Photochromic lenses (Transitions®):

Photochromic lenses are convenient indoor-outdoor eyeglass lenses that automatically darken to a sunglass shade outside when exposed to sunlight, and then quickly return to a clear state indoors. These lenses also provide 100% protection from the sun’s UV rays, and are available in a wide variety of lens materials and designs, including bifocal and progressive lenses.The amount of darkening that most photochromic lenses undergo depends on how much UV radiation they are exposed to. As a general rule, these lenses won't get as dark behind the windshield of your car or truck because the glass blocks out much of the sun’s UV rays that cause the lenses to change color.