Transitions® Lenses for Eyeglasses 
Transitions® Lenses Darken Automatically when Exposed to UV Light
What are Transitions lenses?
Transitions VI® lenses are a special type of lens option to consider when selecting your eyeglasses. They are unique in that they turn darker, similar to sunglasses although not as dark, when you are exposed to sunlight but remain clear indoors and at night. The lenses work by activation to ultraviolet light ("UV"). Transition lenses will only darken when exposed to UV light. This occurs because Transitions lenses are treated with patented photochromic dyes which cause the lenses to activate. The level of darkness of the lenses is dependent on the intensity of light the lenses are exposed to so you get the appropriate darkness for any setting. Darker tinted Transitions lenses (Transitions XTRActive) are available for those that prefer a light tint indoors and an even darker tint outdoors.