Photochemical effect refers to a chemical reaction of molecules absorbs the energy of the photon after external excitation. When ordinary light on biological tissue, under certain conditions, can produce photochemical effects. For example, by rhodopsin bleaching process after illumination. Ergot human skin cholesterol into vitamin D2 under the sun, and in the presence of chloroplasts conditions, sunlight can make water and carbon dioxide synthesis of carbohydrates and oxygen. As one kind of laser energy is highly concentrated, with excellent monochromatic light source, it can cause photochemical effects which can not caused by some ordinary light.