People often refer to photochromic lenses as "transitions" lenses.  Axon Optics photochromic lenses are indoor lenses that darken to outdoor lenses when exposed directly to UV light. A photochromic lens can be a good option for some people. They do darken outside and double as a pair of sunglasses. The darkening element within the lens won’t impede your regular vision, indoors or out. However, it is important to know that photochromic lenses only darken when directly exposed to UV light. They do not darken in bright indoor light or inside cars. These lenses are more reactive in cold weather. Indoor temperatures and warm outdoor weather will be slower.