cathode-ray tube

(click to enlarge)
In a colour-television tube, three electron guns (one each for red, green, and blue) fire electrons … (credit: Encyclopædia Britannica, Inc.)

Vacuum tube that produces images when its phosphorescent surface is struck by electron beams. CRTs can be monochrome (using one electron gun) or colour (typically using three electron guns to produce red, green, and blue images that, when combined, render a multicolour image). They come in a variety of display modes, including CGA (Color Graphics Adapter), VGA (Video Graphics Array), XGA (Extended Graphics Array), and the high-definition SVGA (Super Video Graphics Array).

For more information on cathode-ray tube, visit Britannica.com.

Background

A cathode-ray tube, often called a CRT, is an electronic display device in which a beam of electrons can be focused on a phosphorescent viewing screen and rapidly varied in position and intensity to produce an image. Probably the best-known application of a cathode-ray tube is as the picture tube in a television. Other applications include use in oscilloscopes, radar screens, computer monitors, and flight simulators.

The cathode-ray tube was developed in 1897 by Ferdinand Braun of Strasbourg in what was then the French-German region of Alsace-Lorraine. It was first used as an oscilloscope to view and measure electrical signals. In 1908, A.A. Campbell-Swinton of England proposed using a CRT to send and receive images electronically. It wasn't until the 1920s, however, that the first practical television system was developed. The concept for a color cathode-ray tube was proposed in 1938 and successfully developed in 1949.

Although General Electric introduced their first television set for home use in 1928, commercial television broadcasting remained an experimental technology with only limited range and audience. It took until the late-1940s before television net-works had established themselves sufficiently to start a boom in consumer sales. Black-and-white television sets gave way to the first color sets in the 1960s. In the following decades cathode-ray tubes for televisions got both larger and smaller as manufacturers sought to satisfy consumer wants. Recent developments have included tubes with flatter faces, sharper comers, and higher resolution for better viewing.

A CRT consists of three basic parts: the electron gun assembly, the phosphor viewing surface, and the glass envelope. The electron gun assembly consists of a heated metal cathode surrounded by a metal anode. The cathode is given a negative electrical voltage and the anode a positive voltage. Electrons from the cathode flow through a small hole in the anode to produce a beam of electrons. The electron gun also contains electrical coils or plates which accelerate, focus, and deflect the electron beam to strike the phosphor viewing surface in a rapid side-to-side scanning motion starting at the top of the surface and working down. The phosphor viewing surface is a thin layer of material which emits visible light when struck by the electron beam. The chemical composition of the phosphor can be altered to produce the colors white, blue, yellow, green, or red. The glass envelope consists of a relatively flat face plate, a funnel section, and a neck section. The phosphor viewing surface is deposited on the inside of the glass face plate, and the electron gun assembly is sealed into the glass neck at the opposite end. The purpose of the funnel is to space the electron gun at the proper distance from the face plate and to hold the glass envelope together so that a vacuum can be achieved inside the finished tube.

The CRT used in a color television or color computer monitor has a few additional parts. Instead of one electron gun there are three—one for the red color signal, one for blue, and one for green. There are also three different phosphor materials used on the viewing surface—again, one for each color. These phosphors are deposited in the form of very small dots in a repeated pattern across the screen—red, blue, green, red, blue, green, and so on. The key to a color CRT is a piece of perforated metal, known as the shadow mask, which is placed between the electron guns and the viewing screen. The perforations in the shadow mask are aligned so that the red gun can fire electrons at only the phosphor dots which produce the red color, the blue gun at the blue dots, and the green gun at the green dots. By controlling the intensity of the beam for each color as it scans across the screen, different colors can be produced on different areas of the screen, thus producing a color image. To give an idea of how small the perforations and dots have to be, a 25-inch (63 cm) color television picture tube may have a shadow mask with 500,000 perforations and 1.5 million individual phosphor dots.

Design

The electron gun must be designed for each new application. New screen sizes, new overall glass envelope dimensions, and new image resolution requirements all require a new gun design. Brighter images may require higher power accelerating coils. Finer image resolution may require improved beam focusing coils or plates. While the basic design remains the same, the details are constantly refined.

Likewise the basic design of the phosphor viewing surface is fairly well defined, but the details may change. New image resolution requirements may require a new method of depositing the phosphor dots on the face plate, which in turn may require new material processing techniques. The search for truer colors may result in new material formulations. The amount of time the phosphors emit light, or glow, after being struck by the electron beam is also important and is controlled by the chemical composition of the phosphor. This property is called persistence. In a color television, the electron beam scans the screen 25 times per second. If the persistence is longer than one twenty-fifth of a second (0.04 second), the image would show two scans at the same time and would appear blurred. If the persistence is shorter than this time, the image from the first scan would have disappeared before the second scan came along, and the image would appear to flicker.

Even the glass envelope requires extensive design. Strength, radiation absorption characteristics, temperature tolerance, impact resistance, dielectric properties, and optical clarity are a few of the design criteria used when designing the glass components. Computers may be used to perform finite element analysis to evaluate the stresses in complex envelope shapes. This technique divides the part into a finite number of smaller, more easily definable pieces, or elements, and then performs the calculations for each element to spot unacceptably high stress concentrations. Using the computer, dimensions for contours and wall thickness can easily be adjusted until a satisfactory design is achieved.

Raw Materials

Cathode-ray tubes use an interesting and varied assemblage of raw materials. In many cases, it is the raw materials, not the design or manufacturing process, that determine the performance characteristics of the finished product.

The electron gun is made from a variety of metal pieces. The cathode, or electron emitter, is made from a cesium alloy. Cesium is used as a cathode in many electronic vacuum tube devices because it readily gives off electrons when heated or struck by light. In a CRT, the cathode is heated with a high resistance electrical wire. The accelerating, focusing, and deflection coils may be made from small diameter copper wire. A glass tube protrudes from the rear of the electron gun assembly and is used to evacuate the air from the finished CRT.

The phosphor viewing surface is formed from a continuous layer of a single material in monochromatic CRTs, or is composed of individual dots of three different materials in color CRTs. Zinc sulfide is a common phosphor material. The color is determined by adding a very small amount of material called an activator. Zinc sulfide with 0.01% silver activator emits a blue light. When a 0.001% copper activator is used, it produces a green light. A 50/50 mixture of zinc sulfide and cadmium sulfide with a 0.005% silver activator produces a yellow light. Red light can be produced by adding silver or copper to zinc sulfide mixed with a high percentage of cadmium sulfide. The phosphors are usually ground into a fine powder before they are applied to the inside of the face plate.

The glass envelope uses slightly different raw materials for each of its three component parts. The basic raw material for all of the glass components is silica. Alumina may be added to adjust the flow properties of the molten glass when forming it. Various oxides are used to lower the melting temperature. Barium oxide, strontium oxide, and lead oxide are used to provide radiation protection in the neck and funnel. The face plate, on the other hand, must have a minimum of lead oxide to prevent a discoloration phenomenon known as electron or x-ray browning. Neodymium oxide may be used on the face plate to enhance the contrast of the viewed picture.

In color CRTs, the shadow mask is usually made from a thin sheet of a nickel alloy.

The Manufacturing
Process

The glass envelope or its components are usually formed at a glass manufacturing facility and shipped to the cathode-ray tube manufacturer who forms the phosphor viewing screen, fabricates and assembles the electron gun, and assembles the finished CRT.

Forming the glass envelope

• The glass ingredients are weighed and mixed prior to melting. The glass is melted in gas-fired furnaces about 500-3,000 square feet (46-279 sq m) in size. If this is a continuous process, new ingredients are added to maintain a constant level as the molten glass flows out of the furnace to the forming areas. Before forming, the molten glass must be cooled somewhat and made uniform in temperature throughout.
• The face plate is normally pressed into the desired shape by dropping a gob of molten glass into a mold and pressing on the gob with a plunger. The funnel can be formed either by pressing or by centrifugal casting. In the casting method a gob of molten glass drops into a mold, which then spins rapidly to spread the glass uniformly over the inside surface of the mold. A grooving disk near the top of the mold cuts the soft glass at the desired height so that the excess glass can be removed easily. The neck is made from glass tubing, and one end is flared to facilitate insertion of the electron gun.
• In a monochromatic CRT the three glass components are joined together before they are shipped to the CRT manufacturer. In a color CRT only the neck and funnel are joined, and the face plate is shipped separately for further processing. The glass components are usually joined by heating the mating surfaces to a high temperature with gas jets or electric heaters.

Applying the phosphors

• In monochromatic CRTs the phosphor viewing surface is coated on the inside of the glass face plate. This is done by preparing a liquid suspension of the phosphor and pouring a measured amount into the neck of the glass envelope along with a gelling agent. After about 20 minutes, the coating has set and the excess liquid is poured off. The process for color CRTs is more complicated. First the shadow mask is made by applying a light-sensitive coating to the thin mask material, exposing it to light through a perforated template, and then etching away the exposed coating with an acid to form the millions of holes. The mask is then pressed into a slightly curved shape and attached just behind the face plate. The face plate is placed in a centrifuge and the inside surface is coated with the green phosphor material. The centrifuge spins the face plate to ensure an even coating of phosphor. A strong ultraviolet light is shown through the mask to harden the green phosphor material into hundreds of thousands of dots. The remaining material is then washed off. This process is repeated to form the red and blue phosphor dots, with the ultraviolet light being shifted a small amount each time. When this process is finished, the glass face plate is joined to the funnel. On color tubes, the phosphor dots are sensitive to high temperatures, so instead of using high-temperature gas jets, a mixture of chemical solvent and powdered glass, called a frit, is applied to the joint. This acts like a glass "solder," and the joint can be sealed at a much lower temperature.

Assembling the electron gun

• The metal components of the electron gun are precision formed. If coils are used they are wound from fine copper wire. Some electron guns use metal plates instead of coils, and these plates are stamped and formed. The components are assembled either by hand or with automated machines in a clean environment. The glass tube is sealed into the base, and the base is welded into the gun assembly.

Final assembly and packing

• The inside of the glass envelope neck is lubricated with graphite, and the electron gun is inserted and aligned. The neck is then sealed around the gun. A vacuum pump is attached to the glass tube extending from the rear of the gun, and the inside of the CRT is evacuated of air. When the proper vacuum has been achieved, the glass tube is heated and quickly pinched closed to form a seal.
• The finished CRT is tested for performance and carefully packed to prevent damage. Because the CRT is under a high vacuum, any fracture in the glass envelope could result in an inward explosion known as an implosion.

Quality Control

Although the operating principle of a cathode-ray tube is simple, the manufacturing process requires strict controls and precise alignments. The phosphor materials must be extremely pure to achieve the desired colors. Even a tiny variance in the amount of activator used can result in a significant change in color. Likewise, when you consider that a color television CRT requires the placement of over a million tiny dots side by side on the viewing surface, even a small error in alignment could be disastrous.

Byproducts and Recycling

The principal byproduct of CRT manufacturing is scrap glass. Much of this glass is recycled. Recycled glass with a high content of lead oxide is used to provide radiation protection in CRT funnels and has completely replaced previous sources of lead oxide for this application.

The Future

The worldwide market for cathode-ray tubes was estimated at nearly 400 million units in 1994 and is expected to grow at a 6% annual rate through 2000. The color television market is expected to grow at a 5% annual rate, while the color computer monitor market is expected to grow at a 20% rate. In the television market, the demand for larger television picture tubes with higher image resolution is expected to continue.

One important trend is the development of high definition television (HDTV), which has scanning rates more than twice that of conventional systems. This will require new electron gun designs as well as new glass materials and technologies to handle the doubled radiation rate.

Where To Learn More

Books

Braithwaite, Nicholas and Graham Weaver, eds. Electronic Materials. Butterworths, 1990.

Connelly, J.H. and D.J. Lopata. Engineered Materials Handbook, Volume 4. ASM International, 1991.

Haider, Z. Television Glass Bulb Design and Manufacturing Developments, Glass Production and Technology International. Sterling Publications, Ltd., 1992.

Periodicals

Fleischmann, Mark. "The Big Picture." Popular Science, November 1994, pp. 82-85, 92-95.

Meeks, T. "Inside the CRT: Monitor Technology Explained." PC Novice, July 1993, pp. 40-43.

[Article by: Laurel M. Sheppard/; Chris Cavette]

A device that can produce an image on a screen with electrical impulses.

• A television screen is a sophisticated CRT, as is the screen on which computer output is displayed.

Device whereby electrons are sprayed onto a viewing screen, under the direction of magnetic fields, to form patterns. Examples of CRTs include television screens and computer terminals.

The abbreviation for cathoderay tube.

Capillary refill time.

This article is about the display technology. For other uses, see CRT (disambiguation).

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.

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

Contents 1 History 2 Overview 3 Oscilloscope CRTs 3.1 Phosphor persistence 3.2 Microchannel plate 3.3 Graticules 4 Color CRTs 4.1 Convergence in color CRTs 4.2 Degaussing 5 Vector monitors 6 CRT resolution 7 Gamma 8 Other types of CRTs 8.1 Cat's eye 8.2 Charactrons 8.3 Nimo 9 The future of CRT technology 9.1 Demise 9.2 Causes 9.3 Resurgence in specialized markets 10 Health concerns 10.1 Electromagnetic 10.2 Ionizing radiation 10.3 Toxicity 10.4 Flicker 10.5 High-frequency noise 11 Security concerns 12 See also 13 References 14 Selected patents 15 External links

History

A common CRT used in computer monitors and television sets

The earliest version of the CRT was invented by the German physicist Ferdinand Braun in 1897 and is also known as the Braun tube.^[1] It was a cold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen.

In 1907, Russian scientist Boris Rosing used a CRT in the receiving end of an experimental video signal to form a picture. He managed to display simple geometric shapes onto the screen, which marked the first time that CRT technology was used for what is now known as television.^[2]

The first cathode ray tube 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.

Overview

A cathode ray tube is a vacuum tube which consists of one or more electron guns, possibly internal electrostatic deflection plates, and a phosphor target.^[2] 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. An image is produced by controlling the intensity of each of the three electron beams, one for each primary color (red, green, and blue) with a video signal as a reference.^[3] In all modern CRT monitors and televisions, the beams are bent by magnetic deflection, a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube, although electrostatic deflection is commonly used in oscilloscopes, a type of diagnostic instrument.^[3]

Electron gun

Oscilloscope CRTs

In oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection commonly used with television and other large CRTs. The beam is deflected horizontally by applying an electric field between a pair of plates to its left and right, and vertically by applying an electric field to plates above and below.^[4][5][6]

Phosphor persistence

Various phosphors are available depending upon the needs of the measurement or display application. The brightness, color, and persistence of the illumination depends upon the type of phosphor used on the CRT screen. Phosphors are available with persistences ranging from less than one microsecond to several seconds.^[7] For visual observation of brief transient events, a long persistence phosphor may be desirable. For events which are fast and repetitive, or high frequency, a short-persistence phosphor is generally preferable.^[8]

Microchannel plate

When displaying fast one-shot events the electron beam must deflect very quickly, with few electrons impinging on the screen; leading to a faint or invisible display. Oscilloscope CRTs designed for very fast signals can 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 significant improvement in writing rate (brightness), and improved sensitivity and spot size as well.^[9][10]

Graticules

Most oscilloscopes have a graticule as part of the visual display, to facilitate measurements. The graticule may be permanently marked inside the face of the CRT, or it may be a transparent external plate. External graticules are typically made of glass or acrylic plastic. An internal graticule provides an advantage in that it eliminates parallax error. Unlike an external graticule, an internal graticule can not be changed to accommodate different types of measurements.^[11] Oscilloscopes commonly provide a means for the graticule to be side-illuminated, which improves its visibility when used in a darkened room or when shaded by a camera hood.^[12]

Color CRTs

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).^[13] 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). A grille or mask absorbs the electrons that would otherwise hit the wrong phosphor.^[14] 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.^[13] Another type of color CRT uses an aperture grille to achieve the same result.^[14]

A common misconception is that the three electron beams are different 'colours'. They are not; the only difference between the beams is the signals that they carry. If the 'red' beam were to fall onto the 'green' phosphor, then green light would be produced.

Convergence in color CRTs

The three beams in color CRTs would not strike the screen at the same point without convergence calibration. Instead, the set would need to be manually adjusted to converge the three color beams together to maintain color accuracy.^[15]

Degaussing

Most CRT television sets and 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. This degaussing field is strong enough to remove most cases of shadow mask magnetization.^[16]

Vector monitors

Main article: Vector monitor

Vector monitors were used in early computer aided design systems and in some late-1970s to mid-1980s arcade games such as Asteroids.^[17] They draw graphics point-to-point, rather than scanning a raster.

CRT resolution

Dot pitch defines the maximum resolution of the display, assuming delta-gun CRTs. In these, as the scanned resolution approaches the dot pitch resolution, moiré appears, as the detail being displayed is finer than what the shadow mask can render.^[18] These monitors do not suffer from vertical moiré, however, because the phosphor stripes have no vertical detail. In smaller CRTs, these strips maintain position by themselves, but larger aperture-grille CRTs require one or two crosswise (horizontal) support strips.^[19]

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).^[20]

Other types of CRTs

Cat's eye

In better quality tube radio sets a tuning guide consisting of a phosphor tube was used to aid the tuning adjustment. This was also know as a "Magic Eye" or "Tuning Eye". Tuning would be adjusted until the width of a radial shadow was minimized. This was used instead of a more expensive electromechanical meter, which later came to be used on higher-end tuners when transistor sets lacked the high voltage required to drive the device.^[21]

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.^[22][23]

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.^[24]

The future of CRT technology

Demise

The demand for CRT screens has been falling rapidly,^[25] 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.^[26][27] 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.^[28]. This demise in the developed world, however, has been adapted more slowly in the developing world. According to iSupply, production in units of CRTs was not surpassed by LCDs production until 4Q 2007, owing largely to CRT production at factories in China.

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.^[29]

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.^[30]

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.^[31][32]

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. ^[33]

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.^[34]

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.^[35]

Health concerns

Related terms: Electronic waste

Electromagnetic

See also: Magnetic dipole and VLF

It has been claimed that the electromagnetic fields emitted by CRT monitors constitute a health hazard, and can affect the functioning of living cells.^[36] However, studies that examined this possibility showed no signs that CRT radiation had any effect on health.^[37] Exposure to these fields diminishes considerably at distances of 85 cm or farther according to the inverse square law.^{[citation needed]}

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.^[38]

Toxicity

Older color CRTs may contain toxic phosphors used for production of yellows. The rear glass tube of modern CRTs may be made from leaded glass, which represent an environmental hazard if disposed of improperly.^[39]. By the time personal computers were produced, glass in the front panel (the viewable portion of the CRT) used barium rather than lead, though the rear of the CRT was still produced from leaded glass. Monochrome CRTs typically do not contain enough leaded glass to fail EPA tests.

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.^[40]

In Europe, disposal of CRT televisions and monitors is covered by the WEEE Directive.^[41]

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. 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.^[42]

High-frequency noise

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)).^[43] 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 ringing noise near an operating television CRT.^[44]

Security concerns

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, using a process called Van Eck phreaking.^[45]

See also

Electronics portal

• 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

1. ^ "Cathode Ray Tube". Medical Discoveries. Advameg, Inc.. 2007. http://www.discoveriesinmedicine.com/Bar-Cod/Cathode-Ray-Tube-CRT.html. Retrieved 2008-04-27. 
2. ^ ^{a b} "History of the Cathode Ray Tube". About.com. http://inventors.about.com/od/cstartinventions/a/CathodeRayTube.htm. Retrieved 2009-10-04. 
3. ^ ^{a b} "'How Computer Monitors Work'". http://computer.howstuffworks.com/monitor7.htm. Retrieved 2009-10-04. 
4. ^ "Oscilloscope CRT Clock". http://web.jfet.org/vclk/. Retrieved 2009-10-04. 
5. ^ Bristol, Lloyd R.. "US Patent 4667135 - Z-axis orthogonality compensation system for an oscilloscope". http://www.patentstorm.us/patents/4667135/fulltext.html. Retrieved 2009-10-04. 
6. ^ "The Cathode-Ray Tube". Pearson Education. http://wps.aw.com/wps/media/objects/877/898586/topics/topic07.pdf. Retrieved 2009-10-05. 
7. ^ Doebelin, Ernest (2003). Measurement Systems. McGraw Hill Professional. pp. 972. ISBN 007292201X, 9780072922011. http://books.google.com/books?id=q1mOopLHnTEC&pg=PA972#v=onepage&q=&f=false. 
8. ^ Shionoya, Shigeo (1999). Phosphor handbook. CRC Press. pp. 499. ISBN 0849375606, 9780849375606. http://books.google.com/books?id=lWlcJEDukRIC&pg=PA499#v=onepage&q=&f=false. 
9. ^ Williams, Jim (1991). Analog circuit design: art, science, and personalities. Newnes. pp. 115-116. ISBN 0750696400, 9780750696401. http://books.google.com/books?id=CFoEAP2lwLEC&pg=PA115#v=onepage&q=&f=false. 
10. ^ Yen, William M.; Shigeo Shionoya, Hajime Yamamoto (2006). Practical Applications of Phosphors. CRC Press. p. 211. ISBN 1420043692, 9781420043693. http://books.google.com/books?id=cKMSf0iRVSgC&pg=PA211. 
11. ^ Bakshi, U.A.; A.P.Godse (2008). Electronic Devices And Circuits. Technical Publications. p. 38. ISBN 8184313322, 9788184313321. http://books.google.com/books?id=uJaQML8-MggC&pg=PA38#v=onepage&q=&f=false. 
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Selected patents

• U.S. Patent 1,691,324: Zworykin Television System

External links

Wikimedia Commons has media related to: Cathode ray tube

• The Cathode Ray Tube site
• PCTechGuide: Cathode Ray Tubes
• CRTs at the Virtual Valve Museum
• Samuel M. Goldwasser TV and Monitor CRT (Picture Tube) Information

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