverse the process once the lenses are removed from the sunlight/UV rays returning to the original ‘transparent/ clear’ lens.

Photochromic lenses typically will not darken inside a vehicle because the windshield glass blocks most UV rays. Recent advancements in technology allow some photochromic lenses to activate with both UV and visible light, providing some darkening behind the windshield. Fixed tint sunglasses will al] ways be better for driving.

Photochromic lenses are available in nearly all lens materials and designs, including high-index lenses, bifocals, and multifocal lenses. An added benefit of photochromic lenses is that they shield your eyes from 100 percent of the sun’s harmful UVA and UVB rays.

Because a person’s lifetime exposure to sunlight and UV radiation has been associated with cataracts later in life, it’s a good idea to consider photochromic lenses for children’s eyewear as well as for adults. Polycarbonate is the safest lens material for kids, providing up to 10 times the impact resistance of other lens materials.

Adding an anti-reflective coating to photochromic lenses enhances their performance even further. AR coating allows more light to pass through photochromic lenses for sharper vision in low-light conditions (such as driving at night) and eliminates bothersome reflections of sunlight and other light from the backside of the lenses in bright conditions.

Though photochromic lenses cost more than clear lenses, they offer the convenience of reducing the need to carry a pair of prescription sunglasses with you everywhere you go.