The pRS-SDR has a total reactor volume of 59 mL, with an irradiated volume of approximately 28 mL, located primarily between the quartz window and the rotating disk, which are separated by a 2.0 mm axial gap. The remaining volume, mainly beneath the disk, is considered non-irradiated. When the feed enters at the bottom of the reactor, the flow moves outward through the non-irradiated region (centrifugal flow) and inward in the irradiated region (centripetal flow). The photon flux for this system has been previously reported as 12.5 µmol/s, which allows for the determination of the effective optical path length using an appropriate reactor model (using 3 CSTRs in series rather than 1 CSTR; see Supporting Information, section S1.3). This model yields an optical path length of 4.3 mm. The behavior of gas bubbles in the pRS-SDR is highly dependent on the operating conditions. In the centrifugal flow region, a thin liquid film segregates a continuous gas phase from the liquid. In the centripetal flow region, a dispersed phase containing large bubbles (separated from the rotor and stator by a liquid film) and smaller bubbles is observed. The size of these bubbles in the dispersed region correlates with the rotation speed of the disk: higher rotation speeds lead to smaller bubbles, which can range within the micrometer scale. Figure shows the formation of different bubble sizes when nitrogen is introduced into the actinometric system. At 50 rpm, large bubbles form, spanning most of the axial gap, while at 3000 rpm, smaller bubbles are generated due to the high shear forces exerted on the gas. The kinetic curves in Figure reveal that photon absorption is only slightly affected by the introduction of one equivalent of gas. At high rotation speeds (3000 rpm), photon absorption actually increases. The shortening of liquid residence time is almost fully counteracted, and in some cases even overcompensated, by the concentration of light on the liquid within the reactor. Similar to the Uflow reactor, the expected path length based on the feed ratio is significantly lower than the fitted path lengths (Figure ). The lengthening of the effective optical path length at 3000 rpm is maintained even when the gas flow rate is increased to three times that of the liquid flow rate. At high rotation speeds, the addition of gas enhances photon absorption, thereby increasing the system's throughput.