A simple device for the determination of fluorescence lifetimes

Describes a simple modification to the phase-shift technique for determination of radiative lifetimes. The time delay between the exciting and emitted radiation is measured rather than the phase lag, and this removes the necessity for accurate knowledge of frequency and permits operation at relatively low modulation frequencies. A circuit is presented which provides a modulating and a delayed reference waveform and which compares the phase of a signal derived from the emitted radiation with the reference. Some preliminary tests of the system are described in which it is shown that the time resolution, for a modulation frequency of about 50 kHz, is roughly 5 ns (corresponding to a phase shift of less than 0.1 degrees ).