This work theoretically evaluates light sources currently commercially available for the suitability to drive photoreactions. Comparative evaluation of different light sources reveals significant advantages of light-emitting diodes (LEDs) in the near UV and visible region, underlining the general superiority of narrow-band monochromatic light sources for photochemical processes. A generic analysis based on the average volumetric rate of photon absorption shows the limits resulting from physical fundamentals and the importance of the photon fluence rate for process intensification.