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Cathode ray tube

Cutaway rendering of a color CRT:
1. Three Electron guns (for red, green, and blue phosphor dots)
2. Electron beams
3. Focusing coils
4. Deflection coils
5. Anode connection
6. Mask for separating beams for red, green, and blue part of displayed image
7. Phosphor layer with red, green, and blue zones
8. Close-up of the phosphor-coated inner side of the screen
Magnified view of a shadow mask color CRT
Magnified view of an aperture grille color CRT

The cathode ray tube (CRT) is a vacuum tube containing an electron gun (a source of electrons) and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets and others.

Color CRTs have three separate electron guns (shadow mask-type) or electron guns that share some electrodes for all three beams (Sony Trinitron, and licensed versions)

The CRT uses an evacuated glass envelope which is large, deep, heavy, and relatively fragile. Display technologies without these disadvantages, such as flat plasma screens, liquid crystal displays, DLP, OLED displays have replaced CRTs in many applications and are becoming increasingly common as costs decline.

An exception to the typical bowl-shaped CRT would be the flat CRTs[1][2] used by Sony in its Watchman series (the FD-210 was introduced in 1982). One of the last flat-CRT models was the FD-120A. The CRT in these units was flat with the electron gun located roughly at right angles below the display surface thus requiring sophisticated electronics to create an undistorted picture free from effects such as keystoning.

Contents

History

The earliest version of the CRT was invented by the German physicist Ferdinand Braun in 1897 and is also known as the 'Braun tube'.[3] It was a cold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen. The first version to use a hot cathode was developed by John B. Johnson (who gave his name to the term Johnson noise) and Harry Weiner Weinhart of Western Electric, and became a commercial product in 1922.

The cathode rays are now known to be a beam of electrons emitted from a heated cathode inside a vacuum tube and accelerated by a potential difference between this cathode and an anode. The screen is covered with a crystalline phosphorescent coating (doped with transition metals or rare earth elements), which emits visible light when excited by high-energy electrons. The beam (or beams, in color CRTs) is deflected either by a magnetic or an electric field to move the bright dot(s) to the required position on the screen. External electromagnets deflect the beams magnetically, while internal plates placed near to and alongside the beam deflect it electrostatically. (Electrostatic deflection is used for single-beam tubes only.)

General description

Application

In television sets and computer monitors the entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. A raster is a rectangular array of closely-spaced parallel lines, scanned one at a time, from left to right (and, ever so slightly, "downhill", because the beam is moving steadily down while drawing the image frame). An image is produced by modulating the intensity of each of the three electron beams, one for each primary color (red, green, and blue) with a received video signal (or another signal derived from it). In all CRT TV receivers except some very early models, the beam is deflected by magnetic deflection, a varying magnetic field generated by coils (the deflection yoke), driven by electronic circuits, around the neck of the tube. Some small-screen commercial TV receivers used electrostatic deflection up to the end of the 1940s, typically employing the 7" type 7JP4 CRT.

Electron gun

Construction

The source of the electron beam is the electron gun, which produces a stream of electrons through thermionic emission, and focuses it into a thin beam. Earlier, black-and-white TV CRTs used magnetic focusing, but electrostatic focus has totally superseded focus coils. 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 heaters within the electron gun. When a CRT is operating, the heaters can often be seen glowing orange through the glass walls of the CRT neck. The need for these heaters to 'warm up' causes a delay between the time that a CRT is first turned on, and the time that a display becomes visible. In older tubes, this could take fifteen seconds or more; modern CRT displays have fast-starting circuits which produce 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. The electrodes are often covered with a black layer, a patented process used by all major CRT manufacturers to improve electron density.

The electron gun accelerates not only electrons but also ions present in the imperfect vacuum (some of which result from outgassing of the internal tube components). The ions, being much heavier than electrons, are deflected much less by the magnetic or electrostatic fields used to position the electron beam. Ions striking the screen damage it; to prevent this the electron gun can be positioned slightly off the axis of the tube so that the ions strike the inside of the CRT neck instead of the screen. Permanent magnets (the ion trap) deflect the lighter electrons so that they strike the screen. Some very old TV sets without an ion trap show browning of the center of the screen, known as ion burn. The aluminum coating used in later CRTs eliminated the need for ion traps; they are no longer used.

When electrons strike the poorly-conductive phosphor layer on the glass CRT, it becomes electrically charged, and tends to repel electrons, reducing brightness (this effect is known as "sticking"). To prevent this the interior side of the phosphor layer can be covered with a layer of aluminum connected to the conductive layer inside the tube, which disposes of this charge. It has the additional advantages of increasing brightness by reflecting, towards the viewer, the light emitted towards the back of the tube. The aluminum layer also protects the phosphors from ion bombardment.

Oscilloscope CRTs

For use of an oscilloscope, the design is somewhat different from that in a TV or monitor. Rather than tracing out a raster, the electron beam is directly steered along an arbitrary path, while its intensity is kept constant, so electrostatic deflection is used. Usually the beam is deflected horizontally (X) by a varying potential difference between a pair of plates (inside the neck, of course, part of the electron gun) to its left and right, and vertically (Y) by plates (also inside, part of the gun) above and below, although magnetic deflection is possible (but never seen in modern oscilloscopes). The instantaneous position of the beam will depend upon the X and Y voltages. The first commercial CRT, the WE224, was an electrostatic deflection tube, with a soft vacuum, made by Western Electric intended to be used in the first oscilloscopes.

Fast events

When displaying one-shot fast events the electron beam must deflect very quickly, with few electrons impinging on the screen, leading to a faint or invisible display. A simple improvement can be attained by fitting a hood on the screen against which the observer presses his face, excluding extraneous light, but oscilloscope CRTs designed for very fast signals give a brighter display by passing the electron beam through a micro-channel plate just before it reaches the screen. Through the phenomenon of secondary emission this plate multiplies the number of electrons reaching the phosphor screen, giving a brighter display, possibly with a slightly larger spot.

Phosphor persistence

The phosphors used in the screens of oscilloscope tubes are different from those used in the screens of other display tubes. Phosphors used for displaying moving pictures should produce an image which fades rapidly to avoid smearing of new information by the remains of the previous picture; i.e., they should have relatively-short persistence. An oscilloscope will often display a trace which repeats unchanged, so longer persistence is not a problem; but it is a definite advantage when viewing a single-shot event, so longer-persistence phosphors are used.

There were moving-film oscilloscope cameras in which film movement provided the time base; they functioned much like strip-chart recorders.

Phosphor colors and designations

An oscilloscope trace can be any color without loss of information, so a phosphor with maximum effective luminosity is usually used. The eye is most sensitive to green: for visual and general-purpose use the P31 phosphor. It gives a visually bright trace, and also photographs well and is reasonably resistant to burning by the electron beam. Earlier, the green P1 phosphor was used for the same purposes, but it burned more easily and had a somewhat shorter persistence. For displays meant to be photographed rather than viewed, the blue trace of the P11 phosphor gives higher photographic brightness; it has a very short persistence. For extremely slow displays, such as radar PPIs, very-long-persistence phosphors such as P7, which produce a blue trace followed by a longer-lasting amber or yellow afterimage, are used. P7 is a dual-layer phosphor, and the long-persistence layer is excited by the light from the blue phosphor rather than from the electron beam. Color TV CRT phosphors are collectively designated P22

Graticules

The phosphor screen of most oscilloscope tubes contains a permanently-marked internal graticule, a simple array of squares that serves as a reference grid. It divides the screen using Cartesian coordinates. This internal graticule allows for the easy measurement of signals with no worries about parallax error. Less expensive oscilloscope tubes may instead have an external graticule of glass or acrylic plastic. Most graticules can be side-illuminated for use in a darkened room. The markings disperse the light, appearing bright, while the smooth surface acts like a light pipe.

Implosion protection

Oscilloscope tubes almost never contain integrated implosion protection. External implosion protection must always be provided, either in the form of an external graticule or, for tubes with an internal graticule, a plain sheet of glass or plastic. The implosion protection shield is often colored to match the light emitted by the phosphor screen; this improves the contrast as seen by the user.

Color CRTs (full-color)

Spectra of constituent blue, green and red phosphors in a common CRT

Color tubes use three different phosphors which emit red, green, and blue light respectively. They are packed together in stripes (as in aperture grille designs) or clusters called "triads" (as in shadow mask CRTs). Color CRTs have three electron guns, one for each primary color, arranged either in a straight line or in a triangular configuration (the guns are usually constructed as a single unit). Each gun's beam reaches the dots of exactly one color.A grille or mask absorbs those electrons that would otherwise hit the wrong phosphor. A shadow mask tube uses a metal plate with tiny holes, placed so that the electron beam only illuminates the correct phosphors on the face of the tube. Another type of color CRT uses an aperture grille to achieve the same result. Since each beam starts at a slightly different location within the tube, and all three beams are perturbed in essentially the same way, a particular deflection charge will cause the beams to hit a slightly different location on the screen (called a 'sub pixel'). Color CRTs with the guns arranged in a triangular configuration are known as delta-gun CRTs, because the triangular formation resembles the triangular shape of the Greek letter Δ (delta). While having deep color reproduction, CRTs can often exaggerate red.

Convergence in color CRTs

The three beams in color CRTs would not, by themselves, strike the screen at the same place at the same time. Uncorrected, the three colors in a typical image would be unacceptably misaligned. Elaborate measures are needed to make the beams converge properly over the entire screen. For one, static convergence brings the beams together, while dynamic convergence (such as from auxiliary coils near the deflection yoke) maintains convergence over the whole screen.

Delta-gun CRTs required the electronically driven convergence coils placed on a "triangle" device on the deflection yoke, powered from the deflection oscillators through a complex set of tunable coils, capacitors and resistors. This type had around 15 points of manual adjustment to align the color and was usually located on a board from the side of the TV case, which could be accessed without dismounting the rear cover of the TV, and easily accessed while viewing a test grid picture on the screen.

Flat-gun modern CRTs require no power for the convergence magnets.

Beam-landing trim magnets

Modern shadow-mask color CRTs have a set of six weak ring magnets around their necks, behind the deflection yoke. These magnets have tabs that permit them to be rotated around the neck to correct beam-landing errors that would affect color purity and cannot be corrected otherwise. (Each of the three beams must excite only the color of phosphor it is intended to; this is color purity.)

In the magnet sets, each ring of one pair has N and S poles diametrically opposite, creating a dipolar field. Rotating these together orients the field, while relative rotation between the pair causes the individual ring fields to aid, to have no effect on, or cancel each other, depending upon relative position.

Similarly, a second pair has four poles, alternating N and S around each ring. This pair provides a quadrupolar field, and its magnitude and orientation are adjusted as with the dipole-field magnets.

Finally, the third pair has six poles, again N and S alternating around the ring, and provides a hexapolar field.

When adjusted, the field inside the neck is a composite of the three types of field, and ensures that each beam lands only on the color of phosphor that it is intended to excite.

Magnets

Magnets should never be put next to a color CRT, as they may cause magnetization of the shadow mask, and in severe cases can permanently distort it mechanically, which will cause incorrect colors to appear in the magnetized 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 (demagnetizing) coil, which upon power-up creates a brief, alternating magnetic field which decays in strength over the course of a few seconds. The coil's interaction with the shadow mask, screen band and chassis components is the reason for the characteristic 'hum' associated with turning on many CRT-equipped displays. This degaussing field is strong enough to remove most cases of shadow mask magnetization.

In extreme cases, very strong magnets such as the neodymium iron boron, or NIB magnets, can actually deform (and likely, permanently bend) the shadow mask. This will create an area of impure color rendition on the screen and if the shadow mask has been bent, such damage usually can't be repaired.

Vector monitors

Graphical displays for early computers used vector monitors (also called X-Y displays), a type of CRT similar to the oscilloscope but typically using magnetic, rather than electrostatic, deflection. Here, the beam traces straight lines between arbitrary points, repeatedly refreshing the display as quickly as possible. Vector monitors were limited in that they could display only a shape's outline and only a limited amount of text. The number of shapes and/or textual characters drawn was severely limited, because the speed of refresh was roughly inversely proportional to how many vectors needed to be drawn. Some vector monitors are capable of displaying multiple colors, using two phosphor layers (so-called "penetration color"). By controlling the strength of the electron beam, either or both phosphor layers could be illuminated, typically producing a choice of green, orange, or red.

Vector monitors were used in early computer aided design systems and in some late-1970s to mid-1980s arcade games such as Asteroids.

CRT resolution

Dot pitch defines the "native resolution" of the display, assuming delta-gun CRTs (although this is not really a native resolution like on flat panel displays, because these dots aren't real subpixels). In these, as the scanned resolution approaches the dot pitch resolution, moiré (a kind of soft-edged banding) appears, due to interference patterns between the mask structure and the grid-like pattern of pixels drawn. The term stripe pitch defines resolution of aperture grille monitors. These monitors do not suffer from vertical moiré, however, because the phosphor stripes have no vertical detail. The aperture grille is something like a picket fence, in that it has vertical slots between metal strips. In smaller CRTs, these strips maintain position by themselves, but larger aperture-grille CRTs require one or two crosswise (horizontal) support strips. However, these strips are nearly invisible on the display. Sony Trinitron CRTs use aperture grilles, and their faceplates are toroidal, although the greatly differing radii of curvature make a Trinitron CRT's faceplate seem cylindrical.

Gamma

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

Other types of CRTs

Storage CRTs

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 were quite large, on the order of 20-inch (51 cm) diagonals. Oscilloscopes also used storage tubes, in particular for observing (and, if needed, photographing) single events or very-slowly-changing signals. Some storage tubes have a special plate behind the phosphor display screen. The imaging electron gun writes its trace onto the plate, and where written, the plate permits electrons from another electron gun to pass through it onto the display screen. The latter gun, called a flood gun, covers the whole area of the plate evenly with electrons; it is not at all a focused beam.

Some storage tubes could display continuous-tone images.

Another type of storage tube, used in oscilloscopes and large-screen direct-view X-Y displays, has a special phosphor screen structure that normally blocks electrons from exciting the phosphor, but when written onto, lets the flood-gun's electrons maintain the written trace.

A typical storage CRT's image starts to deteriorate after tens of seconds to minutes; it is by no means permanent.

Charactrons

Some displays for early computers (those that needed to display more text than was practical using vectors, or that required high speed for photographic output) used Charactron CRTs. These incorporate a perforated metal character mask (stencil), which shapes a wide electron beam to form a character on the screen. The system selects a character on the mask using one set of deflection circuits, but that causes the extruded beam to be aimed off-axis, so a second set of deflection plates has to re-aim the beam so it is headed toward the center of the screen. A third set of plates places the character wherever required. The beam is unblanked (turned on) briefly to draw the character at that position. Graphics could be drawn by selecting the position on the mask corresponding to the code for a space (in practice, they were simply not drawn), which had a small round hole in the center; this effectively disabled the character mask, and the system reverted to regular vector behavior. Charactrons had exceptionally-long necks, because of the need for three deflection systems.

Nimo

Nimo tube BA0000-P31

Nimo was the trademark of a family of very small non standard CRTs manufactured by Industrial Electronics Engineers with 10 electron guns, which shaped the electron beam as digits, with a similar principle as the charactron. They were intended as single digit, simple displays, or as 4 or 6 digits by means of a special magnetic deflection system. Having only 3 electrode types (a filament, an anode and 10 different grids), the driving circuit for this tube was very simple, and as the image was projected on the glass face, it allowed a much wider viewing angle than for example nixie tubes which Nimo tried to replace.[4]

Monoscopes

Monoscopes are not display tubes; they create fixed video test patterns.

Dark-trace tubes

These tubes, instead of a light-emitting phosphor, had a faceplate coated with a scotophor, composed of potassium chloride (KCl), and designated P10. Electron impact made the KCl absorb green light, giving a magenta-colored trace. Trace persistence was quite long, but could be shortened by infrared heating of the faceplate. They were used in World War II large-screen radar displays.

Camera tubes

Camera tubes were the image sensors in TV cameras until the 1980s. One of the earliest was the iconoscope, invented and developed by Vladimir Zworykin at RCA. Another early one was the image dissector, invented by Philo T. Farnsworth, and used even recently for monitoring flames in commercial boilers. Philo Farnsworth created the first all-electronic television system.

A tube called the orthicon was refined into the image orthicon, which was the professional camera tube of choice for several decades. Image orthicons were sophisticated, not easy to make, but their image quality was very good.

A much-simpler, much smaller and much less-costly camera tube called the vidicon was popular for small closed-circuit TV cameras, such as security cameras. It spawned many variations, distinguished principally by the material used for its photosensitive surface.

Scan converters

These were uncommon, and used to convert, for example, from radar PPI display video to a raster scan. They were effectively two CRTs facing a common screen. One was a display-type tube that wrote data onto the screen, which stored it for a while (long persistence), and the other was like a camera tube.

The PPI (Plan Position Indicator) is familiar in some TV weathercasts as a decorative animated graphic element—a rotating radial bar of light on a circular screen.

High-voltage considerations

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 electrons impacting the inside of the CRT face. For this reason, the glass of the funnel leaded and that of the front panel contains up to 18.5 of strontium and barium oxide. Color tubes require significantly higher anode voltages than monochrome tubes (as high as 32,000 volts in large tubes), partly to compensate for the blockage of some electrons by the aperture mask or grille; 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 malfunction, the X-ray emission of modern CRTs is well within approved safety limits.

CRT displays accumulate a 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. Cleaning the display with a dry cloth or special cleaning tissue (using ordinary household cleaners may damage the anti-glare protective layer on the screen) will remove the dust and restore the brightness and clarity of the image.

The high voltage (EHT—extra high tension) 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, although monochrome tubes and specialty CRTs may operate at much lower voltages. Typical oscilloscope tubes operate at a few kilovolts. 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 grounded (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 EHT 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 the case of an accident they proved extremely dangerous. The flyback circuit design addressed this: in the case of a fault, the flyback system delivers relatively little current, improving a person's chance of surviving a direct shock from the high voltage anode.

Other early TVs used RF high-voltage supplies, which used a low-loss air-core transformer (something like a well-behaved, comparatively low-voltage-output Tesla coil), fed by a vacuum-tube oscillator. A tube diode rectified the RF. These supplies put out comparatively little current, and were not especially hazardous.

Getting a shock from a CRT device is not so much a danger from the current, which doesn't persist significantly, but from the severe and uncontrolled muscular contractions which can result in physical rather than electrical injury.

The future of CRT technology

Demise

The demand for CRT screens has been falling rapidly,[5] and producers are responding to this trend. For example, in 2005 Sony announced that they would stop the production of CRT computer displays. It has been common to replace CRT-based televisions and monitors in as little as 5–6 years, although they generally are capable of satisfactory performance for a much longer time.

The end of most high-end CRT production in the mid 2000s (including high-end Sony, and Mitsubishi product lines) means an erosion of the CRT's capability.[6][7] Samsung did not introduce any CRT models for the 2008 model year at the 2008 Consumer Electronics Show and on February 4, 2008 Samsung removed their 30" wide screen CRTs from their North American website and has not replaced them with new models.[8]

In the United Kingdom, DSG (Dixons), the largest retailer of domestic electronic equipment, reported that CRT models made up 80–90% of the volume of televisions sold at Christmas 2004 and 15–20% a year later, and that they were expected to be less than 5% at the end of 2006. Dixons ceased selling CRT televisions in 2007.[9]

Causes

CRTs, despite recent advances, remain relatively heavy and bulky compared to other display technologies, and this became a significant disadvantage as consumers considered the thin and wall-mountable flat panels a selling point. CRT screens have much deeper cabinets compared to flat panels and rear-projection displays for a given screen size, and so it becomes impractical to have CRTs larger than 40 inches (102 cm).

Generally, rear-projection displays and LCDs require less power per display area, though plasma displays consume as much as or more than CRTs.[10]

Resurgence in specialized markets

In the first quarter of 2008, CRTs retook the #2 technology position in North America from plasma, due to the decline and consolidation of plasma display manufacturers. DisplaySearch has reported that although in the 4Q of 2007 LCDs surpassed CRTs in worldwide sales, CRTs then outsold LCDs in the 1Q of 2008.[11][12]

The resurgence of CRTs could be attributed to several factors, including increased demand for low-cost digital TVs from consumers, closeout activities from some brands exiting the space or stronger demand in Canada where CRTs are free from the burden of mandated digital tuner requirements. While most well-known electronics companies such as Sony and Panasonic have ended high-end CRT production and often stopped offering CRTs in certain markets altogether since the mid-2000s, many low-cost manufacturers continue to produced small CRTs often as combo units with a built-in VHS or DVD player. [13]

CRTs can be useful for displaying photos with high pixels per unit area and correct color balance. LCDs, as currently the most common flatscreen technology, have generally inferior color rendition due to the fluorescent lights that can be used as backlights, even though they can be brighter overall. Light-emitting diodes (LEDs) are beginning to serve as backlights, however.

CRTs are still popular in the printing and broadcasting industries as well as in the professional video, photography, and graphics fields due to their greater color fidelity and contrast, better resolution when displaying moving images, and better viewing from off-axis. CRTs are often used in psychological research that requires precise recording of reaction times. CRTs still find adherents also in video gaming because of higher resolution per initial cost and fast response time.[14]

In oscilloscopes

Modern high-performance oscilloscopes no longer use CRTs, although the lowest-cost oscilloscopes still use them. Instead, the signals to be observed are digitized by very fast analog-to-digital converters, and the stream of digital data is processed (and stored) for display and analysis. The displays are now color flat panels, and because there is no need to accommodate a long CRT, the overall shape of the instrument is a rectangular block that is likely not as deep as it is tall.

Oscilloscope CRT phosphors had medium persistence, which meant that at a given point, trace brightness was proportional to the amount of electrons hitting that point. This scale of brightness was sometimes quite useful in analyzing the display, but early flat-panel oscilloscope displays lacked this information. However, some modern oscilloscope displays simulate CRT phosphor decay by processing the signal.

Health concerns

Related terms:
Electronic waste

Electromagnetic

It has been claimed that the electromagnetic fields emitted by CRT monitors constitute a health hazard, and can affect the functioning of living cells.[15] However, studies that examined this possibility showed no signs that CRT radiation had any effect on health.[16] Exposure to these fields diminishes considerably at distances of 85 cm or farther according to the inverse square law, which describes the propagation of all electromagnetic radiation.

As the coils in a CRT monitor are extremely inefficient antennas, there is little electromagnetic field radiated.

Ionizing radiation

CRTs can emit a small amount of X-ray radiation as a result of the electron beam's bombardment of the shadow mask/aperture grille and phosphors. The amount of radiation escaping the front of the monitor is widely considered unharmful. The Food and Drug Administration regulations in 21 C.F.R. 1020.10 are used to strictly limit, for instance, television receivers to 0.5 milliroentgens per hour (mR/h) (0.13 µC/(kg·h) or 36 pA/kg) at a distance of 5 cm from any external surface; since 2007, most CRTs have emissions that fall well below this limit.[17] This is one of the reasons CRT equipment sold in the United States is required to have the month and year of manufacture stamped on the back of the set.

Early color television receivers (many of which are now highly collectible, 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 in 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. These stages are universally housed in a metal enclosure called the "high-voltage cage" made from sheet metal to substantially reduce (and effectively eliminate) exposure. For both X-ray and electrical safety reasons, the set should never be operated with the cover of the high voltage cage opened. Many sets incorporated some type of interlock system to prevent operation with the high voltage cage open.

CRTs may emit low levels of beta radiation which can be detectable by a sensitive Geiger counter. The emissions do not come from accelerated electrons in the tube but from radioactive isotopes. The source of this type of radioactivity is mainly Zirconium or other isotopes sometimes used in glass or mask production.

Toxicity

CRTs may contain toxic phosphors within the glass envelope. The glass tube of modern CRTs may be made from heavily leaded glass, which represent an environmental hazard.[18] The front panel contains up to 18.5% of a mixture of strontium and barium oxide. Indirectly heated vacuum tubes (including CRTs) use barium compounds and other reactive materials in the construction of the cathode and getter assemblies; normally this material will be converted into oxides upon exposure to the air.

In some jurisdictions, discarded CRTs are regarded as toxic waste. In October 2001, the United States Environmental Protection Agency created rules stating that CRTs must be brought to special recycling facilities. In November 2002, the EPA began fining companies that disposed of CRTs through landfills or incineration. Regulatory agencies, local and statewide, monitor the disposal of CRTs and other computer equipment.

In Europe, disposal of CRT televisions and monitors is covered by the WEEE Directive.

Flicker

At low refresh rates (below 60 Hz), the periodic scanning nature of the display in most CRTs (particularly raster-oriented displays) may produce an irritating flicker that some people perceive more easily than others, especially when viewed with peripheral vision. High brightness and high contrast in the image accentuate the perception of flicker. A high refresh rate (above 72 Hz) helps to negate these effects, and computer displays and televisions with CRTs driven by digital electronics often use refresh rates of 100 Hz or more to largely eliminate any perception of flicker. Screens that reduce off-axis visibility of the screen can reduce the visibility of the CRT, and any flicker it produces, in peripheral vision (which is more sensitive to flicker than central vision) for persons not looking directly at the screen.

CRTs that scan the screen in a raster pattern require special horizontal and vertical synchronization and scanning circuitry. When this circuitry is incorrectly adjusted, a CRT display may produce a bright flashing and/or moving pattern on the screen at a rate that depends on the degree of improper adjustment. A small percentage of epilepsy sufferers who also suffer from photosensitive epilepsy may experience seizures when exposed to this rapid flickering, which tends to fall within the range of frequencies to which photosensitive epileptics are most vulnerable.

High-frequency noise

In some devices using CRTs that scan in raster patterns, components of the scanning circuitry (transformers, deflection coils, etc.) may mechanically vibrate slightly at frequencies that match the scanning frequency. When these frequencies are in the range of human hearing, they can be heard by some CRT users.

CRTs used for television traditionally have operated at horizontal scanning frequencies of 15,750 Hz (for most NTSC systems) or 15,625 Hz (for most PAL systems)). These frequencies are at the upper range of human hearing and are inaudible to many people, but a significant minority of the population can hear the sounds, and will perceive a high-pitched whistling noise near an operating television CRT. A television CRT operating without a synchronization signal will have a free-running scan rate slightly below these frequencies, making the resulting noise easier to perceive, and making them seem much louder to people able to hear them (because the slight reduction in frequency greatly increases their audibility). People with good hearing may find the noise irritating.

Security

Under some circumstances, the signal radiated from the electron guns, scanning circuitry, and associated wiring of a CRT can be captured and used to remotely reconstruct what is shown on the CRT. When computer displays are used to display text or other characters, special fonts and display techniques that pseudo-randomize the signal can be used to reduce the possibility of the image being successfully reconstructed remotely, although complete prevention requires that the electromagnetic radiation from the CRT be completely blocked. Classified methods are used by government and other organizations to prevent this type of eavesdropping; in the United States, the term TEMPEST is used to refer to the study of these emanations and techniques for reducing or eliminating them.

High voltage

CRTs operate at very high voltages, which can persist long after the device containing the CRT has been switched off and/or 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. Modern circuits contain bleeder resistors, to ensure that the high-voltage supply is discharged to safe levels within a couple of minutes at most. These discharge devices can fail even on a modern unit and leave these high voltage charges present. The final anode connector on the bulb of the tube carries this high voltage.

Implosion

A high vacuum exists within all CRT monitors. If the outer glass envelope is damaged, a dangerous implosion may occur. Due to the power of the implosion, glass pieces may bounce and explode outwards. The fragments can travel at dangerous and potentially fatal velocities. While modern CRTs used in televisions and computer displays have epoxy-bonded face-plates or other measures to prevent shattering of the envelope, CRTs removed from equipment must be handled carefully to avoid personal injury.

Early TV receivers had safety glass in front of their CRTs for protection. Modern CRTs have exposed faceplates; they have tension bands around the widest part of the glass envelope, at the edge of the faceplate, to keep the faceplate's glass under considerable compression, greatly enhancing resistance to impact. The tension in the band is on the order of a ton or more.

See also

  • Charactron
  • Comparison of display technology
  • Crookes tube
  • CRT projector
  • DVBST
  • Flat panel display
  • Image dissector
  • Monitor filter
  • Monoscope
  • Overscan in Television
  • Penetron
  • Raster scan
  • Surface-conduction electron-emitter display
  • Williams tube
  • Display examples
  • TEMPEST
  • photosensitive epilepsy

References

Selected patents

External links