About Photochromatic Lenses
Photochromatic lenses are designed to change their shading properties when exposed to ultraviolet light. The lenses darken when sunlight hits them, protecting the wearer's eyes from ultraviolet radiation. When sunlight is diminished, the lenses revert to their original, clear state, allowing artificial light to pass through. These lenses are useful both for comfort and for medical purposes.
The ingredient that allows the lenses to change shading properties so quickly is known as silver halide. There are several different kinds of silver halides, though the substance most commonly used is silver chloride. This chemical has been used since the 1960s to create photochromatic lenses for eyeglasses, and works by absorbing ultraviolet light waves. The molecules of the silver halide substance react in a specific manner to the short wavelength properties of UV light. This causes the molecules to absorb energy and makes their atoms vibrate faster. Instead of releasing this energy, the atoms use it to form new bonds with surrounding materials, creating a different substance that darkens the lenses and blocks light.
When the lens is moved out of the short wavelength UV light into areas where there are longer wavelengths of light, it begins to release energy, and the atoms revert to their original state. This releasing of energy back into the air depends on temperature and material. In a cool room, it may take 10 to 15 minutes or less for the lens to fully revert to its original transparency. But the exchange is a thermal process, and the more heat the lens can transfer into the air, the faster it can change. By comparison, in hot environments, the glasses will take longer to release heat and lose their shading properties.
There are two ways to apply this darkening material. In the beginning, manufacturers mixed the material in with the glass, so that it permeated the lenses. But the glass versions of photochromatic glasses were heavy and awkward, and as technology advanced, they were replaced with both plastic and polycarbonate materials. The silver halide substance was mixed in with these materials in the same way, but the glasses were much lighter and more flexible.
Since the lens material darkened throughout with this permeation approach, how dark the lens become depended on how thick it was. Thin lenses did not provide as much shading protection, and thick lenses could become too dark for users. To solve this problem, manufactures began also applying the silver halide as a film over the top of lenses, allowing them to control the darkening qualities much more precisely.
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