At 50 rpm, a reduction in photon absorption and effective optical path length is observed, similar to the Uflow reactor, where large gas pockets span a large fraction of the axial gap. Gas holdup experiments indicate that the overall gas holdup increases with higher gas flow rates, and image analysis further confirms an increase in gas holdup within the irradiated region (Figure ). These findings suggest that the decrease in photon absorption at lower rotation speeds can be correlated with the amount of gas in the bubbles in the upper part of the reactor.