Generally, there are two types of photochromic lenses: monomer-based and coating-based. Monomer-based photochromic lenses are made by embedding special photochromic molecules or monomers within the lens material. These molecules undergo a chemical reaction when exposed to UV light, altering their structure or light absorption properties, thus changing the color of the lens. When UV intensity decreases, these molecules return to their original state, and the lens color becomes clear again. This photochromic technology is typically integrated throughout the entire lens material, ensuring a uniform and long-lasting color change effect.