Editing Photomultiplier tube (section)

==Applications==
Photomultipliers were the first [[electric eye]] devices, being used to measure interruptions in beams of light. Photomultipliers are used in conjunction with [[scintillator]]s to detect [[Ionizing radiation]] by means of hand held and fixed radiation protection instruments, and [[particle radiation]] in physics experiments.<ref>{{cite web|url=http://www.drct.com/probes/HP-265_G-M_pancake_probe.htm|title=HP-265 Pancake G-M Probe|website=www.drct.com}}</ref> Photomultipliers are used in research laboratories to measure the intensity and spectrum of light-emitting materials such as [[compound semiconductor]]s and [[quantum dots]]. Photomultipliers are used as the detector in many [[Spectrophotometry|spectrophotometers]]. This allows an instrument design that escapes the [[#High-sensitivity applications|thermal noise limit on sensitivity]], and which can therefore substantially increase the [[dynamic range]] of the instrument.

Photomultipliers are used in numerous medical equipment designs. For example, [[blood analysis]] devices used by clinical medical laboratories, such as [[Flow cytometry|flow cytometers]], utilize photomultipliers to determine the relative concentration of various components in blood samples, in combination with [[optical filter]]s and [[incandescent lamps]]. An array of photomultipliers is used in a [[gamma camera]]. Photomultipliers are typically used as the detectors in [[flying-spot scanner]]s.

===High-sensitivity applications===
After 50 years, during which [[Solid-state (electronics)|solid-state]] electronic components have largely displaced the vacuum tube, the photomultiplier remains a unique and important optoelectronic component. Perhaps its most useful quality is that it acts, electronically, as a nearly perfect [[current source]], owing to the high voltage utilized in extracting the tiny currents associated with weak light signals. There is no [[Johnson noise]] associated with photomultiplier signal currents, even though they are greatly amplified, e.g., by 100 thousand times (i.e., 100&nbsp;dB) or more. The photocurrent still contains [[shot noise]].

Photomultiplier-amplified photocurrents can be electronically amplified by a high-input-impedance electronic amplifier (in the signal path subsequent to the photomultiplier), thus producing appreciable voltages even for nearly infinitesimally small photon fluxes. Photomultipliers offer the best possible opportunity to exceed the Johnson noise for many configurations. The aforementioned refers to measurement of light fluxes that, while small, nonetheless amount to a continuous stream of multiple photons.

For smaller photon fluxes, the photomultiplier can be operated in photon-counting, or [[Geiger counter|Geiger]], mode (see also [[Single-photon avalanche diode]]). In Geiger mode the photomultiplier gain is set so high (using high voltage) that a single photo-electron resulting from a single photon incident on the primary surface generates a very large current at the output circuit. However, owing to the avalanche of current, a reset of the photomultiplier is required. In either case, the photomultiplier can detect individual photons. The drawback, however, is that not every photon incident on the primary surface is counted either because of less-than-perfect efficiency of the photomultiplier, or because a second photon can arrive at the photomultiplier during the "[[dead time]]" associated with a first photon and never be noticed.

A photomultiplier will produce a small current even without incident photons; this is called the [[Dark current (physics)|''dark current'']]. Photon-counting applications generally demand photomultipliers designed to minimise dark current.

Nonetheless, the ability to detect single photons striking the primary photosensitive surface itself reveals the quantization principle that [[Albert Einstein#Photons and energy quanta|Einstein put forth]]. Photon counting (as it is called) reveals that light, not only being a wave, consists of discrete particles (i.e., [[photon]]s).

=== Temperature range ===
It is known that at cryogenic temperatures photo multipliers demonstrate increase in (bursting) electrons emission as temperature lowers. This phenomenon is still [[unsolved problems in physics|unexplained by any physics theory]].<ref>{{Cite journal|last=Meyer|first=H. O.|date=February 2010|title=Spontaneous electron emission from a cold surface|url=https://doi.org/10.1209/0295-5075/89/58001|journal=EPL (Europhysics Letters)|language=en|volume=89|issue=5|pages=58001|doi=10.1209/0295-5075/89/58001|bibcode=2010EL.....8958001M|s2cid=122528463 |issn=0295-5075}}</ref>