Main principles of RET via single-photon excitation

In order for RET to occur over distances of 10-100 A the fluorescence spectrum of D and the absorbance spectrum of A have to overlap adequately the quantum yield of D and the absorption coefficient of A (sa) must be sufficient. Furthermore, for the dipole-dipole interaction to occur, the transition dipoles of D and A must either be oriented favorably relative to each other or one or both of them have to have a certain degree of rapid rotational freedom. 

Forster in 1946, described this resonance energy transfer by the following equation  

---