Photon counting

Photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). A single-photon detector emits a pulse of signal for each detected photon. The counting efficiency is determined by the quantum efficiency and the system's electronic losses.

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

Many photodetectors can be configured to detect individual photons, each with relative advantages and disadvantages.[1][2] Common types include photomultipliers, geiger counters, single-photon avalanche diodes, superconducting nanowire single-photon detectors, transition edge sensors, and scintillation counters. Charge-coupled devices can be used.

Advantages

Photon counting eliminates gain noise, where the proportionality constant between analog signal out and number of photons varies randomly. Thus, the excess noise factor of a photon-counting detector is unity, and the achievable signal-to-noise ratio for a fixed number of photons is generally higher than the same detector without photon counting.[3]

Photon counting can improve temporal resolution. In a conventional detector, multiple arriving photons generate overlapping impulse responses, limiting temporal resolution to approximately the fall time of the detector. However, if it is known that a single photon was detected, the center of the impulse response can be evaluated to precisely determine its arrival time. Using time-correlated single-photon counting (TCSPC), temporal resolution of less than 25 ps has been demonstrated using detectors with a fall time more than 20 times greater.[4]

Disadvantages

Single-photon detectors are typically limited to detecting one photon at a time and may require time between detection events to reset. Photons that arrive during this interval may not be detected. Therefore, the maximum light intensity that can be accurately measured is typically low. Measurements composed of small numbers of photons intrinsically have a low signal-to-noise ratio caused by the randomly varying numbers of emitted photons. This effect is less pronounced in conventional detectors that can concurrently detect large numbers of photons. Because of the lower maximum signal level, either the signal-to-noise ratio will be lower or the exposure time longer than for conventional detection.

Applications

Single-photon detection is useful in fields such as:[1]

Medicine

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 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 a given diagnostic image quality. Photon counting detectors could help, due to their ability to reject noise more easily.[5][6] Photon counting is analogous to color photography, where each photon's differing energy affects the output, as compared to charge integration, which considers only the intensity of the signal, as in black and white photography.[7]

Photon-counting mammography was introduced commercially in 2003. Although such systems are not widespread, some evidence supports their ability to produce comparable images at an approximately 40% lower dose than other digital mammography systems with flat panel detectors.[8][9] Spectral imaging technology was subsequently developed to discriminate between photon energies,[10][6] with the possibility to further improve image quality[11] and to distinguish tissue types.[12] Photon-counting computed tomography is another interest area, which is rapidly evolving and is approaching clinical feasibility.[13][14][15][16]

Fluorescence-lifetime imaging microscopy
Further information: Fluorescence-lifetime imaging microscopy

Time-correlated single-photon counting (TCSPC) precisely records the arrival times of individual photons, enabling measurement of picosecond time-scale differences in the arrival times of photons generated by fluorescent, phosphorescence or other chemical processes that emit light, providing additional molecular information about samples. The use of TCSPC enables relatively slow detectors to measure extremely minute time differences that would be obscured by overlapping impulse responses if multiple photons were incident concurrently.

LIDAR
Further information: LIDAR

Some pulse LIDAR systems operate in single photon counting mode using TCSPC to achieve higher resolution. Infrared photon-counting technologies for LIDAR are advancing rapidly.[17]

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.

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 = 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 · 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

References
  1. "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. K.K, Hamamatsu Photonics. "Detection Questions & Answers". hub.hamamatsu.com. Retrieved 2020-08-14.
  4. "Fast-Acquisition TCSPC FLIM System with sub-25 ps IRF Width" (PDF). Becker and Hickl. Retrieved 17 August 2020.
  5. 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. pp. 2–21. ISBN 9781498766821.
  6. 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. Bibcode:2013MedPh..40j0901T. doi:10.1118/1.4820371. PMC 3786515. PMID 24089889.
  7. "Photon Counting Explained". Direct Conversion. Retrieved 2022-02-10.
  8. 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. PMID 21586506.
  9. 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. PMID 24495234.
  10. 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.
  11. Berglund, Johan; Johansson, Henrik; Lundqvist, Mats; Cederström, Björn; Fredenberg, Erik (2014-08-28). "Energy weighting improves dose efficiency in clinical practice: implementation on a spectral photon-counting mammography system". Journal of Medical Imaging. 1 (3): 031003. doi:10.1117/1.JMI.1.3.031003. ISSN 2329-4302. PMC 4478791. PMID 26158045.
  12. Fredenberg, Erik; Willsher, Paula; Moa, Elin; Dance, David R; Young, Kenneth C; Wallis, Matthew G (2018-11-22). "Measurement of breast-tissue x-ray attenuation by spectral imaging: fresh and fixed normal and malignant tissue". Physics in Medicine & Biology. 63 (23): 235003. arXiv:2101.02755. Bibcode:2018PMB....63w5003F. doi:10.1088/1361-6560/aaea83. ISSN 1361-6560. PMID 30465547. S2CID 53717425.
  13. Yveborg, Moa; Xu, Cheng; Fredenberg, Erik; Danielsson, Mats (2009-02-26). "Photon-counting CT with silicon detectors: feasibility for pediatric imaging". In Samei, Ehsan; Hsieh, Jiang (eds.). Medical Imaging 2009: Physics of Medical Imaging. Vol. 7258. Lake Buena Vista, FL. pp. 704–709. arXiv:2101.09439. doi:10.1117/12.813733. S2CID 120218867.{{cite book}}: CS1 maint: location missing publisher (link)
  14. Pourmorteza, Amir; Symons, Rolf; Sandfort, Veit; Mallek, Marissa; Fuld, Matthew K.; Henderson, Gregory; Jones, Elizabeth C.; Malayeri, Ashkan A.; Folio, Les R.; Bluemke, David A. (April 2016). "Abdominal Imaging with Contrast-enhanced Photon-counting CT: First Human Experience". Radiology. 279 (1): 239–245. doi:10.1148/radiol.2016152601. ISSN 0033-8419. PMC 4820083. PMID 26840654.
  15. "First 3D colour X-ray of a human using CERN technology". CERN. Retrieved 2020-11-23.
  16. "New 3D colour X-rays made possible with CERN technology". CERN. Retrieved 2020-11-23.
  17. Hadfield, Robert H.; Leach, Jonathan; Fleming, Fiona; Paul, Douglas J.; Tan, Chee Hing; Ng, Jo Shien; Henderson, Robert K.; Buller, Gerald S. (2023). "Single-photon detection for long-range imaging and sensing". Optica. 10 (9): 1124. doi:10.1364/optica.488853. S2CID 259687483. Retrieved 2023-08-29.
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