FRET efficiency

The FRET efficiency is the yield of a FRET process. A FRET process transfers energy from the excited state of a donor fluorophore to an acceptor fluorophore. The number of donor molecules in an excited state which transfers energy to an acceptor defines the yield of this energy transfer.

E = \frac{transfered}{excited}

Practically, mostly the donor and acceptor fluorescence intensities are used to recover this yield.