Cathode ray tube

CRT color enhanced.png|right|thumb|300px|Cutaway rendering of a color CRT

Electron guns
Electron beams
Focusing coils
Deflection coils
Anode connection
Mask for separating beams for red, green, and blue part of displayed image
Phosphor layer with red, green, and blue zones
Close-up of the phosphor-coated inner side of the screen

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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, radar displays 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, LCD TVs, DLP, OLED displays, and other technologies. As a result of CRT 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 at its rear end. The released electrons form a beam within the tube due to the voltage difference applied across the two electrodes, and the direction of this beam is then altered either by a magnetic or electric field to trace over the inside surface of the phosphorescent screen (anode), covered by phosphorescent material (often transition metals or rare earths). Light is emitted by that material at the instant that electrons hit it.

In television sets and modern computer monitors, the entire front area of the tube is scanned systematically in a fixed pattern called a raster, and a picture is created by modulating the intensity of the electron beam with the received video signal (or another signal derived from it). 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 wire coils driven by electronic circuits. This usage of electromagnets to change the electron beam's original direction is known as "magnetic deflection".

In the 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. In time-domain mode, the usual mode, the horizontal deflection is proportional to time (measured out by a "sweep oscillator" in the oscilloscope), and the vertical deflection is proportional to the measured signal. In X-Y mode, which is less-used, both the horizontal and vertical deflections are proportional to measured signals. The tube for oscilloscope use is longer and narrower, reducing the deflection angle (presumably for greater accuracy/linearity?), and deflection is done by applying an electrical field via deflection plates built into the tube's neck. The use of an electrical field is known as "electrostatic deflection", and it 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 maximum frequency in the signal that can be accurately represented.

The electron beam source is the electron gun, producing the stream of electrons by thermionic emission and then focusing it into a thin beam. The gun is located in the narrow, cylindrical neck at the extreme rear of a CRT and has electrical connecting pins, usually arranged in a circular configuration, extending from its end. These pins provide external connections to the cathode, to various grid elements in the gun used to focus and modulate the beam, and, in electrostatic deflection CRTs, to the deflection plates. Since the CRT is a hot-cathode device, these pins also provide connections to one or more filament-type heaters within the electron gun. When a CRT is operating, usually the gun heaters can be seen glowing orange through the glass walls of the CRT neck. It is the need for these heaters to achieve their effect that causes a delay between the time that a CRT is first turned on and the time that a display becomes visible; the CRT literally needs time to "warm up". In older tubes, this could take fifteen seconds or more; modern CRT displays have fast-starting circuits that display an image within about two seconds, using either briefly increased heater current or elevated cathode voltage. Once the CRT has warmed up, the heaters stay on continuously to keep the cathode warm. The electrodes are often covered with a thermally black layer, a patented process used by all major CRT-manufacturers to improve electron density.

The electron gun is often mounted slightly off-axis, as it accelerates 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 deflected less by the magnetic field from the deflection coils, and in older constructions with in-axis guns the ions 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, from this bombardment. The combination of an off-axis mounting of the electron gun and permanent magnets bending the electron beam back in the desired direction forms an ion trap; the ions are not deflected enough so they strike the neck of the tube instead of the screen and harmlessly dissipate. This system was later replaced by 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.

Computer displays

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 (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.

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.

The glass envelope

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 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 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).

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, colour 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. Despite these advantages, 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 - 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. 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 colours 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 colour tube.

Health danger

Electromagnetics: Some believe the electromagnetic fields emitted by CRT monitors constitute a health danger to the functioning of living cells. Exposure to these fields diminishes by the inverse square law which describes the propagation of all electromagnetic radiation: double the distance, quarter the power. Likewise, the EM energy is also less intense for the display's user than for a person located behind it because the deflection yoke is behind the display's screen and therefore closer to the rear. Scientific studies have produced conflicting results about the dangers of electromagnetic waves.

Ionizing radiation: 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. ^*

Early color television receivers (many of which are now highly collectable, see CT-100) were especially vulnerable due to primitive high voltage regulation systems. X-ray production is generally negligible in black-and-white sets (due to low acceleration voltage and beam current) and virtually every color display since the late 1960s when systems were added to shut down the horizontal deflection system (and therefore high voltage supply) should regulation of the acceleration voltage fail.

All television receivers and CRT displays equipped with a vacuum tube based high voltage rectifier or high voltage regulator tube also generate X-rays in these stages, though these stages were universally housed in a metal enclosure called the "high voltage cage" to substantially reduce (and effectively eliminate) exposure. As examples, a 1B3 and a 6KB6 vacuum tube would be installed inside this metal enclosure. For both X-ray and electrical safety reasons, the set should never be operated with the cover of the high voltage cage opened. (Photo of HV cage to follow.)

Toxins: 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. 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. Indirectly-heated vacuum tubes (including CRTs) use Barium compounds and other reactive materials in the cionstuction of the cathode and getter assemblies, normaly this material will be converted into oxides upon exposure to the air, but care should be taken to avoid contact with the inside of all broken tubes. In some juristictions, all discarded CRTs are regarded as toxic waste.

Flicker: The constant refreshing of a CRT can cause headaches in migraine sufferers and seizures in epileptics, if they are photosensitive. Screen filters are available to reduce these effects. A high refresh rate (above 75 Hz) also helps to negate these effects.

High voltage: 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. Residual charges of hundreds of volts can also remain in large capacitors in the power supply circuits of the device containing the CRT; these charges may persist for weeks. (Modern circuits contain bleeder resistors to ensure the high-voltage supply is discharged to safe levels within a couple of minutes at most.)

Those working inside CRT-containing equipment should know how and be able to safely discharge these hazards. In particular, the large rubber connector which looks like a suction cup is responsible for supplying accelerating voltage to the bell of the CRT. Under the suction cup is the ultor which couples the accelerating voltage to the inside of the tube. Inside the glass bell is a coating of metallic paint, while the outside of the bell is coated with a conductive graphite coating called Aquadag; between the ultor's connection to the flyback transformer and the Aquadag, there is therefore a capacitance capable of maintaining the full accelerating voltage for weeks. While this accelerating voltage is high (typically from 7kV to 50kV depending on screen size, monochrome or color, direct view or projection), both the capacitance and flyback current are small (on the order of picofarads and nanoamperes respectively), so shocks from the accelerating voltage are typically embarrassing and painful but usually harmless. On the other hand, the voltages and available currents used in the deflection and power supply circuits can result in instantaneous death.

Implosion: All CRTs and other vacuum tubes operate under negative pressure so that air and gas molecules will not interfere with electron streams. CRTs have large viewing areas and proportionally larger bells required to accommodate the deflection of the electron beams to the rear of the screen. As a result, these highly evacuated glass bulbs have a large surface area, with each and every square inch exposed to atmospheric pressure.

As an example, consider a 17 inch (16 inch viewable!) CRT at a mean sea-level atmospheric pressure of 14.7 pounds per square inch. Measuring the visible portion of the CRT and rounding up to the nearest inch (accounting for invisible portions of the face), a Viewsonic model E771 monitor has a screen of 13x10 inches, or 130 square inches. At 14.7 PSI exterior pressure and a near-perfect internal vacuum, the face of this monitor is supporting over 1,900 pounds of air mass on its face alone. The entire CRT is conservatively supporting three times that - or nearly 6,000 pounds, the weight of three typical automobiles - across its entire surface. The larger the CRT, the more surface area, the more total exterior air pressure load.

Therefore, CRTs (outside of finished end-user products) present a hazard to those without proper training and appropriate precautions. While a great deal of research has gone into implosion protective designs for CRTs, all CRTs present an implosion risk. Even CRTs in finished products present a hazard if handled uncautiously. Early television receivers even included a "safety glass" to protect viewers from flying glass due to spontaneous structural failures of the CRT; with modern (early 1960s onward) banded and bonded-face CRTs, the safety glass has become redundant. Safety goggles, leather gloves, and heavy sweaters are considered indispensable safety equipment amongst experienced technicians and preservationists of early television equipment.

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.

Unmounted CRTs should always be carried with its face, the heaviest part, down. Use both hands, and grasp the tube under the face, wrapping your hands around to the sides where the metal mounting frame is attached. Never carry a CRT by the neck! For added safety, carrying the tube in a closed, thick box or with a thick cloth wrapped around it (but not in such a way as to impair your grip on the tube) is a good idea; this will reduce the amount of flying debris should the tube break. Large tubes (over 19 inches) should be carried by two people. In general, you should treat the tube like a hand grenade, thinking that if you handle it carefully and keep your grip on it, there is no serious danger, but that it could cause a disaster if you drop it.

When handling or disposing of a CRT, you must take steps to avoid creating an implosion hazard for yourself 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. Once the vacuum is filled, the tube is destroyed, but it cannot implode. 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.

External links

The Cathode Ray Tube site

Display technology | Television technology | Vacuum tubes

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