Research of fluorescence lifetime imaging technology based on heterodyne method of frequency domain

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.