Photochromic lenses are lenses that are clear indoors and darken automatically to a moderate sunglass tint when exposed to sunlight. The term photochromic refers to this feature of changing color ("chroma") in response to light ("photo-").
Though photochromic is the proper generic term for all lenses that darken in sunlight, these lenses are often referred to by a specific brand name, such as Transitions(R) (made by Transitions Optical) or PhotoGray(R) (made by Corning). Other terms used to describe photochromic lenses include variable tint lenses and lenses with sun-activated tints.
All photochromic lenses block 100 percent of the sun's harmful ultraviolet (UV) rays.
Photochromic lenses are available for both prescription and non-prescription eyewear. Approximately 20 percent of prescription eyeglass lenses sold in the United States are photochromic lenses.
Glass Photochromic Lenses
Photochromic lenses are available in a wide variety lens designs and materials. However, the photochromic process is different for glass and plastic lenses.
Glass photochromic lenses contain millions of silver halide crystals embedded throughout the lens material. When exposed to sunlight, UV rays transform the crystals into light-absorbing particles. As these particles absorb light, the lens darkens and the amount of light passing through the lens is reduced.
When the lenses are removed from sunlight and the stimulating UV radiation is no longer present, the crystals return to their original orientation and the lenses become clear.
Corning, who developed the first glass photochromic lenses in the 1960s, owns the process that creates most of the glass photochromic lenses sold in the United States. The trade names for Corning's glass photochromatic lenses include: PhotoGray(R), PhotoGray(R) Extra, PhotoBrown(R) Extra, and PhotoGray(R) Thin and Dark.
Plastic Photochromic Lenses
Plastic photochromic lenses work in a similar manner, but use organic molecules instead of silver halide crystals. Some plastic photochromic lenses have light-sensitive molecules distributed throughout the lens material (just as silver halide crystals are in glass photochromatic lenses). This type of photochromic process is referred to as "in-mass" technology. Examples of plastic in-mass photochromic lenses include Rodenstock North America's ColorMatic(R) Extra lenses.
The most popular plastic photochromatic lenses sold today are Transitions(R) lenses, made by Transitions Optical. Instead of distributing light-sensitive particles throughout the lens material, Transitions applies a thin layer of photochromic material to the front surface of its lenses. This light-sensitive material penetrates the lens surface evenly to a depth of approximately 0.15 millimeter. This type of photochromic process is called "imbibing" technology.
Corning has recently introduced a lightweight, high index plastic photochromic lens called SunSensors(R). This lens also has photochromic material uniformly distributed into the front surface of the lens, but has a penetration depth that is significantly thicker than the photochromic layer in Transitions lenses. This extra-thick photochromatic layer enables SunSensors lenses to change faster and get darker than other photochromic lenses, according to the company.
Colors Available
Most photochromic lenses have a neutral gray tint. Brown, green, and other tints are also available (but may not be available in all lens materials and prescription types).
"Break-In" Period
It takes several exposures to sunlight before a new pair of photochromic lenses will darken and lighten as designed. Allow your lenses to be exposed to sunlight at least five periods of 30 minutes or longer to "break-in" the photochromic process.
The Effect of Temperature
All photochromic lenses perform better (i.e. get darker and change faster) in cold weather. For standardization purposes, photochromic lenses are usually tested at an ambient temperature of approximately 72 degrees Fahrenheit. At higher temperatures, photochromic lenses may take more time to change and may not darken to the same degree.
Photochromic Lenses and Driving
Photochromic lenses require exposure to ultraviolet (UV) radiation from the sun to darken. Since automobile windshields block some UV rays, photochromic lenses do not darken appreciably inside many cars. For the best vision and comfort while driving in bright sunlight, consider purchasing a pair of clip-on sunglasses to wear over your photochromic lenses or a separate pair of polarized sunglasses for driving.
How Long Do They Last?
Most plastic photochromic lenses gradually lose some of their ability to change in sunlight after 18 to 24 months of use. For the best performance, plastic photochromic lenses should be replaced every two years.
Reducing Glare From Reflections
To eliminate glare from light reflecting off the back surface of photochromic lenses, anti-reflective (AR) coating can be applied, either to both sides of the lens or to the backside only.
AR coating does not adversely affect the photochromic process. Applying AR coating to any photochromic lens will increase the light transmittance in both the indoor (clear) and outdoor (dark) states. Exact percentages will vary depending on the brand of AR coating.
When applied to both surfaces, AR coating can also prolong the effective lifetime of some photochromatic lenses.
Eliminating lens reflections with Anti-reflective coating has been shown to make computer work more comfortable and improve visual acuity for night driving. AR coating, however, does not eliminate glare from light reflecting off other surfaces (e.g. car windshields, pavement, water). To reduce glare from these surfaces on sunny days, polarized sunglasses (or polarized sun clips worn over photochromic lenses) are most effective.
