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Photon counting
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<h2 id="advantages">Advantages</h2> <p>Photon counting eliminates gain noise, where the proportionality constant between analog signal out and number of photons varies randomly. Thus, the <a href="/facts/Excess_noise_factor/x2eUFfgF">excess noise factor</a> of a photon-counting detector is unity, and the achievable <a href="/facts/Signal-to-noise_ratio/qohClhyG">signal-to-noise ratio</a> for a fixed number of photons is generally higher than the same detector without photon counting.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p><p>Photon counting can improve <a href="/facts/Temporal_resolution/9wp61G97">temporal resolution</a>. In a conventional detector, multiple arriving photons generate overlapping <a href="/facts/Impulse_response/l4vaE5er">impulse responses</a>, limiting temporal resolution to approximately the <a href="/facts/Fall_time/vEgFLvVw">fall time</a> 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 <a href="/facts/Time-correlated_single-photon_counting/ghdK61hE">time-correlated single-photon counting</a> (TCSPC), temporal resolution of less than 25 ps has been demonstrated using detectors with a fall time more than 20 times greater.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p> <h2 id="disadvantages">Disadvantages</h2> <p>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 <a href="/facts/Irradiance/fgHwu7uh">intensity</a> that can be accurately measured is typically low. Measurements composed of small numbers of photons intrinsically have a low <a href="/facts/Signal-to-noise_ratio/qohClhyG">signal-to-noise ratio</a> 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. </p> <h2 id="applications">Applications</h2> <p>Single-photon detection is useful in fields such as:<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> <ul><li><a href="/facts/Fiber-optic_communication/dc6S0Hg5">Fiber-optic communication</a></li> <li><a href="/facts/Quantum_information_science/JDuxpHp8">Quantum information science</a></li> <li><a href="/facts/Quantum_encryption/aJ4aQ6yw">Quantum encryption</a></li> <li><a href="/facts/Medical_imaging/pL6JyJEp">Medical imaging</a></li> <li><a href="/facts/Light_detection_and_ranging/b9SPUmLM">Light detection and ranging</a></li> <li><a href="/facts/DNA_sequencing/hsPsNCQW">DNA sequencing</a></li> <li><a href="/facts/Astrophysics/krFPRWLA">Astrophysics</a></li> <li><a href="/facts/Materials_science/N5HrXRYW">Materials science</a></li></ul> <h3>Medicine</h3> <p>In <a href="/facts/Radiology/FMXvDL3u">radiology</a>, one of the major disadvantages of <a href="/facts/X-ray_imaging/ab1bvq3q">X-ray imaging</a> modalities is the negative effects of <a href="/facts/Ionising_radiation/vw0d9GaT">ionising radiation</a>. Although the risk from small exposures (as used in most medical imaging) is thought to be small, the <a href="/facts/Radiation_protection/5LaNNgXe">radiation protection</a> principle of "as low as reasonably practicable" (<a href="/facts/ALARP/zuyQNFeL">ALARP</a>) is always applied. One way of reducing exposures is to make <a href="/facts/X-ray_detectors/gaU0yhyy">X-ray detectors</a> as efficient as possible, so that lower <a href="/facts/Radiation_dose/vw0d9GaT">doses</a> can be used for a given diagnostic image quality. Photon counting detectors could help, due to their ability to reject noise more easily.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> 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.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> </p><p><a href="/facts/Photon-counting_mammography/q8xN2UYz">Photon-counting mammography</a> 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 <a href="/facts/Mammography/mN9jkyf9">digital mammography</a> systems with <a href="/facts/Flat_panel_detector/CtxzbvGM">flat panel detectors</a>.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> <a href="/facts/Spectral_imaging_(radiography)/LydgtPQm">Spectral imaging</a> technology was subsequently developed to discriminate between photon energies,<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a><a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> with the possibility to further improve image quality<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> and to distinguish tissue types.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> <a href="/facts/Photon-counting_computed_tomography/H2LBNxKD">Photon-counting computed tomography</a> is another interest area, which is rapidly evolving and is approaching clinical feasibility.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a><a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p> <h3>Fluorescence-lifetime imaging microscopy</h3> <p class="note">Further information: <a href="/facts/Fluorescence-lifetime_imaging_microscopy/BM0vs5of">Fluorescence-lifetime imaging microscopy</a></p> <p>Time-correlated single-photon counting (<a href="/facts/TCSPC/ghdK61hE">TCSPC</a>) precisely records the arrival times of individual photons, enabling measurement of picosecond time-scale differences in the arrival times of photons generated by <a href="/facts/Fluorescent/qsJVT2qP">fluorescent</a>, <a href="/facts/Phosphorescent/WshJFsKV">phosphorescence</a> 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. </p> <h3>LIDAR</h3> <p class="note">Further information: <a href="/facts/LIDAR/b9SPUmLM">LIDAR</a></p> <p>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.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> </p> <h2 id="measured-quantities">Measured quantities</h2> <p>The number of photons observed per unit time is the <a href="/facts/Photon_flux/Aw3P5Bvb">photon flux</a>. The photon flux per unit area is the <a href="/facts/Photon_irradiance/fgHwu7uh">photon irradiance</a> if the photons are incident on a surface, or <a href="/facts/Photon_exitance/US1T54p8">photon exitance</a> if the emission of photons from a broad-area source is being considered. The flux per unit <a href="/facts/Solid_angle/se8IXufg">solid angle</a> is the <a href="/facts/Photon_intensity/tHNu4aa2">photon intensity</a>. The flux per unit source area per unit solid angle is <a href="/facts/Photon_radiance/5kS1FhpD">photon radiance</a>. SI units for these quantities are summarized in the table below. </p> SI photon units<ul><li>v</li><li>t</li><li>e</li></ul><table><tbody><tr><th scope="col" colspan="2">Quantity</th><th scope="col" colspan="2">Unit</th><th scope="col" rowspan="2">Dimension</th><th scope="col" rowspan="2">Notes</th></tr><tr><th scope="col">Name</th><th scope="col">Symbol<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a></th><th scope="col">Name</th><th scope="col">Symbol</th></tr><tr><th scope="row"><a href="/facts/Photon_energy/Gy4zEAaE">photon energy</a></th><td>n</td><td></td><td></td><td>1</td><td>count of photons n with energy <i>Q</i>p = <a href="/facts/Planck_constant/qGHAe3qq"><i>h</i></a><a href="/facts/Speed_of_light/sKSQwdNx"><i>c</i></a>/<i>λ</i>.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a></td></tr><tr><th scope="row"><a href="/facts/Photon_flux/Aw3P5Bvb">photon flux</a></th><td>Φq</td><td>count per <a href="/facts/Second/jtFxnJXx">second</a></td><td><a href="/facts/Second/jtFxnJXx">s</a>−1</td><td>T−1</td><td>photons per unit time, d<i>n</i>/d<i><a href="/facts/Time_in_physics/5s0grGmk">t</a></i> with <i>n</i> = photon number. also called <i>photon power</i></td></tr><tr><th scope="row"><a href="/facts/Photon_intensity/tHNu4aa2">photon intensity</a></th><td>I</td><td>count per <a href="/facts/Steradian/PRCqT4n1">steradian</a> per second</td><td><a href="/facts/Steradian/PRCqT4n1">sr</a>−1⋅s−1</td><td>T−1</td><td>d<i>n</i>/d<i><a href="/facts/Solid_angle/se8IXufg">ω</a></i></td></tr><tr><th scope="row"><a href="/facts/Photon_radiance/5kS1FhpD">photon radiance</a></th><td><i>L</i>q</td><td>count per <a href="/facts/Square_metre/u5nT9y0W">square metre</a> per steradian per second</td><td><a href="/facts/Square_metre/u5nT9y0W">m−2</a>⋅sr−1⋅s−1</td><td>L−2T−1</td><td>d2<i>n</i>/(d<i><a href="/facts/Area/KsNG1G9t">A</a></i> cos(<i>θ</i>) d<i>ω</i>)</td></tr><tr><th scope="row"><a href="/facts/Photon_irradiance/fgHwu7uh">photon irradiance</a></th><td><i>E</i>q</td><td>count per square metre per second</td><td>m−2⋅s−1</td><td>L−2T−1</td><td>d<i>n</i>/d<i>A</i></td></tr><tr><th scope="row"><a href="/facts/Photon_exitance/US1T54p8">photon exitance</a></th><td>M</td><td>count per square metre per second</td><td>m−2⋅s−1</td><td>L−2T−1</td><td>d<i>n</i>/d<i>A</i></td></tr><tr><td colspan="6">See also: <ul><li>Photon counting</li><li><a href="/facts/SI/NShmfzlD">SI</a></li><li><a href="/facts/Radiometry/w9j9PXSB">Radiometry</a></li><li><a href="/facts/Photometry_(optics)/eo6J7qj3">Photometry</a></li></ul></td></tr></tbody></table> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Single-photon_source/nAmXRF58">Single-photon source</a></li> <li><a href="/facts/Visible_Light_Photon_Counter/jJUdx2yI">Visible Light Photon Counter</a></li> <li><a href="/facts/Transition_edge_sensor/Uw95W6y9">Transition edge sensor</a></li> <li><a href="/facts/Superconducting_nanowire_single-photon_detector/y0jCvWMr">Superconducting nanowire single-photon detector</a></li> <li><a href="/facts/Time-correlated_single_photon_counting/ghdK61hE">Time-correlated single photon counting</a></li> <li><a href="/facts/Oversampled_binary_image_sensor/uISx2V7y">Oversampled binary image sensor</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"High Efficiency in the Fastest Single-Photon Detector System" (Press release). National Institute of Standards and Technology. February 19, 2013. Retrieved 2018-10-11. <a href="https://www.nist.gov/pml/div686/manufacturing/high-efficiency-single-photon-detector.cfm" target="_blank">https://www.nist.gov/pml/div686/manufacturing/high-efficiency-single-photon-detector.cfm</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>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. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>K.K, Hamamatsu Photonics. "Detection Questions & Answers". hub.hamamatsu.com. Retrieved 2020-08-14. <a href="https://hub.hamamatsu.com/us/en/ask-engineer/detection-questions-and-answers/index.html" target="_blank">https://hub.hamamatsu.com/us/en/ask-engineer/detection-questions-and-answers/index.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Fast-Acquisition TCSPC FLIM System with sub-25 ps IRF Width" (PDF). Becker and Hickl. Retrieved 17 August 2020. <a href="https://www.photonicsolutions.co.uk/upfiles/FastAcquisitionTCSPCFLIMSystemApplicationNoteLG21Jun18.pdf" target="_blank">https://www.photonicsolutions.co.uk/upfiles/FastAcquisitionTCSPCFLIMSystemApplicationNoteLG21Jun18.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"High Efficiency in the Fastest Single-Photon Detector System" (Press release). National Institute of Standards and Technology. February 19, 2013. Retrieved 2018-10-11. <a href="https://www.nist.gov/pml/div686/manufacturing/high-efficiency-single-photon-detector.cfm" target="_blank">https://www.nist.gov/pml/div686/manufacturing/high-efficiency-single-photon-detector.cfm</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>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. <a href="9781498766821" target="_blank">9781498766821</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3786515" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3786515</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"Photon Counting Explained". Direct Conversion. Retrieved 2022-02-10. <a href="https://directconversion.com/insight/photon-counting/" target="_blank">https://directconversion.com/insight/photon-counting/</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3473710" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3473710</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>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. <a href="https://doi.org/10.1148%2Fradiol.13131181" target="_blank">https://doi.org/10.1148%2Fradiol.13131181</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>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. <a href="9781439858844" target="_blank">9781439858844</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3786515" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3786515</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4478791" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4478791</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>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. <a href="https://www.repository.cam.ac.uk/handle/1810/289601" target="_blank">https://www.repository.cam.ac.uk/handle/1810/289601</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>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) <a href="http://proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.813733" target="_blank">http://proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.813733</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>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. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4820083" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4820083</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>"First 3D colour X-ray of a human using CERN technology". CERN. Retrieved 2020-11-23. <a href="https://home.cern/news/news/knowledge-sharing/first-3d-colour-x-ray-human-using-cern-technology" target="_blank">https://home.cern/news/news/knowledge-sharing/first-3d-colour-x-ray-human-using-cern-technology</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>"New 3D colour X-rays made possible with CERN technology". CERN. Retrieved 2020-11-23. <a href="https://home.cern/news/news/knowledge-sharing/new-3d-colour-x-rays-made-possible-cern-technology" target="_blank">https://home.cern/news/news/knowledge-sharing/new-3d-colour-x-rays-made-possible-cern-technology</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>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. Bibcode:2023Optic..10.1124H. doi:10.1364/optica.488853. hdl:20.500.11820/4d60bb02-3c2c-4f86-a737-f985cb8613d8. S2CID 259687483. Retrieved 2023-08-29. <a href="https://opg.optica.org/optica/viewmedia.cfm?uri=optica-10-9-1124&html=true" target="_blank">https://opg.optica.org/optica/viewmedia.cfm?uri=optica-10-9-1124&html=true</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>Standards organizations recommend that photon quantities be denoted with a suffix "q" (for "quantum") to avoid confusion with radiometric and photometric quantities. <a href="/wiki/Standards_organization" target="_blank">/wiki/Standards_organization</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>The energy of a single photon at wavelength λ is Qp = hc/λ with h = Planck constant and c = velocity of light. <a href="/wiki/Planck_constant" target="_blank">/wiki/Planck_constant</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> </ol>