Transitional lenses (also called variable tint or photochromic) darken when exposed to UV light. When the ultraviolet light source is removed (for example, walking indoors or driving in a vehicle with a UV blocking windshield), the lenses will return to the original tint. In addition to the flexibility of transition lenses, these safety glasses also shield your eyes from 100% of the sun's harmful UVA and UVB rays. Click here for more information about Variable Tint Lenses