where 〈𝑙𝑙〉, 𝑅𝑅 and 𝐻𝐻 are the geometric average optical path length, the inner radius and height of the tube, respectively. In our analysis, using the geometric average, we calculated an initial spectrumaverage transmittance of ~0.3% -a minuscule value. The negligible difference in transmittance between these two path length measures reveals that the average absorbed fraction (1 -𝑇𝑇, so 0.999 and 0.997, respectively) is practically independent of the path length. This confirms the nature of this setup as a photon-efficient system, wherein almost the entire photon flux is absorbed. In such a reactor, there is a progressive decline in light intensity, inducing variations in reaction kinetics across the reactor's crosssection. Consequently, the reactor wall experiences high light intensities, potentially leading to overirradiation and byproduct formation. In contrast, the reactor's center receives no light, preventing reactions from occurring. This indicates that such reactors may not be optimal for conducting photocatalytic transformations. It also underscores that traditional scaling-up approaches, such as dimension enlargement with corresponding larger diameter vessels, are not ideal for photochemical processes.