TRANSITION LENSES (Potochromic Lenses):

These lenses darken and change color whenever the wearer goes out into the sunlight.  The tint is activated by UV light.  When the wearer goes back indoors, the lenses slowly revert back to clear.  Although these lenses are quite versatile, they do not get as dark as a pair of sunglasses while driving.  Car windows do not allow enough UV light to enter in order to fully darken the lens.