necessary to bring the reactants close to the light source in order to obtain the highest possible luminous efficacy. For this purpose, the reaction mixture can be irradiated either directly or in a flow-through side arm of a reactor with a suitable light source. A disadvantage of photochemical processes is the low efficiency of the conversion of electrical energy in the radiation energy of the required wavelength. In addition to the radiation, light sources generate plenty of heat, which in turn requires cooling energy. In addition, most light sources emit polychromatic light, even though only monochromatic light is needed. A high quantum yield, however, compensates for these disadvantages. Working at low temperatures is advantageous since side reactions are avoided (as the selectivity is increased) and the yield is increased (since gaseous reactants are driven out less from the solvent). The starting materials can sometimes be cooled before the reaction to such an extent that the reaction heat is absorbed without further cooling of the mixture. In the case of gaseous or low-boiling starting materials, work under overpressure is necessary. Due to the large number of possible raw materials, a large number of processes have been described. Large scale reactions are usually carried out in a stirred tank reactor, a bubble column reactor or a tube reactor, followed by further processing depending on the target product. In case of a stirred tank reactor, the lamp (generally shaped as an elongated cylinder) is provided with a cooling jacket and placed in the reaction solution. Tube reactors are made from quartz or glass tubes, which are irradiated from the outside. Using a stirred tank reactor has the advantage that no light is lost to the environment. However, the intensity of light drops rapidly with the distance to the light source due to adsorption by the reactants. The influence of the radiation on the reaction rate can often be represented by a power law based on the quantum flow density, i.e. the mole light quantum (previously measured in the unit einstein) per area and time. One objective in the design of reactors is therefore to determine the economically most favorable dimensioning with regard to an optimization of the quantum current density. Case studies [2+2] Cycloadditions Olefins dimerize upon UV-irradiation. 4,4-Diphenylcyclohexadienone rearrangement Quite parallel to the santonin to lumisantonin example is the rearrangement of 4,4-diphenylcyclohexadienone Here the n-pi* triplet excited state undergoes the same beta-beta