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 fiber-optic communication, quantum information science, quantum encryption, medical imaging, light detection and ranging, DNA sequencing, astrophysics, and materials science.
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.
Photon-counting mammography was introduced commercially in 2003. Although such systems are not widespread, there is some evidence of their ability to produce comparable images at approximately 40% lower dose to the patient than other digital mammography systems with flat panel detectors. The technology was subsequently developed to discriminate between photon energies, so-called spectral imaging, with the possibility to further improve image quality, and to distinguish between different tissue types. Photon-counting computed tomography is another key area of interest, which is rapidly evolving and is at the verge of being feasible for routine clinical use.
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.
|Photon energy||n||1||count of photons n with energy Qp = h⋅c / 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|
- Single-photon source
- Shot noise
- Visible-light photon counter
- Transition edge sensor
- Superconducting nanowire single-photon detector
- Time-correlated single photon counting
- Oversampled binary image sensor
- "High Efficiency in the Fastest Single-Photon Detector System" (Press release). National Institute of Standards and Technology. February 19, 2013. Retrieved 2018-10-11.
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