Photon counting

Source: Wikipedia, the free encyclopedia.
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 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

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

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

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]

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 (

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 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
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:
  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. ^ 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. 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. ISBN 9781498766821
    .
  6. ^
    PMID 24089889
    .
  7. ^ "Photon Counting Explained". Direct Conversion. Retrieved 2022-02-10.
  8. PMID 21586506
    .
  9. PMID 24495234
    .
  10. ISBN 9781439858844
    .
  11. PMID 26158045
    .
  12. S2CID 53717425
    .
  13. S2CID 120218867.{{cite book}}: CS1 maint: location missing publisher (link
    )
  14. 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. S2CID 259687483
    . Retrieved 2023-08-29.
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