Intriguingly, significant chemical conversions -typically observed at prolonged residence times -are essential to discern the notable impact of path length variations. This phenomenon is evident in the initial gradient of the kinetic curves depicted in Figure . At such low conversions, the disparity between different path lengths seems minimal. However, as conversion increases, the curves noticeably diverge, with the effective optical path length playing an increasingly dominant role in the kinetics and the accuracy of the models. Using conventional photon flux determination methods, which assume complete absorption of the incident light, and fitting a linear model to the initial gradient (the traditional fit in Figure ), an average received photon flux of 12.7 μmol s -1 (~4.1 W optical power received by the reaction mixture) is found. This contrasts only marginally with the flux determined via radiometry and ray-tracing, which was less than 2% lower. Interestingly, if one were to apply the known gap distance of 2.0 mm, the needed received photon flux approximates ~19.0 μmol s -1 (~6.2 W optical power received by the reaction mixture). This value exceeds the total optical output power of the floodlight (~5.3 W optical power emitted by the light source) as determined with radiometry and ray-tracing, rendering this as an implausible outcome (all discussed fluxes are shown in Figure ).