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Cathode ray tube

From Academic Kids

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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. The CRT developed from Philo Farnsworth's work was used in all television sets until the late 20th century and the advent of plasma screens, LCDs, DLP displays, and other technologies.

Contents

Apparatus description

The earliest version of the CRT was a cold-cathode diode, a modification of the Crookes tube (used to produce X-rays) with a phosphor-coated screen, sometimes called a Braun tube. The first version to use a hot cathode was developed by J. B. Johnson (who gave his name to the term Johnson noise) and H. W. Weinhart of Western Electric and became a commercial product in 1922.

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 phosphorescent material (usually based on transition metals or rare earths). When the electrons hit this material, light is emitted.

In case of a television and modern computer monitors, 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. The beam in all modern TV sets is scanned with a magnetic field applied to the neck of the tube with a "magnetic yoke", a set of coils driven by electronic circuits. This use of electromagnets to steer the electron beam is referred to as "magnetic deflection".

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 via deflection plates built into the tube's neck. The use of an electrical field (so-called "electrostatic deflection") allows the electron beam to be steered much more rapidly than with a magnetic field, where the inductance of the electromagnets imposes relatively severe limits on the frequency range that can be accurately reproduced.

Graphical displays for early computers used vector monitors, a type of CRT similar to the oscilloscope. Here, the beam would trace straight lines between arbitrary points, repeatedly refreshing the display as quickly as possible. Vector monitors were used in many computer displays as well as by some late 1970s to mid 1980s arcade games such as Asteroids. Vector displays for computers did not noticeably suffer the display artifacts of aliasing and pixelization, but were limited in that they could display only a shape's outline, and only a very small amount of rather largely-drawn text. (Because the speed of refresh was roughly inversely proportional to how many vectors needed to be drawn, "filling" an area using many individual vectors was impractical as was the display of a large amount of text.) Some vector monitors are capable of displaying several colors using either an ordinary tri-color CRT or two phosphor layers (so called "penetration color"). In these dual-layer tubes, by controlling the strength of the electron beam, electrons could be made to reach (and illuminate) either or both phosphor layers, typically producing green, orange, or red.

Other graphical displays used storage tubes including Direct View Bistable Storage Tubes (DVBSTs). These CRTs inherently stored the image and did not require periodic refreshing.

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Shadow mask CRT close-up
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Aperture grille CRT close-up

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 color, 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 leaded glass (sometimes called "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.

CRT displays accumulate static electrical charge on the screen, unless protective measures are taken. This charge does not pose a safety hazard, but can lead to significant degradation of image quality through attraction of dust particles to the surface of the screen. Unless the display is regularly cleaned with a dry cloth or special cleaning tissue (using ordinary household cleaners may damage anti-glare protective layer on the screen), after a few months the brightness and clarity of the image drops significantly.

Other technologies

It is likely that technologies such as plasma displays, liquid crystal displays, and other newer technologies will eventually make CRT-based displays mostly 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. However, color CRTs still find adherents in computer gaming, due to their very quick response time, and in the printing industry for their better color fidelity and contrast.

Magnets

Magnets should never be put next to a color CRT, as they may cause magnetization of the shadow mask, which will cause incorrect colors to appear in the magnetized area and may be expensive to have corrected. Most modern television sets and nearly all newer computer monitors have a built-in degaussing coil. This coil creates a brief, alternating magnetic field from standard 50 or 60 Hz household power upon power-up which decays in strength as a resistor in the circuit increases resistance with its increasing temperature as a result of the current passing through it. The alternating magnetic field created is sufficient enough to shake off most cases of shadow mask magnetization. It is also possible to purchase or to build your own external degaussing coil which can aid in demagnetizing older sets or in cases where the built-in coil was not effective. A soldering gun (a soldering iron will not work as it does not contain a large transformer which produces a large alternating magnetic field) may also be used to degauss a monitor by holding it up to the center of the monitor with the hot tip end facing safely AWAY from the glass (and yourself!) and while holding down the on button, slowly moving the gun in ever wider concentric circles past the edge of the monitor until the shimmering colors can no longer be seen. This may need to be repeated several times to remove severe magnetization.

In extreme cases, high power magnets such as the now popular neodymium iron boron, or NIB magnets, can actually deform the shadow mask. This type of damage is considered permanent and will render the CRT mostly useless. However, subjecting an old black and white television or monochrome (green screen, amber screen) computer monitor to magnets is generally harmless. This can be used as a demonstration tool and children should even be encouraged to do this so that they may see the immediate and dramatic effect of a magnetic field on moving charged particles, provided they are informed to never do the same with a color tube.

Health danger

Some believe the electromagnetic fields emitted by CRT monitors constitute a health danger to the functioning of living cells. Exposure to these fields is far lower at distances of 85 cm or farther. It is also less intensive for the display's user than for a person located behind it.

CRTs also emit very small amounts of X-rays as a result of the electron beam's bombardment of the shadow mask/aperture grille and phosphors. Almost all of this radiation is blocked by the thick leaded glass in the screen so the amount of radiation escaping the front of the monitor is mostly harmless. The Food and Drug Administration regulations in 21 CFR 1020 are used to strictly limit, for instance, television receivers to 0.5 milliroentgens per hour (mR/h) (0.13 C/(kgh) (at a distance of 5 cm from any external surface and as mentioned above, most CRT emissions fall well below this limit.[1] (http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=1020.10)

Because CRTs "contain" a high vacuum and represent a large surface area, the constant compressive force of normal atmospheric pressure means that CRTs also represent a large store of mechanical energy; they can implode very forcefully if the outer glass envelope is damaged. Most modern CRTs used in televisions and computer displays include a bonded, multi-layer faceplate that prevents implosion if the faceplate is damaged, but the bell of the CRT (back portions of the glass envelope) offers no such protection. Certain specialized CRTs (such as those used in oscilloscopes) do not even offer a bonded faceplate; these CRTs require an external plastic faceplate or other cover to render them implosion safe while in use. When handling or disposing of a CRT, you must take steps to avoid creating an implosion hazard for you or your trash removal service.

Old CRTs may also have used toxic phosphors, although that is much less common today. An implosion or other breaking of the glass envelope could release these toxic phosphors. And because of the X-ray hazard, the glass envelopes of most modern CRTs are made from heavily leaded glass. The lead in this glass may represent an environmental hazard, especially in the presence of acid rain leaching through landfills.

The constant refreshing of a CRT can cause seizures in epileptics, if they are photosensitive. Filters are available to reduce these effects. A high refresh rate (above 75 Hz) also helps to negate these effects.

CRTs operate at very high voltages. These voltages can persist long (several days) after the device containing the CRT has been switched off and unplugged. Do not tamper with devices containing CRT tubes unless you have proper engineering training and have taken appropriate precautions. Since the CRT contains a vacuum, care should be taken to prevent implosion.

See also

de:Kathodenstrahlrhre es:Tubo de rayos catdicos fr:Tube cathodique ko:음극선관 id:Tabung sinar kathoda it:CRT nl:Kathodestraalbuis ja:ブラウン管 no:Bilderr pt:Tubo de raios catdicos sv:Katodstrlerr zh:阴极射线管

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