The phenomenon that a substance or a system changes color obviously under the action of an external excitation source is called photochromism. Photochromism means that when compound A is irradiated by light with a certain wavelength, a photochemical reaction takes place to obtain product B with different chemical structure, and the absorption harmonic of visible light will also change. Therefore, the color changes in appearance, and the colors of A and B are obviously different. Photochromism is a reversible chemical inversion. B can be restored to its original form A under the irradiation of other light or heating. This is an important criterion. Some irreversible reactions under the action of light can also lead to color changes, which only belong to the general photochemical category, but not to the photochromic category. There are two kinds of photochromic phenomena: one is that the material changes from colorless or light color to dark color under illumination, which is called positive photochromic; The other is the change from dark color to colorless or light color, which is called reverse photochromic. Substances with photochromic properties are called photochromic organic compounds or photochromic materials, also known as photochromic pigments. People are most familiar with the silver halide system used in photographic photography.