Photon counting


Photon counting is a technique in which individual photons are counted using some single-photon detector. 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,
including a photomultiplier, geiger counter, single-photon avalanche diode, superconducting nanowire single-photon detector, transition edge sensor, or scintillation counter. Charge-coupled devices can also sometimes be used.

Applications

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.

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 is thought to be very small, the radiation protection principle of "as low as reasonably practicable" 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 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.

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.