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cathode-ray tube,special-purpose electron tube in which electrons are accelerated by high-voltage anodes, formed into a beam by focusing electrodes, and projected toward a phosphorescent screen that forms one face of the tube. The beam of electrons leaves a bright spot wherever it strikes the phosphor screen. To form a display, or image, on the screen, the electron beam is deflected in the vertical and horizontal directions either by the electrostatic effect of electrodes within the tube or by magnetic fields produced by coils located around the neck of the tube. Some cathode-ray tubes can produce multiple beams of electrons and have phosphor screens that are capable of displaying more than one color. Cathode-ray tubes are used in television sets, computer monitors, automated teller machinesautomated teller machine
(ATM), device used by bank customers to process account transactions. Typically, a user inserts into the ATM a special plastic card that is encoded with information on a magnetic strip.
..... Click the link for more information. , oscilloscopesoscilloscope
, electronic device used to produce visual displays corresponding to electrical signals. Displays of such nonelectrical phenomena as the variations of a sound's intensity can be made if the phenomena are converted into electrical signals.
..... Click the link for more information. , and radar displays.
(CRT), the general name of a number of electron-beam devices that are intended for the conversion of electric signals or light signals in various ways.
Depending on their purpose, cathode-ray tubes that are used to convert electric signals into visible images may be divided into, for example, the following categories: television picture tubes, or kinescopes; cathode-ray oscillograph tubes; numerical indicator tubes; the indicator tubes used in radar sets (seeINDICATOR); and data displays, including visual storage tubes.
Optical images are converted into video signals by means of television camera tubes.
In some cathode-ray tubes, both the input signals and the output signals are electric. In such CRT’s, the output signals reflect some kind of transformation of the input signals, for example, mathematical processing, a time delay, a change in sequence, or a change in frequency spectrum.
Zhigarev, A. A. Elektronnaia optika i elektronnoluchevyepribory.
V. L. GERUS
cathode-ray tube[′kath‚ōd ¦rā ‚tüb]
An electron tube in which a beam of electrons can be focused to a small cross section and varied in position and intensity on a display surface. In common usage, the term cathode-ray tube (CRT) is usually reserved for devices in which the display surface is cathodoluminescent under electron bombardment, and the output information is presented in the form of a pattern of light. The character of this pattern is related to, and controlled by, one or more electrical signals applied to the cathode-ray tube as input information.
Hundreds of millions of cathode-ray tubes were in service at the end of the twentieth century, and tens of thousands more were manufactured daily. These tubes were commonplace in television sets, computers, homes, hospitals, banks, and airplanes. Even so, the cathode-ray tube is being supplanted in many of its traditional uses by flat-panel electronic devices. This trend is expected to continue until, except for perhaps a few specialized applications, the cathode-ray tube will be primarily of historical interest.
The three elements of the basic cathode-ray tube are the envelope, the electron gun, and the phosphor screen (see illustration).
The envelope is usually made of glass, although ceramic envelopes and metal envelopes have been used. It is typically funnel-shaped. The small opening is terminated by the stem, a disk of glass through which pass metal leads that apply voltages to the several elements of the electron gun. The electron gun is mounted within the neck portion of the envelope and is connected to the leads coming through the stem. The neck is often made sufficiently narrow to allow positioning of deflection and focusing components outside it.
The large end of the funnel is closed by a faceplate, on the inside of which the phosphor screen is deposited. The faceplate is made of high-quality clear glass in order to provide an undistorted view of the display on the phosphor screen.
The electron gun consists of an electrical element called a heater, a thermionic cathode, and an assemblage of cylinders, caps, and apertures which are all held in the proper orientation.
The cathode is a source of electrons when maintained at about 1750°F (1100 K) by thermal radiation from the heater. Electrons emitted by the cathode are formed into a beam, and controlled in intensity by other elements of the electron gun. Means are provided, either within the electron gun itself or externally, to focus the electron beam to a small cross section at its intersection with the phosphor screen and to deflect it to various locations on the screen.
In most cases, monochrome cathode-ray tubes employ a single electron gun. Nearly all color picture tubes employ the shadow-mask principle and use three electron guns.
The deflection path of the electron beam on the phosphor screen depends on the intended use of the cathode-ray tube. In oscillography, a horizontal trace is swept across the phosphor screen, with vertical excursions of the beam which coincide with variations in the strength of some electrical signal. In television, a raster of closely spaced horizontal lines is scanned on the phosphor screen by the electron beam, which is intensity-modulated to produce a visible picture. Radar makes use of a variety of specialized electron-beam scanning patterns to present information to an observer.
In the display of computer output information, two general approaches to beam deflection are used: The raster-scan technique may be identical in format to that used for television or may utilize a greater number of scanning lines for increased definition. The random-scan technique involves computer control to direct the electron beam to locations which may be anywhere on the tube face.
The phosphor screen consists of a layer of luminescent material coated on the inner surface of the glass faceplate. Monochrome cathode-ray tubes generally use a single layer of a homogeneous luminescent material. Color cathode-ray tubes typically utilize a composite screen made up of separate red-, green-, and blue-emitting luminous materials.
Two basic types of deflection are electrostatic deflection and magnetic deflection.
With electrostatic deflection, it is possible to very quickly deflect the beam from one location to any other location on the screen. Operation is possible over a wide frequency range.
Magnetic deflection systems generally require more time, perhaps tens of microseconds, to deflect the electron beam from one location on the screen to another. This is because a change in position requires a change in the value of the current through an inductive coil. Magnetic deflection systems do have an important advantage in that they can deflect the beam through a much wider deflection angle with less distortion in the shape of the cross section of the beam than is possible with electrostatic deflection.
A wide variety of available envelopes, electron guns, and phosphor screens have been combined in different ways to fashion cathode-ray tubes specialized to meet the needs of different applications.
Direct view cathode-ray tubes involve either the presentation on the screen of an actual picture with a full black and white halftone range or with full color, such as is required for television, or the presentation of a computer-generated display which may consist of alphanumerics, graphics, or a variety of pictorial subjects. Tubes for the direct viewing of such presentations are required to have large display sizes, high brightness, high resolution, and in many cases a full halftone range and full color capability. Cathode-ray tubes for these presentations have always employed magnetic deflection and generally electrostatic-focus electron guns operating at high voltages from 15 to 36 kV.
Cathode-ray tubes for computer-generated data-display applications are very similar.
Projection tubes are not intended to be directly viewed. The display on the phosphor screen is projected by using an optical system, such as a lens, onto large screens. Screen sizes vary widely, the largest being those in theaters and sports arenas that are equipped for projection television.
Cathode-ray tubes for projection applications are usually of the general type described above but generally are optimized for extremely high brightness and resolution capability.
On a different scale, projection tubes used in avionics helmet-mounted displays make use of infinity optics to project images directly onto the retina of the aviators. Such cathode-ray tubes must be very small, light, low-voltage, low-power devices.
Another class of cathode-ray tubes which are not intended for direct viewing by human observers comprises photorecording tubes. The applications for these tubes require that the phosphor screen display be projected by an optical system, such as a lens, onto a photosensitive medium, such as photographic film. Applications include electronic phototypesetting and the storage of computer output information on microfilm. Photorecording cathode-ray tubes are required to have extremely high resolution capability, to be extremely stable over long periods of time, and to have accurate and precise display geometry. See Picture tube, Television