Single-photon counting is a productive technique for measuring the intensity of light when the signal is weak. By using a fast photomultiplier tube to detect the light and equally fast electronics to process the signal, the arrival of individual photons can be detected and counted. Photon resolving times of the order of 10 ns accommodate counting rates ranging from a few counts per second up to 106 counts/s with less than 1% nonlinearity in the response to the photon rate. This is a dynamic range in excess of 105:1. The technique can be applied to steady-state signals, or to sources whose intensity varies as a function of time. In the latter case, the time scale of interest can range from nanoseconds to months.
The purpose of this application note is to provide a brief introduction to the techniques and instrumentation that are productive for single-photon counting.