Fluorescence resonance energy donor/acceptor, dipole orientation

Fluorescence Resonance Energy Transfer (FRET), Fig. 1 (continued) the orientation factor from the relative alignment of the donor and acceptor transition dipoles, (d) ERET efficiency as a function of the relative distance between the donor and acceptor... 

In the case of Forster resonance energy transfer (FRET) [13], i.e. energy transfer by the dipole-dipole mechanism, and for randomly oriented donor-acceptor pairs, the depolarization after one transfer step (ensemble average) is almost complete [27]. For this reason, fluorescence anisotropy is a good indicator of energy transfer between identical fluorophores, hence of relative distances. Existence of efficient FRET may therefore reflect an association process. 

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