A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a Photosensitivity photosensitive compound. When this is struck by a quantum of light photon , the absorbed energy causes electron emission due to the photoelectric effect . Coatings Although a plain metallic cathode will exhibit photoelectric properties, the specialized coating greatly increases the effect. A photocathode usually consists of alkali metal s with very low work function s. The coating releases electrons much more readily than the underlying metal, allowing it to detect the low energy photons in infrared radiation. The lens transmits the radiation from the object being viewed to a layer of coated glass. The photons strike the metal surface and transfer electrons to its rear side. The freed electrons are then collected to produce the final image. Photocathode materials Prose date October 2008 Ag O Cs , also called S 1. This was the first compound photocathode material, developed in 1929. Sensitivity from 300 nm to 1200 nm. Since Ag O Cs has a higher dark current than more modern materials photomultiplier tubes with this photocathode material are nowadays used only in the infrared region with cooling. Sb Cs antimony caesium has a spectral response from UV to optical spectrum visible and is mainly used in reflection mode photocathodes ... response range similar to the Sb Cs photocathode, but with higher sensitivity and lower dark current ..., this photocathode operates with very low dark current, making it ideal for use in photon counting applications. Multialkali sodium potassium antimony caesium , Na K Sb Cs . The multialkali photocathode ... to 930 nm by a special photocathode activation processing. GaAs gallium II arsenide . This photocathode ... to noise ratio than Ag O Cs. With special manufacturing techniques this photocathode can operate ... de Photokathode es Fotoc todo fa fr Photocathode nl Fotokathode pl Fotokatoda simple Photocathode ... more details
Near Infrared NIR Signature Management Technology is used to prevent detection of soldiers by NIR Image Converters . These photocathode devices do not detect temperatures, but rather infrared radiation variances. NIR compliant uniforms use a special fabric that allows soldiers to appear at the same radiation level as the surrounding terrain, thus making them more difficult to detect. Category United States Department of Defense US mil stub ... more details
infrared converter tube. This tube consisted of a photocathode in close proximity to a fluorescent screen. Using a simple lens, an image was focused on the photocathode and a potential difference of several thousand volts was maintained across the tube, causing electrons dislodged from the photocathode ... the infra red source. These devices used an S1 photocathode or silver oxygen caesium photocathode ... photocathode had sensitivity peaks in both the infrared and ultraviolet spectrum and with sensitivity over 950 nm was the only photocathode material that could be used to view infrared light ... significant amplification With the discovery of more effective photocathode materials, which increased ... and sodium potassium antimony discovered by A.H. Sommer and his later multialkali photocathode sodium potassium antimony cesium S20 photocathode discovered in 1956 by accident, that the tubes had ... photocathode has a sensitivity or around 150 to 200 A lm. The additional sensitivity made these tubes ... tube, both using electrostatic inversion, a 25mm photocathode of the same material and the same F2.2 ... photocathode that the first generation tubes used, however by using thicker layers of the same materials, the S25 photocathode was developed, which provides extended red response and reduced blue response ... A lm and a higher quantum efficiency than S20 photocathode material. redox Oxidation of the cesium ... angle to the axis of the tube. The micro channel plate fits between the photocathode and screen ... arsenide GaAs cesium oxide CsO Aluminium gallium arsenide AlGaAs photocathode which is more sensitive in the 800 nm 900 nm range than second generation photocathodes. Secondly, the photocathode ... Oxide layer at the edge of the photocathode causes sufficient Band theory of solids band bending. This makes the photocathode very efficient at creating photoelectrons from photons. The achilles heel ... Plate, this led to failure of the photocathode within a short period as little as 100 hours before ... more details
Multiple issues wikify February 2012 cleanup September 2009 no footnotes September 2009 A digicon detector is a spatially resolved light detector using the photoelectric effect directly it uses magnetic and electric fields operating in a vacuum to focus the electrons released from a photocathode by incoming light onto a collection of silicon diodes . It is a photon counting instrument, so most useful for weak sources. Digicon detectors were used on the original instruments for the Hubble Space Telescope, but are very rarely used in new designs, where CMOS active pixel detectors can achieve the same performance without the need for large electric fields or complicated vacuum assemblies. References http www.stsci.edu instrument news handbooks ghrs GHRS 35.html describes with rather ugly diagrams the digicons used in the GHRS instrument on Hubble. Category Optical devices ... more details
Wikify date December 2010 Interventions infobox Name PAGENAME Image Caption ICD10 ICD9 MeshID D010779 OtherCodes Photofluorography sometimes called just fluorography is photography of X ray images from fluorescent screen. ref name mesh http www.nlm.nih.gov cgi mesh 2007 MB cgi?mode &term Photofluorography&field entry Photofluorography Bot generated title ref It is commonly used in some countries for chest X ray screening, e.g. to diagnose tuberculosis or lung cancer see Abreugraphy for more information on such usage of this technique . Method of image formation in photoflourography X Ray beams from the tube get attenuated by the patient producing a transmitted radiation intensity corresponding to the part of the body traversed by the x ray beam. Transmitted intensities now fall on the photocathode stimulating it to produce electrons in quantities external to the light intensities emitted by the input. This is caused by the formation of a light image of the transmitted radiation pattern. Electrons from the photocathode are accelerated and focused electronically out on the output phosphor which emits lights as a result of electron bombardment. This shows a magnified image of what appears on the input phosphor. The semitransparent mirror splits the image in which one part is focused by the camera lens onto the film to form the image. The image size depends on the focal length of the camera lens. At the same time, the TV camera lens focuses the light to form an image on the TV camera photoreceptor where the image is transformed to a complex electronic signal and sent to the electronic image distributor. This then passes the image to the various display or recording devices e.g. a videotape recorder, kinescope, TV or display. See also Chest photofluorography References references Medical imaging Category Fluoroscopy ru uk ... more details
of a semi cylindrical photocathode , a secondary emitter mounted on the axis, and a collector ... field to confine the secondary electrons and relied on the Ag O Cs photocathode which had been demonstrated ... photomultipliers also used a Ag O Cs silver oxide caesium photocathode. They exhibited a peak ... photocathode, Cs sub 3 sub Sb caesium antimony , was reported by P. G rlich. ref cite journal last1 ... ref The caesium antimony photocathode had a dramatically improved quantum efficiency of 12 at 400 ..., the 931 type both as a photocathode and as a secondary emitting material for the dynode s. Different ... the device may be fabricated. Various combinations of photocathode and window materials were assigned ..., S 11 uses the caesium antimony photocathode with a lime glass window, S 13 uses the same photocathode with a fused silica window, and S 25 uses a so called multialkali photocathode Na K Sb Cs, or sodium ... than approximately 1700 nanometers, which can be approached by a special InP InGaAs Cs photocathode ... uses different designations for particular photocathode formulations and introduces modifications ... inside, which houses a photocathode , several dynode s, and an anode . Incident photons strike the photocathode ... s . Each dynode is held at a more positive voltage than the previous one. The electrons leave the photocathode, having the energy of the incoming photon minus the work function of the photocathode ... current pulse indicating the arrival of a photon at the photocathode. There are two common photomultiplier ... the flat, circular top of the tube and passes the photocathode, and the side on design reflection mode , where light enters at a particular spot on the side of the tube, and impacts on an opaque photocathode. Besides the different photocathode materials, performance is also affected by the transmission ... an appropriate design for a particular application. Photocathode materials The photocathodes can be made ... sensitive in infrared cooling the photocathode lowers this thermal noise. The most common ... more details
File phototubes.jpg thumb 300px Two different types of phototubes A phototube is a type of gas filled tube gas filled or vacuum tube that is extremely sensitive to light in the ultraviolet , visible light visible , and near infrared ranges of the electromagnetic spectrum . ref cite web url http www.accessscience.com content.aspx?id 512000 last Weaver first James title Phototube publisher McGraw Hill accessdate 2009 05 08 ref Operating principles Phototubes operate according to the photoelectric effect Incoming photon s strike a photocathode , generating electron s, 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 microampere s. ref cite web url http www.du.edu etuttle electron elect30.htm title Electronics 30 Phototubes author J.B. Calvert publisher University of Denver date 2002 01 16 ref Applications One major application of the phototube was the reading of optical Sound film sound tracks for projected films. Phototubes were used in a variety of light sensing applications until they were superseded by photoresistor s and photodiode s. References reflist Category Optical devices Category Sensors Category Vacuum tubes tech stub ar ca Fototub de Photozelle es Fototubo eo Foto elo it Fotocellula nl Fotocel ja pl Fotokom rka zh ... more details
Refimprove date November 2010 Image Dynodes.jpg thumb right 300 px Two horizontal rows of arc shaped dynodes in a photomultiplier tube. A dynode is one of a series of electrodes within a photomultiplier tube. Each dynode is at a more positive electrical potential than its predecessor. Secondary emission occurs at the surface of each dynode. Such an arrangement is able to amplify the tiny current emitted by the photocathode , typically by a factor of one million. Operation The electron s emitted from the cathode are accelerated toward the first dynode, which is maintained 90 to 100 V positive with respect to the cathode. Each accelerated photoelectron that strikes the dynode surface produces several electrons, that are then accelerated to the second dynode, which is held 90 to 100V more positive than dynode 1. For conventional dynode materials, such as BeO and MgO, a multiplication factor of 10 can normally be achieved by each dynode stage. ref Glenn F Knoll Radiation Detection and Measurement 3rd ed , 1999, P270, ISBN 0 471 07338 5. ref By the time this process has been repeated at each of the dynodes, 10 sup 5 sup to 10 sup 7 sup electrons have been produced for each incident photon, dependent on the number of dynodes. Naming The dynode is so named because it acts as a middle point between the photo cathode and the anode . References references See also Microchannel plate detector Photoelectric effect Particle detector Photodetector Category Electronic amplifiers Category Particle detectors ca Dinode de Dynode es D nodo fr Dynode it Dinodo kk lv Dinods nl Dynode pl Katoda wt rna ru ur ... more details
, and thus the photocathode layer can only receive the remainder of the incoming light the reflection ... the same elements, but orients the wiring behind the photocathode layer by flipping the silicon ... the photocathode layer without passing through the wiring layer. ref name US patent 4266334 cite ... the photocathode layer can lead to a host of problems, such as cross talk , which causes noise , dark ... more details
a photocathode which emits electron s when illuminated, the electrons are then amplified by a chain of dynode s. Phototube s containing a photocathode which emits electron s when illuminated, such that the tube ... more details
Refimprove date November 2008 A particle beam is a stream of charged particle charged or neutral particle s, in many cases moving at near the speed of light . There is a difference between creation and control of charged particle beam s to neutral particle beams, as only the first type can be manipulated to a sufficient extent by devices based on electromagnetism . The manipulation and diagnostics of charged particle beams at high kinetic energies using particle accelerator s are main topics of accelerator physics . Creation Charged particles such as electron s, positron s, and proton s may be separated from their common surrounding. This can be accomplished by e.g. thermionic emission or arc discharge . The following devices are commonly used as sources for particle beams ion source cathode ray tube , or more specifically in one of its parts called electron gun . This is also part of traditional television and computer screens. photocathode s may also be built in as a part of an electron gun , using the photoelectric effect to separate particles from their substrate. ref T. J. Kauppila et al. 1987 , A pulsed electron injector using a metal photocathode irradiated by an excimer laser , Proceedings of Particle Accelerator Conference 1987 ref Neutron beams may be created by energetic proton beam s which impact on a target, e.g. of beryllium material. see article Particle therapy . Acceleration See also Accelerator physics Superconducting Radio Frequency Charged beams may be further accelerated by use of high resonant, sometimes also superconducting , microwave cavity microwave cavities . These devices accelerate particles by interaction with an electromagnetic field . Since the wavelength of hollow macroscopic, conducting devices is in the radio frequency band, the design of such cavities and other RF devices is also a part of accelerator physics. More recently, Plasma acceleration has emerged as a possibility to accelerate particles in a plasma medium, using the elec ... more details
media M. Halmann had first shown CO sub 2 sub photoreduction to formic acid on p GaP as photocathode ... be due to high photovoltage excepted from higher band gap Gallium phosphide p GaP 2.2 eV photocathode ... , methanol and carbon monoxide . On p GaP, p GaAs and p p Si photocathode, the main product is formic acid with small amount of formaldehyde and methanol. However, for p InP and p CdTe photocathode, both ... of mainly formic acid on p GaP, p GaAs and p p Si photocathode owning to no adsorption of singly reduced CO sub 2 sub sup sup radical anion to the surface. In case of p InP and p CdTe photocathode ... more details
Unreferenced date January 2008 File Egun.jpg thumb 250px right Electron gun from a cathode ray tube File Kathodestraalbuis2.jpg thumb 250px right Electron gun from an oscilloscope CRT An electron gun also called electron emitter is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television set s and computer display s which use cathode ray tube CRT technology, as well as in other instruments, such as electron microscope s and particle accelerators . Electron guns may be classified in several ways by the type of electric field generation DC or RF , by emission mechanism thermionic emission thermionic , photocathode , field electron emission cold emission , Plasma physics plasmas source , by focusing pure electrostatic or with magnetic fields , or by the number of electrodes. Characteristics A direct current, electrostatic thermionic electron gun is formed from several parts a hot cathode , which is heated to create a stream of electrons via thermionic emission , electrodes generating an electric field which focus the beam such as a Wehnelt cylinder , and one or more anode electrodes which accelerate and further focus the electrons. A large voltage between the cathode and anode accelerates the electrons. A repulsive ring placed between them focuses the electrons onto a small spot on the anode at the expense of a lower extraction field strength on the cathode surface. Often at this spot is a hole so that the electrons that pass through the anode form a collimated beam and finally reach a second anode called a collector. This arrangement is similar to an Einzel lens . An Ion source ion gun consists of a cylinder where gas enters from one end face, undergoes electron bombardment from the side walls, and is subjected to an extraction voltage from the other end face. The entire cage has the role of the cathode the extractor acts as the anode, and an unnamed ring takes the role of the Wehnelt cylinder . Most ... more details
Other uses Streak disambiguation Streak Incomplete date January 2012 Image with unknown copyright status removed Image Streak Principle.jpg thumb right A streak camera is an instrument for measuring the variation in a pulse of light light s intensity physics intensity with time. They are used to measure the pulse duration of some ultrafast laser systems and for applications such as time resolved spectroscopy and LIDAR . Operation A streak camera operates by transforming the temporal profile of a light pulse into a spatial profile on a detector, by causing a time varying deflection of the light across the width of the detector. In particular, a light pulse enters the instrument through a narrow slit along one direction. It gets then deflected in the perpendicular direction so that photons that arrive first hit the detector at a different position compared to photons that arrive later. ref cite web title Hamamatsu Interactive Java Tutorials Streak Camera url http learn.hamamatsu.com tutorials java streakcamera accessdate 2006 10 15 ref The resulting image forms a streak of light, from which the duration, and other temporal properties, of the light pulse can be inferred. Usually, in order to record periodic phenomena, a streak camera needs to be triggered accordingly, similarly to an oscilloscope . Mechanical types Mechanical streak cameras use a rotating mirror or moving slit system to deflect the light beam. They are limited in their maximum scan speed and thus temporal resolution. Optoelectronic type optoelectronics Optoelectronic streak cameras work by directing the light onto a photocathode , which when hit by photons produces electron s via the photoelectric effect . The electrons are accelerated in a cathode ray tube and pass through an electric field produced by a pair of plates, which deflects the electrons sideways. By modulating the electric potential between the plates, the electric field is quickly changed to give a time varying deflection of the electrons ... more details
targets. Their image intensifier tubes function using an anode and an S 1 photocathode , made primarily ... light instead of an infrared light source. Using an S 20 photocathode , their image intensifiers ... title ref with an S 25 photocathode , ref name autogenerated3 resulting in a much brighter image, especially .... Third generation night vision systems maintain the MCP from Gen II, but now use a photocathode made ... through, diminishing the improvement expected from the Gallium arsenide photocathode. Because ... the photocathode voltage, allowing the NVD to instantaneously adapt to changing light conditions. ref ... of photoelectrons generated by the photocathode. Image Gen3 Vs Gen2 IIT.jpg thumb right A direct photographic ... more details
is called a photocathode and is connected to the negative terminal of a high tension battery ... a charged particle strikes the phosphor, a photon is emitted. This photon strikes the photocathode ... more details
, it is not. When light strikes a charged photocathode plate, electrons are emitted through a vacuum ... in the same pattern as the light that strikes the photocathode, and is on a frequency that the human eye can see. This is much like a Cathode ray tube CRT television , but instead of color guns the photocathode ... more details
an undesirable phenomenon, where the light generated by the phosphor screen feeds back to the photocathode ... has also been experimented with as an electron source, since a photocathode phosphor cell will latch ... more details
photocathode emissions electrons which pass through a scanning aperture to an anode , which serves as an electron ... Operation The optics optical system of the image dissector focuses an image onto a photocathode mounted inside a high vacuum. As light strikes the photocathode, electrons are emitted in proportion to the intensity ... aperture permits only those electrons emanating from a very small area of the photocathode ... the vast majority of electrons emitted by the photocathode are excluded by the scanning aperture ... of the image dissector and the Emitron. It has an efficient photocathode that transforms ... continuous film semitransparent photocathode that transforms the scene light into a light emitted electron ... more details
Infrared vision can be defined as the capability of biological or artificial systems to detect infrared radiation . The terms Thermography thermal vision and Thermography thermal imaging , ref thermal imaging Encarta World English Dictionary North American Edition & P 2007 Microsoft Corporation. 17 Apr. 2008, http encarta.msn.com dictionary thermal 2520imaging.html Encarta . http www.webcitation.org 5kx5SBUG3 Archived 2009 11 01. ref ref thermal imaging Cambridge University Press 2008. 17 Apr. 2008, http dictionary.cambridge.org define.asp?key 82411&dict CALD Cambridge . ref are also commonly used in this context since infrared emissions from a body are directly related to their temperature hotter objects emit more energy in the infrared spectrum than colder ones. The human body, as well as many moving or static objects of military or civil interest, is normally warmer than the surrounding environment. Since hotter objects emit more infrared energy than colder ones, it is relatively easy to identify them with an infrared detector , day or night. Hence, the term night vision is also used sometimes misused in the place of infrared vision , since one of the original purposes in developing this kind of systems was to locate enemy targets at night. ref tank. Encyclop dia Britannica. 2008. Encyclop dia Britannica Online. 17 Apr. 2008, http www.britannica.com eb article 57394 Britannica . ref However, night vision concerns the ability to see in the dark although not necessarily in the infrared spectrum . In fact, night vision equipment can be manufactured using one of two technologies ref How Night Vision Works Howstuffworks. 17 Apr. 2008, http www.howstuffworks.com nightvision.htm HowStuffWorks . ref light intensifiers or infrared vision. The former technology uses a photocathode to convert light in the visible or near infrared portions of the electromagnetic spectrum to electrons, amplify the signal and transform it back to photons. Infrared vision on the other hand, use ... more details
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