Standard PIN and linear-mode avalanche photodiodes are the devices of choice for most light-detection applications. However, at ultralow-light levels, these detectors fall short because the noise of the readout electronics becomes greater than the photodiode signal. In a wide range of research and commercial applications -- such as fluorescence and luminescence detection, DNA sequencing, fluorescence correlation spectroscopy, decay time measurements, flow cytometry, noncontact profilometry, particle sizing and astronomy, as well as military and commercial lidar applications -- the detection of just a few photons is essential.
Photomultiplier tubes and Geiger-mode avalanche photodiodes -- commonly called photon counters -- were developed specifically to excel at ultralow-light levels. Although these devices can detect single photons, it is a mistake to call them photon counters because they cannot resolve photon quantity. Instead, they are trigger devices: They indicate if one or more photons have hit the detector's active area but usually cannot distinguish between the arrivals of one or more photons...

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