Photochromic lenses are optical lenses that darken on exposure to specific types of light of sufficient intensity, most commonly ultraviolet (UV) radiation. In the absence of activating light the lenses return to their clear state. Photochromic lenses may be made of glass, polycarbonate, or another plastic.
They are commonly known as transition lenses because of their transformative feature of darkening when exposed to light. Transmission, meaning your lens will automatically get darker as the sun gets brighter and as the sun goes down, your lenses will automatically lighten.
Transition lenses allow you to play both outdoors and indoors and the best ones to have are clear to brown as brown lenses will offer greater contrast and definition between objects. However, it is important to know that a clear to brown sport sunglasses with transition lens will never be as dark as a typical brown sunglass lens.