In photochemical reactors, the activation of reactants is achieved by irradiating the reaction mixture with a suitable light source (UV radiation, visible light, various wavelength ranges), mostly by absorbing optical radiation from organic compounds. The kinetics of the reactions taking place are largely determined by the intensity of the light and the wavelength. In the selection and design of photochemical reactors, in addition to determining the type and size of the reactor, the dimensioning and arrangement as well as the long-term behavior of the radiators used play a special role.