Page "Photomultiplier" Paragraph 56

For smaller photon fluxes, the photomultiplier can be operated in photon counting or Geiger mode ( see also: single-photon avalanche diode ).

In Geiger mode the photomultiplier gain is set so high ( using high voltage ) that a single photo-electron resulting from a single photon incident on the primary surface generates a very large current at the output circuit.

However, owing to the avalanche of current, a reset of the photomultiplier is required.

In either case, the photomultiplier can detect individual photons.

The drawback, however, is that not every photon incident on the primary surface is counted either because of less-than-perfect efficiency of the photomultiplier, or because a second photon can arrive at the photomultiplier during the " dead time " associated with a first photon and never be noticed.