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Graphical displays for early computers used vector monitors, a type of CRT similar to the oscilloscope but frequently using magnetic, rather than electrostatic, deflection. 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 (with, in advanced vector systems, a limited amount of solid-tone shading), 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 usually 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 then drawing as with other displays.
Many of these various types of early computer display CRTs use "slow" or long-persistence phosphor to reduce flicker for the operator. While it reduces eyestrain for relatively static displays, the drawback of long-persistence phosphor is that when the display is changed, it produces a visible afterimage that can take on the order of a whole second or two to completely fade. This makes it inappropriate for animation or for real-time dynamic information displays.
Shadow mask CRT close-up
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). Color CRTs actually have three electron guns, one for each primary color, arranged either in a straight line or in a triangular configuration. Inside the CRT neck glass, the three guns are usually constructed as a single unit rather than discretely. Each gun can reach only the dots of one color, as the grille or mask absorbs electrons that would otherwise hit the wrong phosphor. Color CRTs with the guns arranged in a triangular configuration are known as delta-gun CRTs, because the triangular formation resembles the shape of the Greek letter delta. Dot pitch defines the "native resolution" of the display. When the scanned resolution nears the dot pitch resolution, moiré appears. Aperture grille monitors, however, don't suffer from vertical moiré, since the phosphor strips have no vertical detail such as gaps.
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 high energy electron beam impacting the inside of the CRT face. For this reason, the glass is leaded (sometimes called "lead crystal"). Color tubes require significantly higher anode voltages (as high as 32,000 volts for large tubes) than monochrome tubes, partly to compensate for the blockage of some electrons by the aperture mask or grille, and the amount of X-rays produced increases with voltage. Because of leaded glass, other shielding, and protective circuits designed to prevent the anode voltage from rising too high in case of malfuction, 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 preventive 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 color 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 Aquadag 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 high voltage capacitor, external to the tube itself. In the earliest televisions, before the invention of the flyback transformer design, a linear high-voltage supply was used; because these supplies were capable of delivering much more current at their high voltage than flyback high voltage systems, in case of accident they proved extremely deadly. The flyback circuit design addressed this; in the case of a fault, the flyback system is capable of delivering relatively little current, making a person's chance of surviving a direct shock from the high voltage anode lead more hopeful (though by no means guaranteed).
In recent years technologies such as liquid crystal displays, and other newer technologies have made CRT-based computer displays mostly obsolete for mainstream users, because the new designs are less bulky, consume less power and have a larger display area. As of mid-2006, LCDs have become directly comparable in price to CRTs of the same display area. However, color CRTs still find adherents in computer gaming, due to their very quick response time and higher resolution per dollar, and in the printing and TV broadcasting industries for their better color fidelity and contrast. Improvements in LCD technology increasingly alleviate these concerns and demand for CRT screens is falling rapidly. Producers are responding to this trend. For instance, in 2005 Sony announced that they would stop the production of CRT computer displays.
This trend is less clear in television CRT displays. Due to the high cost of large LCD panels and plasma displays, a market niche for CRTs still exists as a cheaper alternative to these technologies. However, it is likely that in the future CRT television displays too will be replaced by displays based on other technologies.
Magnets should never be put next to a color CRT, as they may cause magnetisation of the shadow mask, which will cause incorrect colors to appear in the magnetised area - this is called a "purity" problem, because it affects the purity of one of the primary colors, with the residual magnetism causing the undesired deflection of electrons from one gun to the wrong color's phosphor patch. This can be expensive to have corrected, although it may correct itself over a few days or weeks. Most modern television sets and nearly all newer computer monitors have a built-in degaussing coil (variously pronounced "de-gaws-ing" or "de-gow-sing") which upon power-up creates from standard 50 or 60 Hz household power a brief, alternating magnetic field which decays in strength to zero over the course of a few seconds. (Typically, the decay is implemented with a specialized resistor in the circuit which increases resistance with its increasing temperature as a result of the current passing through it.) The coil's interaction with the shadow mask, screen band and chassis components is the reason for the characteristic "HUMMMmmmm" noise associated with turning on many CRT-equipped displays. The decaying alternating field generated is strong enough to remove most cases of shadow mask magnetisation.
Spectra of constituent blue, green and red phosphors in a common CRT
It is also possible to purchase or to build an external degaussing coil which can aid in demagnetising older sets or in cases where the built-in coil is ineffective. 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 the user) 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. (To see the shimmering colors well, you may need to display a white or light colored screen.) This process may need to be repeated several times to fully 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 (unless a discolored area of the screen is acceptable). 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 may 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.
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