As a result, photons can effectively leave the system, leading to a scenario where the system does not achieve complete absorption (i.e., T ≠ 0), as illustrated in Figure . To compensate for these unabsorbed photons, a precise measure of the optical path length becomes essential. This measurement is especially critical when the system's attenuation features vary, such as when determining the quantum yield for different photochemical reactions using an identical setup (see Supplementary Information). Within this context, a photon-efficient photoreactor can be primarily defined by its relatively long effective path length. And interestingly, as the Bouguer-Lambert-Beer law only portrays relative intensity, this classification remains somewhat detached from the employed optical power. It is worth noting, however, that with high-power light sources, the system's attenuation properties (like the concentration of absorbing entities) can undergo rapid alterations. High chemical conversions achieved within short irradiation times can transform a setup into a non-photon-efficient one, causing the light transmission to transition from nearly zero to a notable fraction in a short time span.