Photon counting

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A prototype single-photon detector that was used on the 200-inch Hale Telescope. The Hubble Space Telescope has a similar detector.

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,[1] 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 fiber-optic communication, quantum information science, quantum encryption, medical imaging, light detection and ranging, DNA sequencing, astrophysics, and materials science.[1]

Measured quantities[edit]

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.

SI photon units

Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol
Photon energy n 1 count of photons n with energy Qp = hc / λ.[nb 2]
Photon flux Φq count per second s−1 T−1 photons per unit time, dn/dt with n = photon number.
also called photon power.
Photon intensity I count per steradian per second sr−1⋅s−1 T−1 dn/dω
Photon radiance Lq count per square metre per steradian per second m−2⋅sr−1⋅s−1 L−2⋅T−1 d2n/(dA cos(θ) dω)
Photon irradiance Eq count per square metre per second m−2⋅s−1 L−2⋅T−1 dn/dA
Photon exitance M count per square metre per second m−2⋅s−1 L−2⋅T−1 dn/dA
See also: Photon counting · SI · Radiometry · Photometry
  1. ^ Standards organizations recommend that photon quantities be denoted with a suffix "q" (for "quantum") to avoid confusion with radiometric and photometric quantities.
  2. ^ The energy of a single photon at wavelength λ is Qp = h⋅c / λ with h = Planck's constant and c = velocity of light.

See also[edit]