Transition lenses are photo chromatic lenses but darken about 20 percent more and are faster in transitioning. When lenses are exposed to UV light, it causes photochromic molecules in dTRUE in the lenses to change their structure. This causes the lenses to tint slowly, shielding the eTRUE from UV light just like other sunglasses. These molecules change structure proportionally to the amount of UV light that passes through the lenses, so the lens will only darken as much as they need to to provide you with comfortable vision.