Photochromic lenses darken when exposed to ultraviolet light. When the UV light source is removed (for example, by walking indoors or driving in a vehicle with UV blocking windshield), the lenses return to the original tint. An added benefit of photochromic lenses is that they shield your eyes from 100% of the sun’s harmful UVA and UVB rays. Click here for more information about Photochromic Eyewear