Photochromic sunglasses are like a UV sensor on your face. They darken or brighten, depending on ambient light conditions. The brighter it is outside, the darker they get. And vice versa. This useful technology has been around for decades (mostly in prescription lenses), but now lower production costs are bringing it into the mainstream; a wide range of photochromic sunglasses are now available. Here’s what you need to know.
The chemistry and application of photochromic, or photochromatic, molecules is remarkably complex (naphthopyrans and oxazines, anybody?), but the basics are not. Photochromic lenses darken when exposed to ultraviolet (UV) light. UV rays come from the sun. UV levels vary with conditions, from bright cloudless sun (maximum UV) to overcast grays (less UV), to indoors (no UV). As UV light diminishes, lenses approach their natural, brightest state, which varies depending on the underlying tint of the sunglasses. Note that because windshields and other types of glass block UV rays, photochromic sunglasses change little while driving or indoors.
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How fast do they change?
Most photochromic lenses transition rapidly to about 90 percent of their maximum darkness (or brightness) within 60 seconds, then slowly continue to their darkest or lightest state over the next five minutes or so. This is a consideration if you’re regularly switching between very bright and very dark environments, such as running or cycling through shadowy woodlands. For general outdoor use, however, the transition is certainly quick enough.
How bright? How dark?
Photochromic lenses vary significantly in darkness, as measured by the percent of light that passes through the lens. Most sunglasses for active outdoor use in bright conditions have light transmissions of roughly 10 to 20 percent. Glasses designed for ultra-bright conditions and glare—water, snow, high-elevation—will go as low as 5 percent. Casual and fashion shades can range anywhere from 15 to 50 percent. For photochromic sunglasses, transmission levels span a broad range depending on intended use, such as 16-56 percent (all-purpose, around-town), 12-18 percent or 8-20 percent (outdoor use in a range of bright conditions), or 7-41 percent (mountaineering, sailing, and other very bright activities).
Temperature and time
Photochromic lenses are affected by temperature. They get slightly darker when it’s cold, and brighter when it’s hot. Expect maximum darkness on a cold-weather adventure in the snow, not on a hot summer day on the water. (Cold lenses also take a bit longer to transition than warm lenses.) Photochromic chemicals degrade over time and lose darkness over the course of many years—if you manage to hold on to a pair of sunglasses long enough for that to matter, congratulations!
Expect to pay $100 to $200-plus for a quality pair of photochromic sunglasses.