In photochemical reactions, not every emitted photon by the light source interacts with the reaction mixture. Factors such as the positioning of the reactor relative to the light source, reflection losses, and other material interferences play a role. Those photons that do penetrate the reaction mixture contribute to the initial intensity (𝐼𝐼 0 ), defined at the inner surface of the reactor. This is the boundary where the reactor material meets the reaction mixture. Subsequently, the decline in light intensity within the reaction mixture is governed by the concentration of the attenuating species and its molar attenuation coefficient. It is also influenced by the distance the light covers within the medium. This distance, termed the optical path length, serves as an average descriptor of the setup's layout, and is independent of the absorbing species properties.