How Does an Auto Darkening Lens Work?

Background

Prescription eyeglasses that change from light to dark are known as photochromatic glasses. They were first invented in the 1960s, but didn't gain popularity until the 1990s. They were made famous by a company called Transitions, and, as a result, the glasses are often referred to as Transitions lenses.

Silver Chloride or Silver Halide

The lenses are embedded with silver chloride or silver halide molecules. These molecules react to UV rays by transforming their shape, and this new shape absorbs light, which then causes the lenses to darken. The more intense the UV rays are, the more molecules will change shape. When you go back inside, the UV light dissipates, and the molecules shift back into their original shape. The lenses then appear light again.

Pros and Cons

If you're going to use photochromatic lenses, it's best to know what you're in for. The Transitions brand of photochromatic sunglasses can react to the UV rays rather quickly, becoming dark within about 30 seconds. However, when the UV rays become absent again, they can take up to five minutes to return to their normal transparency. Another problem is that your car's windshield and windows block out most of the UV rays, so the lenses don't change while you're driving.