E., they darken. These processes are reversible; when the lens is removed from robust sources of UV rays the photochromic compounds return to their clear state. The preferred manufacturer of photochromic lenses in the U.S. is Transitions Optical, Which explains why photochromic lenses are generally called transition lenses Appear your https://travismkihf.theisblog.com/15981070/not-known-facts-about-photochromic-sunglasses