Amongst other techniques, using resonant inductive coupling (RIC) has emerged as a suitable method to power WLE for photochemical applications (Figure ). This technique promised ready usability in traditional cylindrical reaction vessels and the option of a conventional scale-up in volumetric reactors. However, the performance data published in the first reports was still orders of magnitude below the proposed theoretical limits. 24 The main characteristics for the performance and usability of a photochemical reaction setup are the scale, productivity and energy efficiency. The productivity is typically expressed as the space time yield (STY) which together with the scale denominates the amount of product produced in a given reactor. While often not a critical factor for high-value products, the energy efficiency is still an important parameter regarding environmental impact and costs of the process. Besides the overall energy or wall-plug efficiency, the photochemical or photocatalytic space time yield (PSTY) has recently been introduced as a figure of merit for the energy efficiency of photochemical reaction systems. Assuming unity quantum yield, a maximum PSTY of about 300 mmol d -1 W -1 are theoretically possible for UVA to blue light.