Photochromic lenses were invented by Roger Araujo at the Corning Glass Works Inc. in the 1960s. These lenses darken with exposure to certain types of light, especially ultraviolet (UV) light. In the absence of light, the darkened lenses revert to their original transparent appearance. These lenses are also called photochromatic or transitional glasses. They are generally made of glass, polycarbonate, or some other kind of plastic.
How They Change Color
The reason behind the ability of photochromic lenses to change their color from light to dark is their molecular structure and the effect of light on it. Photochromatic glass contains millions of molecules of silver chloride or silver halide. These molecules are transparent to visible light or in the absence of UV light. When they are exposed to UV rays, a chemical reaction takes place causing the molecules to change shape. The changed molecular structure absorbs portions of visible light. This causes the lenses to darken.
The number of the molecules which change shape differs from the intensity of the UV rays. When these glasses are not exposed to UV rays anymore, the reverse chemical reaction occurs. The sudden absence of UV light results in the snapping back of the molecules to their original shape. This, in turn, results in the loss of their light absorbing properties. The entire process of change in the molecular structure takes place quickly, and the darkening of the glass is directly proportional to the intensity of the sunlight.
Their main advantage is that they work as sunglasses due to UV exposure, hence, they are particularly helpful to bikers. Many motorcyclists are forced to carry 2-3 pairs of sunglasses or interchangeable lens kits with clear, charcoal, and amber color lenses. With photochromic lenses, they can buy and carry just one pair of eyewear for the day and the same one even for night use.
Even though these lenses are so convenient to use, they have two small disadvantages. They do not adjust themselves to their darkened form immediately. They take about two minutes to completely change from clear to dark form or vice versa, and they do not change color when worn inside a vehicle, as UV rays do not pass through the glass of the vehicle.