(of glass) changing colour with the intensity of incident light, used, for example, in sunglasses that darken as the sunlight becomes brighter
photochromic in American English
(ˌfoʊtoʊˈkroʊmɪk)
adjective
designating or of a material, as certain glass or film, which turns dark when exposed to light and returns to its normal transparency with the removal of the light source