Cathode ray tube

The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is the display device used in most computer displays, video monitors, televisions and oscilloscopes.

Cathode rays are streams of high-speed electrons emitted from the heated cathode of a vacuum tube. In a cathode ray tube, the electrons are carefully directed into a beam, and this beam is deflected by a magnetic field to scan the surface at the viewing end (anode), which is lined with luminescent material (usually phosphor). When the electrons hit this material, light is emitted.

In case of a television, the entire front area of the tube is scanned in a fixed pattern called a raster, and a picture is created by modulating the intensity of the electron beam according to the programme's video signal.

In case of an oscilloscope, the intensity of the electron beam is kept constant, and the picture is drawn by steering the beam along an arbitrary path. Usually, the horizontal deflection is proportional to time, and the vertical deflection is proportional to the signal. The tube for this kind of use is longer and narrower, and deflection is done by applying an electrical field.

Color tubes use three different materials which specifically emit red, green, and blue light, closely packed together in strips, (in aperture grille designs) or clusters (in shadow mask CRTs). There are three electron guns, one for each color, and each gun can reach only the dots of one colour, as the grille or mask absorbs electrons that would otherwise hit the wrong phosphor.

The outer glass allows the light generated by the phosphor out of the monitor, but (for color tubes) it must block dangerous X-rays generated by the impact of the high-energy electron beam. For this reason, the glass is made of lead crystal. Because of this and other shielding, and protective circuits designed to prevent the anode voltage rising too high, the X-ray emission of modern CRTs is well within safety limits.

CRTs have a pronounced triode characteristic, which results in significant gamma (a nonlinear relationship between beam current and light intensity). In early televisions, screen gamma was an advantage because it acted to compress the screen contrast. The gamma characteristic exists today in all digital video systems. However, in some systems where a linear response is required, as in desktop publishing, gamma correction is applied.

It is likely that technologies such as plasma displays, liquid crystal displays, and other newer technologies will eventually make CRT based displays obsolete, because the new designs are less bulky and consume less power. As of mid-2003, LCDs are becoming directly comparable in price to CRTs, with LCDs forming 30% of the computer display market by value.

Magnets should never be put next to a CRT tube, as they may cause permanent magnetization which will stop the tube from working properly and may be impossible to correct.

WARNING: CRTs operate at very high voltages. These voltages can persist long after the device containing the CRT has been switched off. Do not tamper with devices containing CRT tubes unless you have proper engineering training and have taken appropriate precautions.

See also LCD




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