Photochromic Lenses look like regular clear lenses when indoors but automatically darken when moving into a brighter area. Ultraviolet (UV) rays from the sun affect the molecules in the photochromic lenses so they change colour. They will darken in the light even on overcast days as UV rays still penetrate clouds.
This provides you with the best possible vision in all lighting through different shades of tint, without you even needing to think about it.