text: Förster resonance energy transfer efficiency () is the quantum yield of the energy-transfer transition, i.e. the probability of the energy-transfer event occurring per donor excitation event:
  where 
  * is the rate of energy transfer, 
  * the radiative decay rate (fluorescence) of the donor, 
  * are non-radiative relaxation rates (e.g., internal conversion, intersystem crossing, external conversion etc.).