Photon counting
{{Short description|Counting photons using a single-photon detector}}
File:Prototype photon counting system, c 1980s. (9660571969).jpg. The Hubble Space Telescope has a similar detector.]]
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.
Many photodetectors can be configured to detect individual photons, each with relative advantages and disadvantages.{{cite press release |author= |title=High Efficiency in the Fastest Single-Photon Detector System |url=https://www.nist.gov/pml/div686/manufacturing/high-efficiency-single-photon-detector.cfm |publisher=National Institute of Standards and Technology |date=February 19, 2013 |access-date=2018-10-11}}{{cite journal |first=RH |last=Hadfield |title=Single-photon detectors for optical quantum information applications |journal=Nature Photonics |volume=3 |issue=12 |page=696 |date=2009|doi=10.1038/nphoton.2009.230 |bibcode=2009NaPho...3..696H }} 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.{{Cite web|last=K.K|first=Hamamatsu Photonics|title=Detection Questions & Answers|url=https://hub.hamamatsu.com/us/en/ask-engineer/detection-questions-and-answers/index.html|access-date=2020-08-14|website=hub.hamamatsu.com|language=en}}
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.{{cite web |title=Fast-Acquisition TCSPC FLIM System with sub-25 ps IRF Width |url=https://www.photonicsolutions.co.uk/upfiles/FastAcquisitionTCSPCFLIMSystemApplicationNoteLG21Jun18.pdf |publisher=Becker and Hickl |accessdate=17 August 2020}}
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
=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.{{cite book |last1=Shikhaliev |first1=M |editor1-last=Iwanczyk |editor1-first=Jan S. |title=Radiation Detectors for Medical Imaging |date=2015 |publisher=CRC Press |location=Boca Raton, FL |isbn=9781498766821 |pages=2–21 |url=https://books.google.com/books?id=7wYZCwAAQBAJ |chapter=Medical X-ray and CT Imaging with Photon-Counting Detectors}}{{cite journal |last1=Taguchi |first1=Katsuyuki |last2=Iwanczyk |first2=Jan S. |title=Vision 20/20: Single photon counting x-ray detectors in medical imaging |journal=Medical Physics |date=12 September 2013 |volume=40 |issue=10 |pages=100901 |doi=10.1118/1.4820371 |pmid=24089889 |pmc=3786515 |bibcode=2013MedPh..40j0901T}} 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.{{Cite web|title=Photon Counting Explained|url=https://directconversion.com/insight/photon-counting/|access-date=2022-02-10|website=Direct Conversion|language=en-US}}
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.{{cite journal |last1=McCullagh |first1=J B |last2=Baldelli |first2=P |last3=Phelan |first3=N |title=Clinical dose performance of full field digital mammography in a breast screening programme |journal=The British Journal of Radiology |date=November 2011 |volume=84 |issue=1007 |pages=1027–1033 |doi=10.1259/bjr/83821596 |pmid=21586506 |pmc=3473710}}{{cite journal |last1=Weigel |first1=Stefanie |last2=Berkemeyer |first2=Shoma |last3=Girnus |first3=Ralf |last4=Sommer |first4=Alexander |last5=Lenzen |first5=Horst |last6=Heindel |first6=Walter |title=Digital Mammography Screening with Photon-counting Technique: Can a High Diagnostic Performance Be Realized at Low Mean Glandular Dose? |journal=Radiology |date=May 2014 |volume=271 |issue=2 |pages=345–355 |doi=10.1148/radiol.13131181|pmid=24495234 |doi-access=free }} Spectral imaging technology was subsequently developed to discriminate between photon energies,{{cite book|chapter-url=https://books.google.com/books?id=kgJ-DwAAQBAJ&pg=PT51|title=Electronics for Radiation Detection|last1=Iwanczyk|first1=Jan S|last2=Barber|first2=W C|last3=Nygård|first3=Einar|last4=Malakhov|first4=Nail|last5=Hartsough|first5=N E|last6=Wessel|first6=J C|date=2018|publisher=CRC Press|isbn=9781439858844|editor1-last=Iniewski|editor1-first=Krzysztof|chapter=Photon-Counting Energy-Dispersive Detector Arrays for X-Ray Imaging}} with the possibility to further improve image quality{{Cite journal|last1=Berglund|first1=Johan|last2=Johansson|first2=Henrik|last3=Lundqvist|first3=Mats|last4=Cederström|first4=Björn|last5=Fredenberg|first5=Erik|date=2014-08-28|title=Energy weighting improves dose efficiency in clinical practice: implementation on a spectral photon-counting mammography system|journal=Journal of Medical Imaging|volume=1|issue=3|pages=031003|doi=10.1117/1.JMI.1.3.031003|issn=2329-4302|pmc=4478791|pmid=26158045}} and to distinguish tissue types.{{Cite journal|last1=Fredenberg|first1=Erik|last2=Willsher|first2=Paula|last3=Moa|first3=Elin|last4=Dance|first4=David R|last5=Young|first5=Kenneth C|last6=Wallis|first6=Matthew G|date=2018-11-22|title=Measurement of breast-tissue x-ray attenuation by spectral imaging: fresh and fixed normal and malignant tissue|journal=Physics in Medicine & Biology|volume=63|issue=23|pages=235003|doi=10.1088/1361-6560/aaea83 | arxiv=2101.02755|pmid=30465547|bibcode=2018PMB....63w5003F |s2cid=53717425 |issn=1361-6560|url=https://www.repository.cam.ac.uk/handle/1810/289601}} Photon-counting computed tomography is another interest area, which is rapidly evolving and is approaching clinical feasibility.{{Cite book|last1=Yveborg|first1=Moa|last2=Xu|first2=Cheng|last3=Fredenberg|first3=Erik|last4=Danielsson|first4=Mats|title=Medical Imaging 2009: Physics of Medical Imaging |date=2009-02-26|editor-last=Samei|editor-first=Ehsan|editor2-last=Hsieh|editor2-first=Jiang|chapter=Photon-counting CT with silicon detectors: feasibility for pediatric imaging|volume=7258 |chapter-url=http://proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.813733|location=Lake Buena Vista, FL|pages=704–709|doi=10.1117/12.813733|arxiv=2101.09439|s2cid=120218867 }}{{Cite journal|last1=Pourmorteza|first1=Amir|last2=Symons|first2=Rolf|last3=Sandfort|first3=Veit|last4=Mallek|first4=Marissa|last5=Fuld|first5=Matthew K.|last6=Henderson|first6=Gregory|last7=Jones|first7=Elizabeth C.|last8=Malayeri|first8=Ashkan A.|last9=Folio|first9=Les R.|last10=Bluemke|first10=David A.|date=April 2016|title=Abdominal Imaging with Contrast-enhanced Photon-counting CT: First Human Experience|journal=Radiology|volume=279|issue=1|pages=239–245|doi=10.1148/radiol.2016152601|issn=0033-8419|pmc=4820083|pmid=26840654}}{{Cite web|title=First 3D colour X-ray of a human using CERN technology|url=https://home.cern/news/news/knowledge-sharing/first-3d-colour-x-ray-human-using-cern-technology|access-date=2020-11-23|website=CERN|language=en}}{{Cite web|title=New 3D colour X-rays made possible with CERN technology|url=https://home.cern/news/news/knowledge-sharing/new-3d-colour-x-rays-made-possible-cern-technology|access-date=2020-11-23|website=CERN|language=en}}
=Fluorescence-lifetime imaging microscopy=
{{Further|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|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.{{Cite journal |url=https://opg.optica.org/optica/viewmedia.cfm?uri=optica-10-9-1124&html=true |access-date=2023-08-29 |journal=Optica |doi=10.1364/optica.488853 | title=Single-photon detection for long-range imaging and sensing | date=2023 | last1=Hadfield | first1=Robert H. | last2=Leach | first2=Jonathan | last3=Fleming | first3=Fiona | last4=Paul | first4=Douglas J. | last5=Tan | first5=Chee Hing | last6=Ng | first6=Jo Shien | last7=Henderson | first7=Robert K. | last8=Buller | first8=Gerald S. | volume=10 | issue=9 | page=1124 | s2cid=259687483 | doi-access=free |bibcode=2023Optic..10.1124H | hdl=20.500.11820/4d60bb02-3c2c-4f86-a737-f985cb8613d8 | hdl-access=free }}
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}}