The key technology of fluorescence lifetime imaging is introduced in this article. Time-correlated single photon counting
(TCSPC) in Time-domain method is adopted to record fluorescence lifetime . The principle of TCSPC and the
Multi-wavelength imaging method are introduced in the paper. The counting efficiency and the amount of information in
the data can be increased by recording the fluorescence in several wavelength channels simultaneously.
Multi-wavelength imaging was also successfully used to separate different chromophores in stead-state images.
Principle of frequency -domain method to implement fluorescence lifetime imaging is introduced in the paper. Referred
to the excitation light, the fluorescence light has a phase shift and a reduced modulation degree. Both the phase and the
modulation can be used to determine the fluorescence lifetime. However, phase measurements are much more accurate
than measurements of the modulation degree. So the phase is used for lifetime measurements. Taking into consideration
the high frequency, the wide amplitude range of the fluorescence signal and the signal-to noise ratio, the better solution is
provided. As high frequency mixers do not work well for DC output signals, heterodyne principle is used. The
heterodyne method was used in the signal modulation technique so that the signal-to-noise ration is improved. The
advantage of the method is that the output signals of the mixers can be directly digitized so that effective digital filtering
algorithms may be applied and the phase can be determined via fast Fourier transform.