text: The emitted photons from excited lanthanides are detected by highly sensitive devices and techniques such as single-photon detection. If the lifetime of the excited emitting level is long enough, then time-resolved detection (TRD) can be used to enhance the signal-to-noise ratio. The instrumentation used to perform LRET is relatively simple, although slightly more complex than conventional fluorimeters. The general requirements are a pulsed UV excitation source and time-resolved detection.
  Light sources which emit short duration pulses can be divided into the following categories:
  * Flash tubes
  * Mechanically or electronically chopped discharge lamps
  * Spark gaps
  * Pulsed lasers
  The most important factors in the selection of the pulsed light source for are the duration and intensity of the light. Pulsed lasers for the 300 to 500 nm range have now replaced spark caps in fluorescence spectroscopy. There are four general types of pulsing lasers used: lasers with pulsed excitation, lasers with G-switching, mode locked lasers and cavity dumped lasers. Pulsed nitrogen lasers (337 nm) have often been used as an excitation source in time resolved fluorometry.
  In time resolved fluorometry the fast photomultiplier tube is the only practical single photon detector. Good single photon resolution is also an advantage in counting photons from long decay fluorescent probes, such as lanthanide chelates.
  These commercial instruments are available in the market today: Perkin-Elmer Micro Filter Fluorometer LS-2, Perkin-Elmer Luminescence Spectrometer Model LS 5, and LKB-Wallac Time-Resolved Fluorometer Model 1230.