cathode ray tube

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cathode ray tube

(CRT) An electrical device for displaying images by exciting phosphor dots with a scanned electron beam. CRTs are found in computer VDUs and monitors, televisions and oscilloscopes. The first commercially practical CRT was perfected on 29 January 1901 by Allen B DuMont.

A large glass envelope containing a negative electrode (the cathode) emits electrons (formerly called "cathode rays") when heated, as in a vacuum tube. The electrons are accelerated across a large voltage gradient toward the flat surface of the tube (the screen) which is covered with phosphor. When an electron strikes the phosphor, light is emitted. The electron beam is deflected by electromagnetic coils around the outside of the tube so that it scans across the screen, usually in horizontal stripes. This scan pattern is known as a raster. By controlling the current in the beam, the brightness at any particular point (roughly a "pixel") can be varied.

Different phosphors have different "persistence" - the length of time for which they glow after being struck by electrons. If the scanning is done fast enough, the eye sees a steady image, due to both the persistence of the phospor and of the eye itself. CRTs also differ in their dot pitch, which determines their spatial resolution, and in whether they use interlace or not.
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(1) (CRunTime) See runtime library.

(2) (Cathode Ray Tube) A vacuum tube used as a display screen in a computer monitor or TV. The viewing end of the tube is coated with phosphors, which emit light when struck by electrons.

In the past, CRT was a popular term for a computer display terminal. Today, "monitor" is the correct term as computer displays have long since migrated from CRTs to LCD panels (see flat panel display). Likewise, TV sets no longer use CRTs (see flat panel TV).

Electrons and Phosphors
The CRT works by heating a cathode which causes electrons to flow. Accelerating and focusing anodes turn the electrons into a fine beam that is directed to the phosphors by magnetic fields that are generated by steering coils. The viewing end of a color CRT tube is coated with red, green and blue phosphor dots, and separate "electron guns" bombard their respective colors a line at a time in a prescribed sequence (see raster scan).

The resulting color displayed on screen is derived by the intensity of the electron beams as they strike the red, green and blue phosphors and cause them to glow at each pixel location. See cathode and vacuum tube.

Back to the 1800s
The first oscilloscope tube was developed in 1897 by German scientist Ferdinand Braun. Using a fluorescent screen and still known as a "Braun tube" in Germany, his "cathode-ray oscilloscope" was used to display the patterns of electronic signals. Although better known for inventing the CRT, Braun shared the Nobel Prize in 1909 with Guglielmo Marconi for wireless telegraphy.

The Braun Tube - 1897
Using a bellows, it took a strong man to evacuate the air from this tube. The successor to Sir William Crookes' vacuum tubes some 20 years earlier, these tubes used unheated "cold" cathodes that required a huge voltage. (Image courtesy of O'Neill's Electronic Museum)

Bulky But Magic in the 1950s
Although clunky by comparison to today's color screens, millions of people were thrilled when they first watched CRT-based monochrome TV. (Image courtesy of Vintage Vibe,

CRT vs. Flat Panel
The CRT gave way to LCD panels in the late 1990s, taking less space, less power and emitting less radiation. This high-quality EIZO LCD monitor was state-of-the-art in 1999. (Image courtesy of EIZO Nanao Technologies Inc.)

CRT Front Projection
The first data and TV projectors used CRTs, and although mostly abandoned, they continue to provide the highest quality. In 2006, this home theater was built by a serious video enthusiast. See front-projection TV. (Images courtesy of Kal of

CRT Rear Projection
Although big and bulky, the Pioneer Elite Pro-107 was perhaps the best CRT-based rear-projection TV ever made. Still working fine after 17 years, this unit was sold for a pittance in 2010. See rear-projection TV.
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