Cathode ray tube

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Cathode ray tube or CRT is a vacuum tube device used to display graphic information on a screen (as opposed to performing purely electronic operations such as amplifying, oscillating, or switching). It is distantly descended from the Crooke's tube.

In a cathode ray tube, a series of electrodes produces a narrow, tightly focussed beam of electrons. This beam is then deflected horizontally and vertically, either by other electrodes or by external magnetic fields. The beam travels a distance, typically on the order of one or two feet, and strikes a flat screen covered with phosphors. The phosphors glow where the beam strikes, causing a bright dot of light.

Cathode-ray tubes were very important electronic devices throughout most of the twentieth century. They found application in the early part of the century in oscilloscopes, a piece of technical equipment used in scientific research and electrical engineering. During the Second World War, they were used as radar screens. After the war, they became ubiquitous as television "picture tubes."

The deflection system is used to move the light rapidly across the screen. In an oscilloscope, the beam traces a wiggly line on the phosphor. In a television set, the beam traces a pattern of adjacent straight lines, called a "raster." In each case, because the deflection speed is so high, and because the phosphor glow persists for some time (milliseconds to seconds, depending on the phosphor), a viewer does not see the beam as moving, but sees a solid line or an illuminated picture (though sometimes with some detectable "flicker").

A curious application of the CRT was as a form of computer memory, the Williams tube. In this application, the CRT "writes" by depositing a pattern of charge on the screen, and "reads" by scanning the screen and detecting small differences in current caused by the pattern of charge deposited during the write cycle. Williams tubes were temperamental and unreliable, but enjoyed a brief vogue circa 1950.

Driven by the huge volume of consumer applications, cathode-ray tubes advanced enormously in the second half of the century. Very large tubes were developed in order to meet the need for large television screens. The glass envelopes, originally circular in shape, became rectangular to save space. Various forms of color CRTs were developed, the commonest employed three separate electron beams, three different colors of phosphor dots, and a "shadow mask" insure that each beam struck only the dots of its corresponding color. As computers became common in the last quarter of a century, color CRTs became sharper and higher in resolution, and the screen itself—original shaped like a section of a sphere centered on the electron gun—became shallower and, eventually, completely flat, to minimize reflections.

Circa the year 2000, solid state flat-panel displays began to drop in price to the point of becoming competitive with CRTs, and as of 2007 the era of the CRT appears to be drawing to a close.

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