https://doi.org/10.1351/goldbook.L03516
The time required for the @[email protected] intensity to decay from some initial value to \(\frac{1}{\text{e}}\) of that value (\(\text{e} = 2.718\,...\)). Lifetimes can be measured by @[email protected] (@[email protected]) where the phase shift between the sinusoidally modulated exciting light and the emitted light is measured.