What does distance have to do with FRET efficiency?

FRET efficiency depends strongly on the distance between the donor and acceptor fluorophores. FRET efficiency increases as the intermolecluar distance decreases. The ideal distance is within the range of 1–10 nm for fluorescent proteins.

The limitation of FRET is that the transfer process is only effective when the distance between the donor and acceptor is less than 10 nanometers.