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CRT

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Project Owner : NATARAJP
Created Date : Mon, 27/06/2011 - 20:27
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History

A common CRT used in computer monitors andtelevision sets

The experimentation of cathode rays is largely accredited to J.J. Thomson, an Englishphysicist who, in his three famous experiments, was able to deflect cathode rays, a fundamental function of the modern CRT. The earliest version of the CRT was invented by the German physicist Ferdinand Braun in 1897 and is also known as the Braun tube.[1] It was acold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen.

In 1907, Russian scientist Boris Rosing used a CRT in the receiving end of an experimentalvideo signal to form a picture. He managed to display simple geometric shapes onto the screen, which marked the first time that CRT technology was used for what is now known astelevision.[2]

The first cathode ray tube to use a hot cathode was developed by John B. Johnson (who gave his name to the term Johnson noise) and Harry Weiner Weinhart of Western Electric, and became a commercial product in 1922.[citation needed]

[edit]Overview

A cathode ray tube is a vacuum tube which consists of one or more electron guns, possibly internal electrostatic deflection plates, and a phosphor target.[2] In television sets and computer monitors, the entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of each of the three electron beams, one for each additive primary color (red, green, and blue) with a video signal as a reference.[3] In all modern CRT monitors and televisions, the beams are bent bymagnetic deflection, a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube, although electrostatic deflection is commonly used in oscilloscopes, a type of diagnostic instrument.[3]

Electron gun

[edit]Oscilloscope CRTs

In oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection commonly used with television and other large CRTs. The beam is deflected horizontally by applying an electric field between a pair of plates to its left and right, and vertically by applying an electric field to plates above and below. Oscilloscopes use electrostatic rather than magnetic deflection because the inductive reactance of the magnetic coils would limit the frequency response of the instrument.[4][5][6]

[edit]Phosphor persistence

Various phosphors are available depending upon the needs of the measurement or display application. The brightness, color, and persistence of the illumination depends upon the type of phosphor used on the CRT screen. Phosphors are available with persistences ranging from less than one microsecond to several seconds.[7] For visual observation of brief transient events, a long persistence phosphor may be desirable. For events which are fast and repetitive, or high frequency, a short-persistence phosphor is generally preferable.[8]

[edit]Microchannel plate

When displaying fast one-shot events the electron beam must deflect very quickly, with few electrons impinging on the screen; leading to a faint or invisible image on the display. Oscilloscope CRTs designed for very fast signals can give a brighter display by passing the electron beam through a micro-channel plate just before it reaches the screen. Through the phenomenon of secondary emission this plate multiplies the number of electrons reaching the phosphor screen, giving a significant improvement in writing rate (brightness), and improved sensitivity and spot size as well.[9][10]

[edit]Graticules

Most oscilloscopes have a graticule as part of the visual display, to facilitate measurements. The graticule may be permanently marked inside the face of the CRT, or it may be a transparent external plate. External graticules are typically made of glass or acrylic plastic. An internal graticule provides an advantage in that it eliminates parallax error. Unlike an external graticule, an internal graticule can not be changed to accommodate different types of measurements.[11] Oscilloscopes commonly provide a means for the graticule to be side-illuminated, which improves its visibility when used in a darkened room or when shaded by a camera hood.[12]

[edit]Color CRTs

Spectra of constituent blue, green and red phosphors in a common CRT

Color tubes use three different phosphors which emit red, green, and blue light respectively. They are packed together in stripes (as in aperture grille designs) or clusters called "triads" (as inshadow mask CRTs).[13] Color CRTs have three electron guns, one for each primary color, arranged either in a straight line or in a triangular configuration (the guns are usually constructed as a single unit). A grille or mask absorbs the electrons that would otherwise hit the wrong phosphor.[14] A shadow mask tube uses a metal plate with tiny holes, placed so that the electron beam only illuminates the correct phosphors on the face of the tube.[13] Another type of color CRT uses an aperture grille to achieve the same result.[14]

[edit]Convergence in color CRTs

The three beams in color CRTs would not strike the screen at the same point without convergence calibration. Instead, the set would need to be manually adjusted to converge the three color beams together to maintain color accuracy.[15]

[edit]Degaussing

Most CRT television sets and computer monitors have a built-in degaussing (demagnetizing) coil, which upon power-up creates a brief, alternating magnetic field which decays in strength over the course of a few seconds. This degaussing field is strong enough to remove most cases of shadow mask magnetization.[16]

[edit]Vector monitors

Vector monitors were used in early computer aided design systems and in some late-1970s to mid-1980s arcade games such asAsteroids.[17] They draw graphics point-to-point, rather than scanning a raster.

[edit]CRT resolution

Dot pitch defines the maximum resolution of the display, assuming delta-gun CRTs. In these, as the scanned resolution approaches the dot pitch resolution, moiré appears, as the detail being displayed is finer than what the shadow mask can render.[18] Aperture grille monitors do not suffer from vertical moiré, however, because their phosphor stripes have no vertical detail. In smaller CRTs, these strips maintain position by themselves, but larger aperture grille CRTs require one or two crosswise (horizontal) support strips.[19]

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