phototube

An electron tube comprising a photocathode and an anode mounted within an evacuated glass envelope through which radiant energy is transmitted to the photocathode. A gas phototube contains, in addition, argon or other inert gas which provides amplification of the photoelectric current by partial ionization of the gas. The photocathode emits electrons when it is exposed to ultraviolet, visible, or near-infrared radiation. The anode is operated at a positive potential with respect to the photocathode. See also Electrical conduction in gases; Electron tube.

A phototube responds to radiation over a limited range of the spectrum that is determined by the photocathode material. Radiant sensitivity is the photoelectric current emitted per unit of incident monochromatic radiant power. See also Photoemission.

Quantum efficiency, or photoelectron yield, is the number of electrons emitted per incident photon. For photometric applications a useful parameter is luminous sensitivity: the photoelectric current per lumen incident from a specified source of light. A source commonly used is a tungsten-filament lamp operated at a color temperature of 4700°F (2870 K). See also Incandescence; Luminous flux; Photon.

Photocathodes are semiconductors which contain one or more of the alkali metals sodium, potassium, rubidium, or cesium chemically combined with bismuth, antimony, or silver oxide. The cathode surface contains a critical excess of the alkali metal which enhances photoelectric emission by decreasing the affinity of the surface for electrons. Negative affinity for electrons is achieved with the gallium arsenide:cesium (GaAs:Cs) and indium gallium arsenide:cesium (InGaAs:Cs) photocathodes used in photomultipliers. Phototubes also emit electrons thermionically at ambient temperatures. This “dark current,” observed in the absence of all irradiance, increases almost exponentially with temperature. Thermionic emission from the cesium antimonide (CsSb) photocathode is about 10⁻¹⁵ A/cm² at 68°F (20°C). See also Photomultiplier; Semiconductor.

Vacuum phototubes are used as detectors of radiant energy in the spectral range from 200 to 1100 nanometers. Since the photoelectric current is directly proportional to the intensity of the radiation, these tubes are used in radiometers, photometers, and colorimeters. By virtue of their narrow pulse response, vacuum phototubes are also used to measure the intensity of very short pulses of light generated by lasers and visible nuclear radiation. Gas phototubes can be used in light-operated relays and for the reproduction of sound from motion picture film, although their response to intensity-modulated light is limited to frequencies below 15 kHz. Vacuum as well as phototubes have been replaced in many applications by semiconductor photodiodes and photovoltaic cells. See also Colorimetry; Laser; Photodiode; Photometer; Photovoltaic cell; Radiometry.

A phototube is a type of gas-filled or vacuum tube that is extremely sensitive to light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum.^[1]

Operating principles

Phototubes operate according to the photoelectric effect: Incoming photons strike a photocathode, generating electrons, which are attracted to an anode. Thus current flow is dependent on the frequency and intensity of incoming photons. Unlike photomultiplier tubes, no amplification takes place, so the current generated is typically on the order of microamperes.^[2]

Applications

One major application of the phototube was the reading of optical sound tracks for projected films. Phototubes were used in a variety of light-sensing applications until they were superseded by photoresistors and photodiodes.

References

1. ^ Weaver, James. "Phototube". McGraw-Hill. http://www.accessscience.com/content.aspx?id=512000. Retrieved 2009-05-08. 
2. ^ J.B. Calvert (2002-01-16). "Electronics 30 - Phototubes". University of Denver. http://www.du.edu/~etuttle/electron/elect30.htm. 

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