Abstract: A phase fluorometer employs a baseband frequency generator to generate a sample baseband signal at a first frequency f.sub.o and a reference signal correlated to the sample baseband signal. A second signal generator generates a carrier signal having a second frequency f. The sample baseband signal and the carrier signal are combined to form an up-converted sample signal having a frequency f.+-.f.sub.o. A light source is modulated at the up-converted sample signal frequency f.+-.f.sub.o to illuminate a sample. A photo detector detects fluorescence emissions from a sample excited by the excitation light and converts the emission into a fluorescence signal. A down-converter combines the carrier signal and fluorescence signal to form a phase-shifted and down-converted sample signal. A processor digitally processes the phase-shifted and demodulated sample signal and the reference signal to determine the fluorescence lifetime.