Photochromic Lenses

What Are Photochromic Lenses?

Most glasses and sunglasses are one color. Brown, grey, black, yellow, red, or green, the lenses stay the same shade no matter what intensity of light (or lack thereof) the wearer wanders into. Photochromatic lenses are different. They appear clear in low-light conditions, but darken when the light gets brighter. First developed by Corning in the 1960s, the lenses became popular when the Transitions brand was introduced to the mainstream market.

How Do They Work?

Photochromatic lenses react to ultraviolet (UV) light. When the wearer is exposed to UV, photochromatic molecules (silver halide or silver chloride) that are deeply embedded in the lens begin to change shape. The new structure makes it possible for the lenses to absorb light. When UV exposure ends, the molecules resume their normal shape and lose their ability to absorb light. The process if swift -- often the lenses change within 60 seconds.

Benefits

The primary benefit of these lenses is convenience. With photochromic lenses, you do not have to have a separate pair of sunglasses. There is no switching back and forth between normal, vision-correcting glasses and prescription sunglasses. There is no need to keep your pair of sunglasses accessible, and if you frequently carry large bundles or packages into and out of buildings, the danger of tripping over something because the sunglasses you needed outside are stuck over your eyes until you can put down your load and remove them.

A secondary benefit is the protection from UV light that they offer. As they are activated by UV rays, the slightest change in light intensity will automatically cause the lenses to darken appropriately.

Drawbacks

The most important drawback is that photochromic lenses generally do not work well for driving because the car's windscreen blocks most UV light, so that there is no trigger to change the lenses.

Another, sometimes amusing, drawback is that very strong indoor light can make the lenses appear dark -- people who are frequently in bright indoor lights (like performers under spotlights) should avoid these kinds of lenses, as they may darken during a performance and obscure the performer's face.

Unless the lens is polycarbonate or plastic, a prescription lens will not change color evenly. Thicker parts of the lens will appear darker, and, if each eye has a different prescription, each lens in the pair will have a slightly different tint.

Lens Brands

While Transitions brand is the oldest and best-known, a number of other sunglasses manufacturers make photochromic lenses available on select styles.

• Transitions has, in addition to its regular line, the "Splitz" line of unusual-colored lenses that change to other colors -- red changes to purple, for example. Transitions also offers a new product, SunLenses, with a green lens color. These SunLenses have the advantage of being dark enough in the absence of UV to allow you to drive safely and comfortably while wearing them.

• Corning still offers both PhotoGray and PhotoBrown lenses, as well as Thin & Dark lenses, which are thinner and lighter than traditional glass lenses, and SunSensors, a lightweight plastic lens.

• Rodenstock offers ColorMatic Extra lenses, a gray plastic lens with a photochromic dye mixed into the matrix.

• Hoya makes the Sungray line, another plastic lens with the photochromic molecules blended into the plastic.

• KB Co. offers Transhades, which are both photochromic and polarized.

Sunglass Manufacturers

Many sunglasses brands offer photochromic lenses, including Serengeti and Smith. If you like a more fashionable set of frames, it might be worth researching whether your favorite high-end sunglasses brand has a line of photochromic lenses available.

Related Links

• Serengeti Sunglasses
• Smith Sunglasses
• Choosing Sunglasses
• Prescription Sunglasses
• Polarized Sunglasses