Photochromic Technology.
Explained.

Transitions lenses employ photochromic technology to change the way you see the world. UV light from the sun causes a chemical reaction inside our lenses to produce varying degrees of darkness. On a cloudy day, in shade, or when the sun is setting and the amount of UV light is minimal, the molecules only slightly activate. This moderate activation is intentional to allow just the right amount of light to pass through the lens to reach your eyes so that you can see clearly and comfortably, regardless of the time of day and lighting conditions.

In the presence of UV rays, our clear, polycarbonate lenses start growing darker in only a matter of seconds, typically reaching 70% activation within 35 seconds. When UV rays are no longer present, our lenses immediately begin to fade back and become clear lenses in just a few minutes.

Regardless of the amount of light present, Transitions lenses always block harmful UVA and UVB rays.