Also found in: Dictionary, Thesaurus, Wikipedia.


(foh-toh-kath -ohd) An electrode in an electronic device, such as a photocell, photomultiplier, or image tube, that emits electrons when a beam of electromagnetic radiation strikes the surface. By a suitable choice of photocathode material, a reasonable response may be obtained from near-infrared wavelengths to low-energy X-ray wavelengths. The electrons result from the photoelectric effect. As many as 30% of the incident photons can liberate electrons, although the percentage is usually lower when taken over a wide spectral region. The current of resulting electrons increases linearly with radiation intensity over a wide range of intensities.



a cathode in certain vacuum-tube devices that emits electrons when exposed to light. Photocathodes are usually made of substances based on compounds consisting of elements from groups I and V or groups I and VI of the periodic system of elements.

The most widely used types of photocathodes are cesium oxide-silver, cesium antimonide (Cs3Sb), and trialkali photocathodes. Cesium oxide-silver photocathodes consist of Cs2O containing free cesium and free silver. Trialkali photocathodes are made of Sb-Cs, Sb-K, and Sb-Na compounds. The emissive material is deposited as a monomolecular layer on a metal or glass substrate. A photocathode may be either opaque or semitransparent. An opaque photocathode is exposed to light through the vacuum; a semitransparent photocathode is exposed through the substrate.

The main parameter characterizing the efficiency of a photocathode is the luminous sensitivity, which is equal to the ratio of the photoelectric current and the luminous flux that produces the current. For example, the luminous sensitivity of opaque cesium oxide-silver and cesium antimonide photocathodes is 100–120 microamperes per lumen (µA/lm); the luminous sensitivity of opaque trialkali photocathodes may be as high as 1,000 µA/lm, and that of semitransparent trialkali photocathodes is 600 µA/lm.

A new type of photocathode, called the negative-electron-affinity (NEA) photocathode, was developed in the 1960’s (seeELECTRON AFFINITY). NEA photocathodes include photocathodes made of III-V compounds—for example, GaAs photocathodes, which are sensitive to visible light, and InAsP and InGaAs photocathodes, which are sensitive to visible light and to infrared radiation at wavelengths of up to 1.5 micrometers. The luminous sensitivity of opaque NEA photocathodes may be as high as or even exceed 1,500 µA/lm. The luminous sensitivity of semitransparent NEA photocathodes is relatively low. Thus, the luminous sensitivity of GaAs photocathodes with a film thickness of 1–2 micrometers does not exceed 400(µA/lm; that is, it is lower than the luminous sensitivity of semitransparent trialkali photocathodes.

The production technology for NEA photocathodes is considerably more complex than that for conventional photocathodes. Hence, NEA photocathodes are not widely used.



A photosensitive surface that emits electrons when exposed to light or other suitable radiation; used in phototubes, television camera tubes, and other light-sensitive devices.
References in periodicals archive ?
The electrostatic fields used to move electrons from the photocathode to the screen electrode are also moving any positive ions within the image intensifier tube toward the photocathode.
The ion barrier film used in Gen III systems is utilised to block stray absorbed gasses dislodged from the surface of the MCPs by electron-to-MCP wall collisions since these gasses became positively charged ions that could damage the photocathode surface and reduce the life of the I2 tube.
where g is the PMT gain, e is the elementary charge, Q ([lambda]) is the quantum efficiency of the PMT photocathode at wavelength [lambda], and [E.
Those applications are: i) maximizing the electron beam quality from a radiofrequency photocathode for its application in the SLAC X-ray laser facility currently under development, the first of its kind in the world; ii) accurately modeling electron beam loss in the SLAC SPEAR3 electron storage ring with a goal to further increase efficiency at the synchrotron X-ray laboratory; and iii) optimizing local electron orbit correction in an electron storage ring for enhanced, robust control.
1982 -- Gen 3 technology introduced a more efficient conversion of light to electrical energy at extremely low light levels due primarily to advances in photocathode technology.
Very schematically, a Gen III tube operates as follows: available photons strike a photocathode plate that in turn generates a proportional amount of electrons.
The new sensor, under development for the Department of Energy by Sandia National Laboratories and Los Alamos National Laboratory, will combine Intevac's patented LIVAR(R) photocathode with an advanced CMOS chip, developed by Rockwell Scientific Company, utilizing advanced ultra-high vacuum packaging technology developed by Intevac.
The NightHawk viewer uses advanced digital technology to enhance the quality and performance of the viewer, as well as eliminating the image intensifiers and photocathode tubes used in older night vision products.
Multiplication of the electrons emitted from the photocathode is achieved by a thin microchannel plate sandwiched between the photocathode and the phospor screen.
The EBAPS camera is compact, smaller than a pack of matches, and incorporates a proprietary camera chip, proprietary packaging and a high efficiency photocathode.
The second project includes a manufacturing technology effort for improving the performance and reducing the fabrication cost of the transferred electron photocathode tube, the primary component of the camera.
Three major, parallel, technology development efforts will be conducted and coordinated under this CECOM project including Transferred Electron EBAPS (TE-EBAPS) camera development, the CMOS APS chip development to support the camera, and thermal hardening of the TE photocathode to enable military and industrial storage temperature requirements to be met.