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Photochromic Lenses

Photochromic lenses change their degree of tint as you move between dim light and sunlight, and they will correct your vision as effectively as plain lenses. Because of this, ads often claim that they are “2 in 1”—regular glasses and sunglasses rolled into one. But this is not necessarily the case. If you are considering Photochromic glasses, be sure to understand what they are before purchasing them.

How do they Work?

Photochromic glass contains a dye whose molecules are sensitive to the ultraviolet portion of sunlight. When ultraviolet rays strike these molecules, they darken and absorb part of the light that passes through the glass. Since ultraviolet is not emitted by indoor artificial light, they do not darken in response to this kind of light.

On bright, sunny days, most Photochromic lenses darken to absorb 75-80% of the light, which makes them nearly as effective as many ordinary sunglasses. On cloudy days, most of them absorb about 40% of the light. The drawback is that the lenses absorb light—usually 15-20%—so they always look tinted. At their clearest, they are no lighter than light sunglasses. They may interfere with vision in dim light and can make night driving less safe.

Other Considerations

Although most photochromics darken quickly on exposure to sunlight, reaching their maximum darkness can take 5 to 10 minutes. The reversal of this process of clearing to maximum lightness can take 20 to 30 minutes. This is the nature of the photochromic dye. If the delay is inconvenient, you may not want these to be your only glasses. In addition, they do not darken well inside a car, since much of the ultraviolet light is filtered out by the windshield, especially if it is tinted.

The lenses are made in shades of gray and brown, but the dyes weaken with age and the color may change or become less intense. Older lenses may also be slower to change in both directions. Photochromic are presently made only in glass, the process does not work in plastic.