Interestingly, this fitted path length is significantly larger than the inner diameter of 0.8 mm, suggesting that the experimental setup using a capillary coil contributes to an extension of the optical path length. A different capillary (8.9 mL reactor volume, 3.2 mm OD, 1.6 mm ID) was tested under the same conditions, and similarly resulted in a path length much greater than the inner diameter (3.9 mm). Notably, the change in reactor volume induced by the different capillaries had a marginal effect on conversion (Figure ). This is due to a proportional change in photon flux per reactor volume and comparable path lengths (2.6 mm and 3.9 mm). The use of a smaller capillary size effectively concentrates light on the reaction volume, significantly increasing space-time yield-achieving higher conversions at the same liquid residence time (Figure ). This observation confirms that the number of absorbed photons per unit of liquid flow rate, rather than per unit of liquid residence time, is the determining metric. Introducing an equivalent volume of gas into the system led to the formation of slug flow, characterized by alternating gas and liquid slugs. In this flow regime, a thin film of liquid separates the gas bubbles from the capillary wall. As illustrated in Figure , introducing one equivalent volume of gas resulted in a decrease in photon absorption, which is attributed to a reduction in the effective optical path length. However, in terms of the feed ratio and actual gas holdup, photons are concentrated on the liquid inside the reactor. This is supported by a comparison with the expected path length, which serves as a reference point. The expected path length accounts for the volume occupied by the gas in the reactor, reducing the system's ability to absorb photons. Assuming an equal volumetric ratio of gas to liquid in both the feed and the reactor suggests that the original path length of 2.6 mm is reduced proportionally (i.e., to 1.3 mm).