Photochromism is a phenomenon where the molecular structure of a single chemical species changes with light action, without changing the molecular weight, reversibly generating two isomers with each different color. Photochromic compounds can be classified into two major categories in terms of thermal stability. In P-type compounds, both isomerization processes occur only by light irradiation, not by heat. In most cases, the structure changes to a colored isomer by UV light and becomes a colorless isomer by visible light irradiation. Typical examples of P-type include diarylethene and fulgide. In T-type compounds, conversion occurs by heat, as well as by light. Typical examples include azobenzene and spiropyran.