What are photochromic lenses
Photochromic lenses are the tint changing lenses in eyeglasses that darken when exposed to UV rays from the sun. With the many brands and styles available, these lenses combine fashion with comfort and offer protection from the damaging effects of UV radiation and glare while at the same time enhancing patients' quality of vision.
One of the problems facing those who wear corrective eyewear is how to protect their eyes from the glare of the sun. Not only is bright sunlight difficult to see in, excessive exposure to ultraviolet radiation can cause permanent damage to the cornea and conjunctiva of the eye.
Before the introduction of photochromic lenses, patients were forced to purchase a set of eyeglasses for indoor wear as well as a set of prescription sunglasses for outdoor wear, often having to carry both and switch when traveling from inside to outside. Thanks to photochromic lenses, eyeglass wearers can now purchase one set of glasses that will allow them to see inside, as well as reduce the glare of the sun when outdoors.
How they work
Photochromic lenses have millions of molecules of substances such as silver chloride and silver halide, embedded into them. When there is no UV light present, such as the case with artificial lighting, the molecules are transparent to visible light. This is why the lenses stay clear while indoors. However, when the lenses are exposed to UV rays, such as the kind found in sunlight, the molecules undergo a chemical process that causes them to change shape.
This new molecular structure absorbs portions of the visible light and causes the lenses to darken. The number of molecules that change shape varies with the strength of the UV rays. When a patient wearing photochromic lenses travels from the outdoors to the indoors a different chemical reaction occurs and causes the molecules to bounce back to their original shape which results in the loss of their light absorbing properties allowing the lenses to lighten.
How they are made
Early photochromic lenses were made only from glass, but now are also made from regular plastic, polycarbonate and high-index glass and plastic. Recent technology has changed to improve the way the lenses darken. The active ingredient that causes the lenses to darken is called silver halide, or silver chloride and it is mixed throughout the lens. This will make the entire lens change color when exposed to light. It also means that with a particularly strong prescription, the strongest and thickest part of the lens will be darker than the thinner parts.
In addition, if the prescription difference between the two eyes is large, the lens with the stronger prescription will be darker than the weaker. New technology has tackled that issue and now, many lenses are made by immersing the lens in a chemical bath so that the photochromic molecules are actually absorbed to a depth of approximately 150 microns into the plastic. This is much improved over the simple coating that is only about 5 microns thick. Having the molecules mixed throughout the lens material, rather than just on the top layer results in a uniform color density throughout the lenses regardless of thickness or prescription.
Answering the question what are photochromic lenses is complicated due to the many types of lenses available on the market and the options they provide. From lenses that darken even behind the UV protected glass of a car windshield to glass photosensitive lenses used to enhance the vision of patients who have various ocular pathologies, the options vary. Visit your eye care professional for more information to help you decide which type is right for you.