Sketch of the fluorescent collector geometries compared in the present paper (seen from the bottom). (a) Classical design with the solar cells mounted at each side of the collector with length l and thickness d. (b) Modified classical system where only a fraction of the respective collector sides is covered with a solar cell area A_cell = ds. (c) Collector with solar cells with an area A_cell = s² mounted at the bottom. In all cases the (remaining) back side is covered with a mirror. (d) Systems (b) and (c) are assumed to be periodic in space. Exemplary, a detail of the bottom-mounted system is shown.