Light guiding behavior of a fluorescent collector covered with solar cells at the sides and a mirror at its back side. (a) Definition of photon ray angle θ. (b) Absorbed photons are reemitted spatially randomized. The system leads rays with θ > θ_c to the sides of the collector. (c) Rays with angle θ < θ_c for total internal reflection leave the top surface. (d) Applying a photonic band structure (PBS) keeps rays with θ < θ_pbs in the system as well. This PBS is energy selective with θ_pbs = θ_c. Therefore, rays with energies E ≤ E₁ are kept in the system only. (e) For an energy and angular selective PBS a reflection cone is assumed such that only rays with E ≤ E₁ and θ < θ_pbs are kept in the system.