The lenses of photochromic sunglasses are lenses made of glass, plastic or polycarbonate. They will darken when exposed to glare from ultraviolet light. The lenses are also called transition lenses. They can respond to ultra-violet radiation. The tint is a patented dye that activates the lens. Variation in light conditions, temperature and lens material interact with the tint or coating embedded in the lens and cause the lens to darken and lighten. The stronger and brighter the UV light, the darker the lens will be. The process can cause the lens to fade into various stages of light and dark, depending on the amount of UV rays they are interacting with. The process is automatic and wearers of transition lenses say they hardly notice the changes as they occur.