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The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is the display device that was traditionally 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, OLED displays, and other technologies. As a result of this technology, television continues to be referred to as "The Tube" well into the 21st century, even when referring to non-CRT sets.
Apparatus description
The earliest version of the CRT was a cold-cathode diode, a modification of the Crookes tube 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 exist in the form of streams of high speed electrons emitted from the heating of cathode inside a vacuum tube. The released electrons form a beam within the cathode ray tube due to the voltage difference applied in the two electrodes, and the direction of this beam is then altered either by a magnetic or electric field to swap over the surface at the fluorescent screen (anode), covered by phosphorescent material (often transition metals or rare earths). Light is emitted at the instant that electrons hit the surface of that material.
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 usage of electromagnets to change the electron beam's original direction is known to be "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.
The electron beam source is the electron gun, producing the stream of electrons by thermionic emission and then focusing it to a thin beam. The gun was often mounted slightly off-axis, as it accelerated not only electrons but also ions resulting from outgassing of the internal tube components and from an imperfect vacuum. The ions are heavier than electrons, therefore they are less likely to be deflected by the magnetic field from the deflection coils, and in older constructions with in-axis guns they were bombarding the phosphor in the center of the screen and causing its deterioration; some very old black and white TV sets show browning of the center of the screen, known as ion burn. The combination of an off-axis mounting of electron guns and permanent magnets bending the electron beam back in the desired direction forms an ion trap; the ions were not deflected enough so they struck the neck of the tube instead of the screen and harmlessly dissipated. This system was later replaced with aluminium coating of the phosphor.
The internal side of the phosphor layer is often covered with a layer of aluminium. The phosphors are usually poor electrical conductors, which leads to deposition of residual charge on the screen, effectively decreasing the energy of the impacting electrons due to electrostatic repulsion (an effect known as "sticking"). The aluminium layer is connected to the conductive layer inside the tube, disposing of this charge. It also reflects the phosphor light in the desired direction towards the viewer, and protects the phosphor from ion bombardment.
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.
Some displays for early computers (those that needed to display more text than was practical using vectors, or required high speed for photographic output) used Charactron CRTs. These used a perforated metal character mask ("Stencil") to shape a wide electron beam to form a selected character shape on the screen. The electronics could quickly select a character on the mask with one set of deflection circuits, while selecting the position to display the character at with a second set of deflection circuits, and then just turn on the beam briefly to draw that character. Graphics could still be drawn by selecting the unneeded position on the mask corresponding to the code for a space (when drawing a space the beam was simply kept off), which had a small round hole in the center instead of being solid, and draw this as with other displays.
Many of these various types of early computer display CRTs use "slow" or long persistance phosphor, to reduce flicker for the operator.
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.
The high voltage (E.H.T.) used for accelerating the electrons is provided by a transformer. For CRTs used in televisions, this is usually a flyback transformer that steps up the line (horizontal) deflection supply to as much as 32,000 volts for a colour tube. (Monochrome tubes may operate at a somewhat lower voltage and specialty CRTs may operate at much lower voltages.) The output of the transformer is rectified and the pulsating output voltage is smoothed by a capacitor formed by the tube itself: the accelerating anode being one plate, the glass being the dielectric, and the earthed coating on the outside of the tube being the other plate. Before all-glass tubes, the structure between the screen and the electron gun was made from a heavy metal cone which served as the accelerating anode. Smoothing of the E.H.T. was then done with a massive capacitor, external to the tube itself.
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 and TV broadcasting industries for their better color fidelity and contrast. In 2005 Sony announced that they would stop the production of CRT computer displays.
Magnets
Magnets should never be put next to a colour CRT, as they may cause magnetisation of the shadow mask, which will cause incorrect colours to appear in the magnetised area and may be expensive to have corrected (although this will correct itself over a few days or weeks). Most modern television sets and nearly all newer computer monitors have a built-in degaussing coil (pronounced "de-gow-sing"). 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 magnetisation. It is also possible to purchase or to build your own external degaussing coil which can aid in demagnetising 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 colours can no longer be seen. This may need to be repeated several times to remove severe magnetisation.
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 colour 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 µC/(kg·h) (at a distance of 5 cm from any external surface and as mentioned above, most CRT emissions fall well below this limit.
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 leaking 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. (Modern circuits contain bleeder resistors to ensure the high-voltage supply is discharged to safe levels within a couple of minutes at most.)
CRT tubes present a hazard to those without proper training and appropriate precautions. Since the CRT contains a vacuum, there is also risk of implosion, as well as electrocution from any residual charge.
High vacuum safety
Because of the strong vacuum within a CRT, they store a large amount 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. Before the use of bonded faceplates one of the hazards would be that a broken neck or envelope would cause the neck and electron gun to be propelled by atmosperic pressure at such a velocity that it would erupt through the face of the tube.
When handling or disposing of a CRT, you must take steps to avoid creating an implosion hazard for you or your trash removal service. The most simple and safe method to make the tube safe is to identify the small sealed glass nib at the far back of the tube (this may be obscured by the electrical connector) and then (while wearing safety glasses and gloves) filing a small nick across this and then to break it off using a pair of pliers. A loud sucking sound will be heard as the air enters the tube, filling the vacuum. One must be very cautious not to break the neck of the tube when it is evacuated since there is no plastic coating preventing shattering of the glass. High vacuum and high voltage can be dangerous.
See also
External links
- The Cathode Ray Tube site
- Free IC DataSheet Search Site : http://www.Datasheet4U.com