Sketch of the absorption and emission behavior as assumed in this paper. The dye absorption is given by a step function. Incoming photons have the energy E₁ and a high absorption coefficient α₁. The lower absorption coefficient α₂ holds for the lower energy E₂ and leads with Kirchhoffs law (Eq. (1)) to a high emission coefficient e₂. The model also features the possibility of an energy selective photonic band stop (PBS) that keeps the emitted photons in the FC system.