The time required for the
luminescence intensity to decay from some initial value to

of that value
(

). Lifetimes can be measured by
phase fluorimetry (
phosphorimetry) where the phase shift between the sinusoidally modulated exciting light and the
emitted light is measured.
Source:
PAC, 1984, 56, 231
(Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI:
molecular luminescence spectroscopy)
on page 242