Self-Darkening Eyeglasses
National Eye Care Month: These shades aren’t just cool, they help protect eyes from the sun's high-energy UV light.
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Two for one eyeglass lenses that darken when exposed to ultraviolet rays from the sun and return to their clear state indoors have come a long way since they were first invented in the 1960s. Early versions of these so-called photochromic lenses were plagued by slow darkening or lightening, and sometimes the lenses would get stuck midway. Newer models of the changeling spectacles are much quicker to activate and fade, and they come in both glass and lightweight plastic.
The first photochromic lenses were made of glass and developed by scientists in the U.S. at Corning. The company has made improvements to its original process over the years, but in general, the science remains the same, says Lionel M. Tanguy, a glass application engineer at Corning Ophthalmic, in France.
Glass photochromic lenses contain silver halide crystals embedded in a glass substrate. In the presence of UV-A light (wavelengths of 320–400 nm), electrons from the glass combine with the colorless silver cations to form elemental silver. Because elemental silver is visible, the lenses appear darker. Back in the shade, this reaction is reversed. The silver returns to its original ionic state, and the lenses become clear.
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Excerpted with permission, Chemical & Engineering News
Copyright © 2009 American Chemical Society