FRET efficiency falls steeply with donor-acceptor separation — it scales as the sixth power of distance, making FRET a molecular ruler for the 1–10 nm range.
R₀ (Förster radius) depends on spectral overlap J, quantum yield Φ_D, and orientation factor κ²:
The blue photons emitted by the donor decrease while red acceptor emission grows as distance shrinks below R₀.