A custom continuous-flow reactor was assembled using a microcapillary (1.6 mm inner diameter, 3.2 mm outer diameter) coiled around a cylindrical base, which was then placed inside a larger cylindrical chamber. The chamber's inner wall was fitted with an LED strip aligned to irradiate the coiled capillary (Figure ). To characterize the light source, light intensity was measured at specified input powers at the chamber's center, with the coiled reactor being absent during the assessment. The LED strip, with its helical design, was then digitally modeled to calibrate the optical input power essential for the photon flux simulations (see Supplementary Information). For the simulations, a three-dimensional digital replica of the coiled capillary tube, mounted on its cylindrical base, was created. This replica was virtually illuminated by the calibrated LED strip through ray-tracing, as illustrated in Figure . In our study, the LED strip's input current and resultant potential were adjustable, enabling us to test and simulate two distinct power settings. These simulations revealed received photon fluxes of 0.85 and 1.8 μmol s -1 (equivalent to 0.27 and 0.56 W, optical power received by the reaction mixture) for the two settings. We juxtaposed the experimental outcomes with modeled kinetic curves (Figure ), produced using the molar balance for the tubular reactor under the assumption of it functioning as an ideal plug flow reactor (see Supplementary Information). In these findings, we differentiate between two path lengths: the effective optical path length, denoted as 𝑙𝑙 ̅ , and the geometric average path length, ⟨𝑙𝑙⟩, which is defined in Equation ( ) for any cylinder irradiated on one side. For the capillary tube, which has an inner diameter of 1.6 mm and an outer diameter of 3.2 mm, the expected geometric average path length is 1.25 mm. This translates to an initial absorbed fraction of roughly 0.53, underlining the significant transmittance. However, the effective optical path lengths determined for this system -which should ideally be consistent across different irradiation powers -stood at 1.9 mm and 1.8 mm for the 21 W and 52 W settings, respectively. These determined effective optical path lengths notably exceed the geometric average path length (i.e., 1.25 mm), leading to a greater initial absorbed fraction of approximately 0.66 and, consequently, a higher conversion rate. As can be seen in Figure , employing the effective optical path length yielded a strong alignment between the simulation and experimental data, whereas utilizing the traditionally adopted geometric average path length led to less accurate outcomes. Moreover, in light of the radiometric data, the photon fluxes required to attain the optimal kinetic curve fit for a 1.25 mm geometric average path length are unattainable with the current settings of the light source (see Figure ).