Photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). In contrast to a normal photodetector, which generates an analog signal proportional to the photon flux, a single-photon detector emits a pulse of signal every time a photon is detected. The total number of pulses (but not their amplitude) is counted, giving an integer number of photons detected per measurement period. 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. 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 also sometimes be used.
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 will usually be higher than if the same detector were operated without photon counting.
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 the arrival time of the photon. 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 longer.
Single-photon detectors are typically limited to detecting one single photon at a time and may require a "dead time" between detection events to reset. If additional photons arrive during this interval, they may not be detected. Therefore, the maximum light intensity that can be accurately counted is typically very low. Images or measurements composed of low numbers of photons intrinsically have low signal to noise ratio due to shot noise caused by the randomly varying numbers of emitted photons. This effect is less pronounced in conventional detectors which can concurrently detect large numbers of photons, mitigating shot noise. Therefore the signal to noise ratio with photon counting is typically much lower than conventional detection, and obtaining usable images may require very long acquisition times to accumulate photons.
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.
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.
Some pulse LIDAR systems operate in single photon counting mode using TCSPC to achieve higher resolution.
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 · Radiometry · Photometry|
- Single-photon source
- Visible-light photon counter
- Transition edge sensor
- Superconducting nanowire single-photon detector
- Time-correlated single photon counting
- Oversampled binary image sensor
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