When Transition lenses are exposed to UV light, trillions of photochromic molecules in the lens begin to change structure. This reaction is what causes the lenses to darken. Only Transitions Signature lenses use exclusive Chromea7 technology, an advanced formulation that allows the lenses to be more responsive than ever before.Chromea7 Technology:This exclusive, patented formulation consists of unique dyes that are integrated into the surface of the lens. The dyes are made up of molecules that constantly and smoothly recalibrate as the light around you changes. They darken and lighten to just the right degree so the optimal amount of light reaches your eyes.