Measurement and Applications of Fluorescence Lifetimes in the Nanosecond Range Using the Time Correlated Single Photon Technique

Abstract

The time correlated single photon (TCSP) technique is more commonly known as the “single photon counting” method, a term that has led to confusion with conventional photon counting detection. Both are spectroscopic detection methods but they differ greatly in principle, instrumentation, and application. Perhaps the distinction between the two techniques will be served by first briefly describing photon counting. Photon counting is an instrumental technique for measuring low levels of light or particle beam densities (1–5). Individual photons incident on a photomultiplier tube (PMT) create discrete pulses or bursts of electrons arriving at the anode at a rate directly proportional to the photon flux. The instrumentation is relatively straightforward. The current pulse from the PMT anode is converted to a voltage, amplified, and sent to a pulse height discriminator. Only those pulses within preset voltage limits are passed on to a counter. Advantages of photon counting include: 1) the signal-to-noise (S/N) ratio is enhanced compared to the DC current measurement method, 2) the data are in digital form, 3) the sensitivity is greater at low light levels, 4) the measurements may be integrated over long periods of time resulting in higher precision and accuracy(2).