Photochromic lenses, also known as transition lenses, utilize the sun’s ultraviolet rays to perform their color-changing process. A chemical reaction incited by the rays allows photochromic lenses to darken smoothly, easing bright natural light before it reaches the eyes. Despite the advanced technologies used in their creation, plastic photochromic lenses -- the industry standard -- are subject to highs and lows in their performance and may ultimately lose some of their tinting abilities over time. Lifespan The ultraviolet light that sparks the darkening process in photochromic lenses is also the primary factor in the deterioration of lens functionality. Because of this, frequency of usage can play a role in determining how long plastic photochromic lenses retain their abilities. A pair of plastic photochromics may begin to lose its ability to darken after anywhere from one to four years. Consumers using plastic photochromic lenses should replace them every two years for optimum performance. Glass Photochromics The original photochromic lenses -- created in the 1960s -- were made of glass and made use of a different chemical process known as “silver halide” to achieve the same darkened result. Silver halide involved specialized color-changing crystals being distributed throughout the glass lens. This process, the same one used in traditional photography, allowed the lightening and darkening effects to continue without deterioration. These lenses’ popularity was a direct result of the success of the silver halide implementation. Plastic Photochromics Since plastic lenses have replaced their glass counterparts, a new type of process is used to make photochromics. “Spiral helix” has replaced the former process and is implemented in several variations. The most popular process is imbibing, in which manufacturers implant a photochromic resin into the outermost layers of the lenses up to approximately 0.15 millimeters deep. Dip coating is a less effective method in which the front and back surfaces are coated with the chemicals needed to incite lens darkening. The process with the longest-lasting effects is known as “in-mass,” achieved when the molecules causing the reaction are distributed throughout the entirety of the lens. Other Factors that Inhibit Darkening New plastic photocromic lenses have a brief delay before reaching their maximum performance -- 30 minutes of ultraviolet exposure on five separate occasions is required before lenses will work as expected. Temperature also plays a major factor in color-changing ability. Lenses are tested at temperatures in the low 70s, and warmer temperatures will diminish performance, while lower temperatures will enhance it. Since photochromic lenses darken in ultraviolet light, use inside vehicles with UV-blocking windshields will hinder the desired effects.