ABSTRACT
In time-domain technology, the sample is excited with a pulse of light. The width of the pulse is made as short as possible and is preferably much shorter than the fluorescence lifetime of the sample. The time-dependent intensity decay is measured following the excitation pulse. In the frequency-domain or phase-modulation method, the sample is excited with intensity modulated light. Typically, a sine wave modulation of light is used for excitation. The intensity modulated excitation forces the fluorescence intensity to respond at the same modulation frequency. Because of the finite fluorescence
lifetime, the fluorescence intensity is delayed in time relative to the excitation. In effect, the delayed response results in a phase shift and demodulation of the signal compared to the excitation.
