Why Does Transition Lens Change

It’s a transmutation of matter worthy of the ancient alchemists.

How Do Transitions Lenses Change?

Why Does Transition Lens ChangeClear prescription lenses that darken into sunglasses when exposed to the sun, incrementally adjusting for the amount of light blockage required; then returning to their crystal-clear state when sunlight is removed. A quick peek behind the curtain will prove that the trick is not magic… just pure science.

Photochromic (or transition) lenses receive a transitional coating that imbues them with their light-morphing capability. This coating is applied in one of two ways; either the lenses are treated with a permanent external coating, or they have this coating applied between layers of glass or plastic. The coating itself is embedded with millions of molecules that are transparent when exposed to most forms of light – the main exception being ultraviolet (UV) light, a component of direct sunlight. When UV rays strike these molecules, they change shape, and this altered shape begins to absorb more of the light spectrum. This effectively results in a darkening of the lenses that will remain in effect for as long as the UV light is present.

The intensity of the UV light source determines how many molecules change shape. Slight exposure will cause only a layer or two to change over, resulting in a lightly darkened tint; full exposure to UV light causes all the molecules in the coating to adapt, causing your lenses to reach their darkest possible state. When exposure to UV light lessens or stops entirely, the molecules reverse the process – they revert to their nascent shape, causing the lenses to clear again. And they can repeat this process over and over again without wearing out or wearing down.

Photochromic lenses are available in two different colors, brown and gray, both of which function exactly the same way, so color choice is strictly a matter of aesthetics. They’re compatible with just about every form and type of lens material: glass or plastic, high index or low, polycarbonate or Trivex. Multi-vision systems like bifocals, trifocals, or progressives do not interfere with transition lenses, and they can be obtained in just about every prescription format and strength imaginable. To top it all off, photochromic lenses provide 100% protection from both UVA and UVB rays, the ultraviolet spectrums that can damage the retina if the eye is exposed to it for long periods of time.

Before you rush out and purchase this miracle coating, however, be mindful that it does have a few potential drawbacks:

  • Photochromic lenses do not darken fully when worn during driving, as most vehicle windshields and motorcycle windscreens already block the UV light that causes photochromics to change. If this is an issue for you, look into ordering DriveWear lenses (which were developed to adjust to both visible and invisible light) or consider obtaining a separate pair of sunglasses or clip-ons to achieve a comfortable level of light protection while driving.
  • Photochromics take some time to adjust. Standard sunglasses provide instantaneous coverage, but photochromic lenses need a few minutes to do their work.
  • While most of the transition takes place in the first minute, photochromics can take up to fifteen minutes to reach their fully darkened state and another fifteen minutes to fully clear again. When you walk indoors after being out in the sun, you will have deal with shaded lenses for a brief period.
  • Higher temperatures can prevent photochromic lenses from reaching their darkest states, and even under standard temperature conditions, photochromics only darken to a certain degree…a level that some people consider too light. Transitions® Xtractive® lenses have been developed to combat these problems.

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Written by Tony Albarella

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