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][2] 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]


In radiology, one of the major disadvantages of X-ray imaging modalities is the negative effects of ionising radiation. Although the risk from small exposures (as used in most medical imaging) is thought to be very small, the radiation protection principle of "as low as reasonably practicable" (ALARP) is always applied. One way of reducing exposures is to make X-ray detectors as efficient as possible, so that lower doses can be used for the same diagnostic image quality. Photon counting detectors could help, due to their ability to reject noise more easily, and other advantages compared to conventional integrating (summing) detectors.[3][4]

Commercial photon counting mammography machines have been produced. Although such systems are not widespread, there is some evidence of their ability to produce comparable images at lower doses than other digital mammography systems with flat panel detectors.[5][6] Photon-counting computed tomography is another key area of interest, where the ability to discriminate between photon energies could significantly improve the ability to distinguish tissue types when reconstructing an image.[7][4]

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.

Metric 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 · UCUM · 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]


  1. ^ a b "High Efficiency in the Fastest Single-Photon Detector System" (Press release). National Institute of Standards and Technology. February 19, 2013. Retrieved 2018-10-11.
  2. ^ Hadfield, RH (2009). "Single-photon detectors for optical quantum information applications". Nature Photonics. 3 (12): 696. Bibcode:2009NaPho...3..696H. doi:10.1038/nphoton.2009.230.
  3. ^ Shikhaliev, M (2015). "Medical X-ray and CT Imaging with Photon-Counting Detectors". In Iwanczyk, Jan S. (ed.). Radiation Detectors for Medical Imaging. Boca Raton, FL: CRC Press. p. 2-21. ISBN 9781498766821.
  4. ^ a b Taguchi, Katsuyuki; Iwanczyk, Jan S. (12 September 2013). "Vision 20/20: Single photon counting x-ray detectors in medical imaging". Medical Physics. 40 (10): 100901. doi:10.1118/1.4820371. PMC 3786515.
  5. ^ McCullagh, J B; Baldelli, P; Phelan, N (November 2011). "Clinical dose performance of full field digital mammography in a breast screening programme". The British Journal of Radiology. 84 (1007): 1027–1033. doi:10.1259/bjr/83821596. PMC 3473710.
  6. ^ Weigel, Stefanie; Berkemeyer, Shoma; Girnus, Ralf; Sommer, Alexander; Lenzen, Horst; Heindel, Walter (May 2014). "Digital Mammography Screening with Photon-counting Technique: Can a High Diagnostic Performance Be Realized at Low Mean Glandular Dose?". Radiology. 271 (2): 345–355. doi:10.1148/radiol.13131181.
  7. ^ Iwanczyk, Jan S; Barber, W C; Nygård, Einar; Malakhov, Nail; Hartsough, N E; Wessel, J C (2018). "Photon-Counting Energy-Dispersive Detector Arrays for X-Ray Imaging". In Iniewski, Krzysztof (ed.). Electronics for Radiation Detection. CRC Press. ISBN 9781439858844.