Photon correlation system for fluorescence lifetime measurements

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Abstract
The construction and testing of a dual‐channel photoncorrelator is reported for the frequency domain imaging of fluorescence lifetimes using photon‐counting detection. A light source modulated at radio frequency excites fluorescence, which is detected using an imaging single‐photon detector. After discrimination, single‐photon events are processed in parallel by the correlation circuit, the purpose of which is to allow both the mean phase delay and the demodulation of fluorescence to be calculated relative to a reference signal derived from the modulated excitation source. Outputs from the correlator are integrated in a computer, resulting in accumulation of images which have been statistically filtered by sine and cosine transforms, and which can be manipulated within the computer to generate a resultant image where contrast depends on fluorescence lifetime rather than fluorescence intensity.