Förster Resonance Energy Transfer (FRET) is a process where the energy of an excited dye (named donor) is transferred to a second dye (named acceptor). This energy is transferred emission free and is dependent on the distance between the donor and the acceptor. It can be observed up to a distance of about 10 nm.
The acceptor molecule can emit the energy via photons (depending on the dye used in the acceptor) which can be used to determine the FRET efficiency. But the FRET efficiency also influences the lifetime of the donor which can be measured via TCSPC (with our quTAG):
The FRET efficiency is then calculated from the lifetime of the donor with and without the presence of the acceptor. The measurement of the lifetime is independent from the intensity and therefore independent from any intensity fluctuations which makes it more accurate and reliable than the standard FRET based on the emitted intensity.