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Photon counting is a technique in which individual photons are counted using some single-photon detector (SPD). The counting efficiency is determined by the quantum efficiency and any electronic losses that are present in the system.
Many photodetectors can be configured to detect individual photons, each with relative advantages and disadvantages, including a photomultiplier, geiger counter, single-photon avalanche diode, superconducting nanowire single-photon detector, transition edge sensor, or scintillation counter. Charge-coupled devices can also sometimes be used.
Single-photon detection is useful in many fields including interplanetary communications, fiber-optic communication, quantum information science, quantum encryption, medical imaging, light detection and ranging, DNA sequencing, astrophysics, and materials science.
Measured quantities
The number of photons observed per unit time is the photon flux. The photon flux per unit area is the photon irradiance if the photons are incident on a surface, or photon exitance if the emission of photons from a broad-area source is being considered. The flux per unit solid angle is the photon intensity. The flux per unit source area per unit solid angle is photon radiance. SI units for these quantities are summarized in the table below.