光致变色镜片是一种当爆露在某特定波长的光（一般是紫外光）时会变得深色的镜片，而当不再被此光源照射便会回复原有的透光率而变得清透。光致变色镜片可以由玻璃或包括聚碳酸酯在内的塑胶制成。 其变色效果是由添加在镜片内的氯化银或其他卤化物，此类物质在没有被紫外光照射时对可见光是透明的，当被紫外光照射时就会起化学反应，化学反应后变成会吸收部份可见光而令镜片呈现得深色。此化学反应属可逆反应，所以当不再被紫外光照射时镜片就会回复原有透明的状态。

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 principally used in eyeglasses that are dark in bright sunlight, but clear in low ambient light conditions. They darken significantly within about a minute of exposure to bright light, and take somewhat longer to clear. A range of clear and dark transmittances are available; one manufacturer makes one glass with transmittance reducing from 87% to 20%, and another reducing from 45% to 9%.