Sunglasses which change with the light

You carry your glasses and sunglasses wherever you go, switching back and forth each time you go outside and head back in. A simple alternative is to wear glasses with photochromic or "transition" lenses. These lenses automatically darken in bright sunlight and return to a regular tint in dimly lit environments. Photochromic lenses have been around for decades. Years ago, all transition lenses were made of glass and came in a single color: grey.

These lenses contain proprietary dyes that undergo chemical changes and darken when exposed to UV light. Photochromic technology has come a long way since the very first transition lenses, which darkened when silver chloride or silver halide in the glass lens reacted with UV light.

Glasses with photochromic lenses certainly offer convenience, but they have some drawbacks too. The molecules are transparent to visible light in the absence of UV light , which is normal for artificial lighting. But when exposed to UV rays, as in direct sunlight, the molecules undergo a chemical process that causes them to change shape. The new molecular structure absorbs portions of the visible light , causing the lenses to darken.

The number of molecules that change shape varies with the intensity of the UV rays. When you go indoors and out of the UV light, a different chemical reaction takes place. The absence of the UV radiation causes the molecules to "snap back" to their original shape, resulting in the loss of their light absorbing properties. In both directions, the entire process happens very rapidly. In the original PhotoBrown and PhotoGrey products made by Corning, the lenses are made of glass, and the molecules are distributed evenly throughout the entire lens.

Protects your eyes — Transitional lenses do more than function as sunglasses. They actually filter out a good deal of the harmful UV rays emitted from the sun, leading to healthier and happier eyes. The cons So does the bad outweigh the good? These are some of the drawbacks of photochromic lenses: Ineffective in cars — Photochromic lenses darken in reaction to UV rays, which your windshield blocks.

Differences in brands — Another issue that can arise with photochromic lenses is that different brands have different levels of darkness and different reaction times. Speaking with your eye doctor about this can help you find the brand that works best for you. Affected by cold weather — Transitional lenses are also affected by colder weather, meaning they take a bit longer to react to UV rays in winter.

Check with your eye doctor to see what option is best for you. On average, photochromic lenses will begin to wear out in about three years. How will you notice your photochromic lenses are becoming less effective? They will develop a yellowish tinge while in their clear state and no longer darken fully outdoors. Add photochromic lenses when you shop for eyeglasses at an optical store near you or an online eyewear retailer.