FRET is a part of TR-FRET, leading to the thinking that the basis for both types of measurements is similar. FRET stands for Fluorescence Resonance Energy Transfer. This means that two fluorophores are involved, a donor and an acceptor dye. A molecule is fused to both dyes and the interaction between these molecules (e.g. proteins) is investigated. The donor dye is excited and the resulting transmission energy is transferred to the acceptor that gives then an emission signal as well. In FRET measurements donor and acceptor can be any combination of fluorescent substances that overlap in the emission of the donor and excitation of the acceptor.
In contrast, for TR-FRET measurements the donor is well defined. It must be a substance that gives a TR-signal, a Time-Resolved signal. This is a quite unique property of certain elements, the so-called lanthanides. The most commonly used donors in TR-FRET assays are Europium or Terbium compounds.
For more information on TR-FRET, visit: http://www.bmglabtech.com/technology/tr-fret.cfm
But whatever you want to measure FRET or TR-FRET, the PHERAstar series of microplate readers was designed to measure both kinds of assays quickly and reliably.