Too much ultraviolet (UV) light can lead to eye diseases like photokeratitis or cataracts, so protection for your eyes isn't just for comfort, to cut down glare or to have a fashionable tint of sunglasses. UV light can damage the retina as well as give you a painful sunburn. Before UV light was discovered, tinted glasses were used to help correct vision and limit the amount of light for sensitive eyes.
Types
According to Lenntech.com, soda-lime glass (common commercial glass) does not allow UV light (lower than 400 nanometers [nm] wave length) to pass through, but pure SiO2 glass does. The soda-lime glass is about 75 percent silica, 12 percent soda, and 12 percent lime. Photochromic eyeglass lenses respond to UV light in different ways depending on whether they are made of glass or plastic polymers. (UV light isn't visible to the eye, which has about a 400 to 750 nm wave length spectrum.)
Origin
Photochromic eyeglass lenses were first developed by Corning in 1964 for prescription use. Silver-based crystals embedded in the glass were found to activate when exposed to UV rays and darken to limit glare and uncomfortably bright sunlight. The reversible chemical process involves the gain or loss of an electron in silver crystals when exposed to UV light rays. The brighter the light and amount of UV rays present, the quicker the darkening of the eyeglass lenses. Heat also affects the process, so cold bright light activates the lenses better than hot bright light.
Developments
About 30 years later a process of coating plastic lenses was developed which also produced a darkening of lenses when exposed to UV light. Rather than the gain or loss of an electron, there is an opening or closing of an organic molecule present in certain dyes in plastic coated lenses. Because the stability of these dyes is less than the stability of the silver crystals in glass, the process loses effectiveness over time.
Considerations
Since UV rays are not present indoors and are mostly blocked through automobile glass, there is no activation or special protection with photochromic glasses for drivers from bright headlights or indoors. On the other hand, because UV rays are present on cloudy days, photochromic lenses will react outside and darken even when glare is not a problem.
Significance
Since glass lenses are heavier than plastic lenses, the discomfort involved should be weighed against the advantage of proven durability of glass photochromic lenses. Plastic lenses are lighter, but tend to scratch easier than glass and may lose some photochromic ability over time.
