fluorescence resonance energy transfer time resolved fluorescence resonance energy transfer TR-FRET Time resolved-fluorescence resonance energy transfer (TR-FRET) unites TRF (Time-Resolved Fluorescence) and FRET (Fluorescence Resonance Energy Transfer) principles. This combination brings together the low background benefits of TRF with the homogeneous assay format of FRET. Introducing a time delay (50-150 micro seconds) between the system excitation and fluorescence measurement allows the signal to be cleared of all non-specific short lived emissions. TR-FRET uses lanthanide donors which are fluorophores with long emission half-lives.