Lambert Instruments

The FRET efficiency indicates the percentage of the excitation photons that contribute to FRET. It depends strongly (sixth-power relationship) on the distance between the donor and acceptor fluorophores. The lifetime is inversely proportional to the FRET efficiency, thus the higher the efficiency the lower the lifetime of the donor fluorophore becomes.
However, the FRET efficiency in one pixel of your image does not give exact conclusions about the interactions between fluorophores. Namely, in case of 50% FRET efficiency in one pixel of your image, it is possible that 50% of the donor fluorophores have had 100% energy transfer to acceptor fluorophores, but it is also possible that 100% of the donor fluorophores have had 50% energy transfer to acceptor fluorophores.

The efficiency E depends on the donor-to-acceptor separation distance r with an inverse 6th order law due to the dipole-dipole coupling mechanism:

with R0 being the Förster distance of this pair of donor and acceptor at which the FRET efficiency is 50%.