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(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.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
If the timing is right, the ultracold helium electrons from this plasma photocathode are then rapidly captured by the plasma wave and produce a new, much brighter beam of electrons.
These facts would indicate that the main routes of photocathode optimization and refinement of dye-sensitizer design (Figure 13) must be necessarily accompanied by a systematic research on the selection and eventually the definition of new redox mediators in order to improve the performance of p-DSCs.
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A high-voltage differential between the electrodes accelerates electrons into the glass fibers, and collisions with the wall elicit many more electrons, multiplying electrons coming from the photocathode. Finally, the amplified electrons from the micro-channel plate are projected onto a phosphor screen.
Intevac's digital night vision cameras and goggles use an electron bombarded active pixel sensor, or EBAPS, which contains a "photocathode" that takes available light and magnifies it two to three hundred times, said Bill Maffucci, vice president and general manager for mission systems.
The new JCAP photocathode construct consists of the semiconductor gallium phosphide and a molecular cobalt-containing hydrogen production catalyst from the cobaloxime class of compounds.
The MCP amplifies electronically an image focused onto a photocathode. The intensified image is then transmitted from the intensifier phosphor screen to a CCD sensor by means of a proprietary fiber optic bundle coupling.
Optocoupler consists of LEDs on one side and a photodiode, phototransistor, or triac photocathode on the other side.
A catadioptric lens sucks ambient light from near darkness and funnels it to an image-intensifier-tube photocathode. The tube amplifies brightness and delivers a viewable image to the primary optical sight (your scope) behind it.
Sniperscope and German Vampir and their infrared projectors; Generation 1, as exemplified by the cascade tube and AN/PVS-2 Starlight Scope; Generation 2 with the introduction of the micro channel plate (MCP); and Generation 3, which came about with the addition of the gallium arsenide (GaAs) photocathode and the ion-barrier film on the MCP.