Cathode-ray tube: Difference between revisions

Cutaway rendering of a color CRT:
1. Three electron emitters (for red, green, and blue phosphor dots)
2. Electron beams
3. Focusing coils
4. Deflection coils
5. Anode (collector)
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

The cathode ray tube (CRT) is a vacuum tube containing one or more electron guns, and a phosphorescent screen used to view images. It has a means to accelerate and deflect the electron beam(s) onto the screen to create the images. The images may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets or others. CRTs have also been used as memory devices, in which case the visible light emitted from the fluorescent material (if any) is not intended to have significant meaning to a visual observer (though the visible pattern on the tube face may cryptically represent the stored data)james love marline.

The CRT uses an evacuated glass envelope which is large, deep (i.e. long from front screen face to rear end), fairly heavy, and relatively fragile. As a matter of safety, the face is typically made of thick lead glass so as to be highly shatter-resistant and to block most X-ray emissions, particularly if the CRT is used in a consumer product. Since the late 2000s, CRTs have largely been superseded by newer display technologies such as LCD, plasma display, and OLED screens, which have lower manufacturing costs and power consumption, and significantly less weight and bulk. Newer display technologies can also be made in larger sizes; whereas 38" to 40" was about the largest size of a CRT TV, new display technologies are available in 50" to 60" and even larger sizes. The vacuum level inside the tube is high vacuum on the order of 0.01 Pa to 133 nPa.

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 additive primary color (red, green, and blue) with a video signal as a reference. 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.

A 14-inch cathode ray tube showing its deflection coils and electron guns

Typical 1950s United States television set

A flat CRT assembly inside a 1984 Sinclair FTV1 pocket TV

Electron gun

History

Braun's original cold-cathode CRT, 1897

Cathode rays were discovered by Johann Hittorf in 1869 in primitive Crookes tubes. He observed that some unknown rays were emitted from the cathode (negative electrode) which could cast shadows on the glowing wall of the tube, indicating the rays were traveling in straight lines. In 1890, Arthur Schuster demonstrated cathode rays could be deflected by electric fields, and William Crookes showed they could be deflected by magnetic fields. In 1897, J. J. Thomson succeeded in measuring the mass of cathode rays, showing that they consisted of negatively charged particles smaller than atoms, the first "subatomic particles", which were later named electrons. The earliest version of the CRT was known as the "Braun tube", invented by the German physicist Ferdinand Braun in 1897. 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.

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.

It was named by inventor Vladimir K. Zworykin in 1929. RCA was granted a trademark for the term (for its cathode ray tube) in 1932; it voluntarily released the term to the public domain in 1950.

The first commercially made electronic television sets with cathode ray tubes were manufactured by Telefunken in Germany in 1934.

Oscilloscope CRTs

An Oscilloscope showing Lissajous curve

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. Televisions use magnetic rather than electrostatic deflection because the deflection plates obstruct the beam when the deflection angle is as large as is required for tubes that are relatively short for their size.

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. 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.

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 image on the 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.

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 made of glass or acrylic plastic. An internal graticule eliminates parallax error, but cannot be changed to accommodate different types of measurements. Oscilloscopes commonly provide a means for the graticule to be illuminated from the side, which improves its visibility.

Image storage tubes

The Tektronix Type 564: First Mass Produced Analog Phosphor Storage Oscilloscope

These are found in analog phosphor storage oscilloscopes. These are distinct from digital storage oscilloscopes which rely on solid state digital memory to store the image.

Where a single brief event is monitored by an oscilloscope, such an event will be displayed by a conventional tube only while it actually occurs. The use of a long persistence phosphor may allow the image to be observed after the event, but only for a few seconds at best. This limitation can be overcome by the use of a direct view storage cathode ray tube (storage tube). A storage tube will continue to display the event after it has occurred until such time as it is erased. A storage tube is similar to a conventional tube except that it is equipped with a metal grid coated with a dielectric layer located immediately behind the phosphor screen. An externally applied voltage to the mesh initially ensures that the whole mesh is at a constant potential. This mesh is constantly exposed to a low velocity electron beam from a 'flood gun' which operates independently of the main gun. This flood gun is not deflected like the main gun but constantly 'illuminates' the whole of the storage mesh. The initial charge on the storage mesh is such as to repel the electrons from the flood gun which are prevented from striking the phosphor screen.

When the main electron gun writes an image to the screen, the energy in the main beam is sufficient to create a 'potential relief' on the storage mesh. The areas where this relief is created no longer repel the electrons from the flood gun which now pass through the mesh and illuminate the phosphor screen. Consequently, the image that was briefly traced out by the main gun continues to be displayed after it has occurred. The image can be 'erased' by resupplying the external voltage to the mesh restoring its constant potential. The time for which the image can be displayed was limited because, in practice, the flood gun slowly neutralises the charge on the storage mesh. One way of allowing the image to be retained for longer is temporarily to turn off the flood gun. It is then possible for the image to be retained for several days. The majority of storage tubes allow for a lower voltage to be applied to the storage mesh which slowly restores the initial charge state. By varying this voltage a variable persistence is obtained. Turning off the flood gun and the voltage supply to the storage mesh allows such a tube to operate as a conventional oscilloscope tube.

Data storage tubes

Further information: williams tube

Color CRTs

Magnified view of a delta-gun shadow mask color CRT

Magnified view of a Trinitron color CRT

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 an equilateral triangular configuration (the guns are usually constructed as a single unit). (The triangular configuration is often called "delta-gun", based on its relation to the shape of the Greek letter delta Δ.) A grille or mask absorbs the 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; the holes are tapered so that the electrons that strike the inside of any hole will be reflected back, if they are not absorbed (e.g. due to local charge accumulation), instead of bouncing through the hole to strike a random (wrong) spot on the screen. Another type of color CRT uses an aperture grille of tensioned vertical wires to achieve the same result.

Convergence and purity in color CRTs

Due to limitations in the dimensional precision with which CRTs can be manufactured economically, it has not been practically possible to build color CRTs in which three electron beams could be aligned to hit phosphors of respective color in acceptable coordination, solely on the basis of the geometric configuration of the electron gun axes and gun aperture positions, shadow mask apertures, etc. The shadow mask ensures that one beam will only hit spots of certain colors of phosphors, but minute variations in physical alignment of the internal parts among individual CRTs will cause variations in the exact alignment of the beams through the shadow mask, allowing some electrons from, for example, the red beam to hit, say, blue phosphors, unless some individual compensation is made for the variance among individual tubes.

Color convergence and color purity are two aspects of this single problem. Firstly, for correct color rendering it is necessary that regardless of where the beams are deflected on the screen, all three hit the same spot (and nominally pass through the same hole or slot) on the shadow mask. This is called convergence. More specifically, the convergence at the center of the screen (with no deflection field applied by the yoke) is called static convergence, and the convergence over the rest of the screen area is called dynamic convergence. The beams may converge at the center of the screen and yet stray from each other as they are deflected toward the edges; such a CRT would be said to have good static convergence but poor dynamic convergence. Secondly, each beam must only strike the phosphors of the color it is intended to strike and no others. This is called purity. Like convergence, there is static purity and dynamic purity, with the same meanings of "static" and "dynamic" as for convergence. Convergence and purity are distinct parameters; a CRT could have good purity but poor convergence, or vice versa. Poor convergence causes color "shadows" or "ghosts" along displayed edges and contours, as if the image on the screen were intaglio printed with poor registration. Poor purity causes objects on the screen to appear off-color while their edges remain sharp. Purity and convergence problems can occur at the same time, in the same or different areas of the screen or both over the whole screen, and either uniformly or to greater or lesser degrees over different parts of the screen.

The solution to the static convergence and purity problems is a set of color alignment magnets installed around the neck of the CRT. These movable weak permanent magnets are usually mounted on the back end of the deflection yoke assembly and are set at the factory to compensate for any static purity and convergence errors that are intrinsic to the unadjusted tube. Typically there are two or three pairs of two magnets in the form of rings made of plastic impregnated with a magnetic material, with their magnetic fields parallel to the planes of the magnets, which are perpendicular to the electron gun axes. Each pair of magnetic rings forms a single effective magnet whose field vector can be fully and freely adjusted (in both direction and magnitude). By rotating a pair of magnets relative to each other, their relative field alignment can be varied, adjusting the effective field strength of the pair. (As they rotate relative to each other, each magnet's field can be considered to have two opposing components at right angles, and these four components [two each for two magnets] form two pairs, one pair reinforcing each other and the other pair opposing and canceling each other. Rotating away from alignment, the magnets' mutually reinforcing field components decrease as they are traded for increasing opposed, mutually cancelling components.) By rotating a pair of magnets together, preserving the relative angle between them, the direction of their collective magnetic field can be varied. Overall, adjusting all of the convergence/purity magnets allows a finely tuned slight electron beam deflection or lateral offset to be applied, which compensates for minor static convergence and purity errors intrinsic to the uncalibrated tube. Once set, these magnets are usually glued in place, but normally they can be freed and readjusted in the field (e.g. by a TV repair shop) if necessary.

On some CRTs, additional fixed adjustable magnets are added for dynamic convergence or dynamic purity at specific points on the screen, typically near the corners or edges. Further adjustment of dynamic convergence and purity typically cannot be done passively, but requires active compensation circuits.

Dynamic color convergence and purity are one of the main reasons why until late in their history, CRTs were long-necked (deep) and had biaxially curved faces; these geometric design characteristics are necessary for intrinsic passive dynamic color convergence and purity. Only starting around the 1990s did sophisticated active dynamic convergence compensation circuits become available that made short-necked and flat-faced CRTs workable. These active compensation circuits use the deflection yoke to finely adjust beam deflection according to the beam target location. The same techniques (and major circuit components) also make possible the adjustment of display image rotation, skew, and other complex raster geometry parameters through electronics under user control.

Degaussing

If the shadow mask becomes magnetized, its magnetic field deflects the electron beams passing through it, causing color purity distortion as the beams bend through the mask holes and hit some phosphors of a color other than that which they are intended to strike; e.g. some electrons from the red beam may hit blue phosphors, giving pure red parts of the image a magenta tint. (Magenta is the additive combination of red and blue.) This effect is localized to a specific area of the screen if the magnetization of the shadow mask is localized. Therefore, it is important that the shadow mask is unmagnetized. (A magnetized aperture grille has a similar effect, and everything stated in this subsection about shadow masks applies as well to aperture grilles.)

Most color CRT displays, i.e. television sets and computer monitors, each have a built-in degaussing (demagnetizing) circuit, the primary component of which is a degaussing coil which is mounted around the perimeter of the CRT face inside the bezel. Upon power-up of the CRT display, the degaussing circuit produces a brief, alternating current through the degaussing coil which smoothly decays in strength (fades out) to zero over a period of a few seconds, producing a decaying alternating magnetic field from the coil. This degaussing field is strong enough to remove shadow mask magnetization in most cases. In unusual cases of strong magnetization where the internal degaussing field is not sufficient, the shadow mask may be degaussed externally with a stronger portable degausser or demagnetizer. However, an excessively strong magnetic field, whether alternating or constant, may mechanically deform (bend) the shadow mask, causing a permanent color distortion on the display which looks very similar to a magnetization effect.

The degaussing circuit is often built of a thermo-electric (not electronic) device containing a small ceramic heating element and a positive thermal coefficient (PTC) resistor, connected directly to the switched AC power line with the resistor in series with the degaussing coil. When the power is switched on, the heating element heats the PTC resistor, increasing its resistance to a point where degaussing current is minimal, but not actually zero. In older CRT displays, this low-level current (which produces no significant degaussing field) is sustained along with the action of the heating element as long as the display remains switched on. To repeat a degaussing cycle, the CRT display must be switched off and left off for at least several seconds to reset the degaussing circuit by allowing the PTC resistor to cool to the ambient temperature; switching the display off and immediately back on will result in a weak degaussing cycle or effectively no degaussing cycle.

This simple design is effective and cheap to build, but it wastes some power continuously. Later models, especially Energy Star rated ones, use a relay to switch the entire degaussing circuit on and off, so that the degaussing circuit uses energy only when it is functionally active and needed. The relay design also enables degaussing on user demand through the unit's front panel controls, without switching the unit off and on again. This relay can often be heard clicking off at the end of the degaussing cycle a few seconds after the monitor is turned on, and on and off during a manually initiated degaussing cycle.

Vector monitors

Main article: Vector monitor

Vector monitors were used in early computer aided design systems and are in some late-1970s to mid-1980s arcade games such as Asteroids. They draw graphics point-to-point, rather than scanning a raster. Either monochrome or color CRTs can be used in vector displays, and the essential principles of CRT design and operation are the same for either type of display; the main difference is in the beam deflection patterns and circuits.

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. Aperture grille monitors do not suffer from vertical moiré; however, because their 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.

Gamma

CRTs have a pronounced triode characteristic, which results in significant gamma (a nonlinear relationship in an electron gun between applied video voltage and beam intensity).

Other types

Cat's eye

Main article: Magic eye tube

In better quality old-fashioned tube radio sets, a tuning guide consisting of a phosphor tube was used to aid the tuning adjustment. This was also known 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. The same type of device was used with tape recorders as a recording level meter, and for various other applications including electrical test equipment.

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

Main article: Nimo tube

Nimo tube BA0000-P31

Nimo was the trademark of a family of small specialised CRTs manufactured by Industrial Electronics Engineers. These had 10 electron guns which produced electron beams in the form of digits in a manner similar to that of the charactron. The tubes were either simple single-digit displays or more complex 4- or 6- digit displays produced by means of a suitable magnetic deflection system. Having little of the complexities of a standard CRT, the tube required a relatively simple driving circuit, and as the image was projected on the glass face, it provided a much wider viewing angle than competitive types (e.g., nixie tubes).

Williams tube

Main article: Williams tube

The Williams tube or Williams-Kilburn tube was a cathode ray tube used to electronically store binary data. It was used in computers of the 1940s as a random-access digital storage device. In contrast to other CRTs in this article, the Williams tube was not a display device, and in fact could not be viewed since a metal plate covered its screen.

Flood beam CRT

Flood beam CRT's are small tubes that are arranged as pixels for large screens like Jumbotrons. The first screen using this technology was introduced by Mitsubishi Electric for the 1980 Major League Baseball All-Star Game. It differs from a normal CRT in that the electron gun within does not produce a focused controllable beam. Instead, electrons are sprayed in a wide cone across the entire front of the phosphor screen, basically making each unit act as a single light bulb. Each one is coated with a red, green or blue phosphor, to make up the color sub-pixels. This technology has largely been replaced with light emitting diode displays. A similar device has been proposed by one manufacturer as a lamp.

Zeus thin CRT display

In the late 1990s and early 2000s Philips Research Laboratories experimented with a type of thin CRT known as the Zeus display which contained CRT-like functionality in a flat panel display. The devices were demonstrated but never marketed.

Current usage

Demise

Although a mainstay of display technology for decades, CRT-based computer monitors and televisions constitute a dead technology. The demand for CRT screens has dropped precipitously since 2007, and this falloff had accelerated in the last two years of that decade. The rapid advances and falling prices of LCD flat panel technology, first for computer monitors and then for televisions, has been the key factor in the demise of competing display technologies such as CRT, rear-projection, and plasma display.

The end of most high-end CRT production by around 2010 (including high-end Sony and Mitsubishi product lines) means an erosion of the CRT's capability. In Canada and the United States, the sale and production of high-end CRT TVs (30-inch screens) in these markets had all but ended by 2007. Just a couple of years later, inexpensive combo CRT TVs (20-inch screens with an integrated VHS player) disappeared from discount stores. 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.

Companies are responding to this trend. Electronics retailers such as Best Buy have been steadily reducing store spaces for CRTs. In 2005, Sony announced that they would stop the production of CRT computer displays. Samsung did not introduce any CRT models for the 2008 model year at the 2008 Consumer Electronics Show, and on 4 February 2008 Samsung removed their 30" wide screen CRTs from their North American website and has not replaced them with new models.

However, the demise of CRTs has been happening 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 2006.

Causes

CRTs, despite recent advances, have remained relatively heavy and bulky and take up a lot of space in comparison to other display technologies. 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). The CRT disadvantages became especially significant in light of rapid technological advancements in LCD and plasma flat-panels which allow them to easily surpass 40 inches (102 cm) as well as being thin and wall-mountable, two key features that were increasingly being demanded by consumers.

Slimmer CRT

A comparison between 21 inch Superslim and Ultraslim CRT

Some CRT manufacturers, both LG Display and Samsung Display, have innovated CRT technology by creating a slimmer tube. Slimmer CRT has a trade name Superslim and Ultraslim. A 21-inch flat CRT has 447.2 millimeter depth. The depth of Superslim is 352 millimeters and Ultraslim is 295.7 millimeters.

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.

CRTs are 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 (despite having greater overall brightness) due to the fluorescent lights commonly used as a backlight.

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, contrast, and better viewing from off-axis (wider viewing angle). CRTs also still find adherents in vintage video gaming because of their higher resolution per initial cost, lowest possible input lag, fast response time, and multiple native resolutions such as 576p. Many retro gamers also prefer to use older CRTs with lower resolutions and scan rates due to the distinct scan line effect created when displaying the 240p analog signal generated by most retro consoles. CRT TVs are also the only way for almost the entire library of light gun games to be played, as newer TVs have a different signal latency which prevents light guns from working.

CRT monitors are still widely used in the study of the brain's visual processing (e.g. in psychophysics). The speed and fidelity of their response, combined with the simplicity of their design, makes them well-suited for experiments where scientists need to have very fine control over stimuli which are presented to an observer. As of 2016, some inexpensive motel chains still use CRT TVs, as these televisions are less expensive, harder to steal (due to their weight) and they are much less attractive to thieves.

Health concerns

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 not to be harmful. The Food and Drug Administration regulations in 21 CFR 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 (2 in) from any external surface; since 2007, most CRTs have emissions that fall well below this limit.

Toxicity

Older color and monochrome CRTs may contain toxic substances, such as cadmium, in the phosphors. The rear glass tube of modern CRTs may be made from leaded glass, which represent an environmental hazard if disposed of improperly. 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 TCLP tests. While the TCLP process grinds the glass into fine particles in order to expose them to weak acids to test for leachate, intact CRT glass does not leache (The lead is vitrified, contained inside the glass itself, similar to leaded glass crystalware).

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

Main article: Flicker (screen)

At low refresh rates (60 Hz and below), the periodic scanning of the display may produce a flicker that some people perceive more easily than others, especially when viewed with peripheral vision. Flicker is commonly associated with CRT as most televisions run at 50 Hz (PAL) or 60 Hz (NTSC), although there are some 100 Hz PAL televisions that are flicker-free. Typically only low-end monitors run at such low frequencies, with most computer monitors supporting at least 75 Hz and high-end monitors capable of 100 Hz or more to eliminate any perception of flicker. Non-computer CRTs or CRT for sonar or radar may have long persistence phosphor and are thus flicker free. If the persistence is too long on a video display, moving images will be blurred.

High-frequency audible noise

50 Hz/60 Hz CRTs used for television operate with horizontal scanning frequencies of 15,734 Hz (for NTSC systems) or 15,625 Hz (for PAL systems). These frequencies are at the upper range of human hearing and are inaudible to many people; however, some people (especially children) will perceive a high-pitched tone near an operating television CRT. The sound is due to magnetostriction in the magnetic core and periodic movement of windings of the flyback transformer.

This problem does not occur on 100/120 Hz TVs and on non-CGA computer displays, because they use much higher horizontal scanning frequencies (22 kHz to over 100 kHz).

Implosion

High vacuum inside glass-walled cathode ray tubes permits electron beams to fly freely—without colliding into molecules of air or other gas. If the glass is damaged, atmospheric pressure can collapse the vacuum tube into dangerous fragments which accelerate inward and then spray at high speed in all directions. The implosion energy is proportional to the evacuated volume of the CRT. Although modern cathode ray tubes used in televisions and computer displays have epoxy-bonded face-plates or other measures to prevent shattering of the envelope, CRTs must be handled carefully to avoid personal injury.

Electric shock

To accelerate the electrons from the cathode to the screen with sufficient velocity, a very high voltage (EHT or Extra High Tension) is required, from a few thousand volts for a small oscilloscope CRT to tens of kV for a larger screen color TV. This is many times greater than household power supply voltage. Even after the power supply is turned off, some associated capacitors and the CRT itself may retain a charge for some time.

Security concerns

Under some circumstances, the signal radiated from the electron guns, scanning circuitry, and associated wiring of a CRT can be captured remotely and used to reconstruct what is shown on the CRT using a process called Van Eck phreaking. Special TEMPEST shielding can mitigate this effect. Such radiation of a potentially exploitable signal, however, occurs also with other display technologies and with electronics in general.

Recycling

As electronic waste, CRTs are considered one of the hardest types to recycle. CRTs have relatively high concentration of lead and phosphors (not phosphorus), both of which are necessary for the display. There are several companies in the United States that charge a small fee to collect CRTs, then subsidize their labour by selling the harvested copper, wire, and printed circuit boards. The United States Environmental Protection Agency (EPA) includes discarded CRT monitors in its category of "hazardous household waste" but considers CRTs that have been set aside for testing to be commodities if they are not discarded, speculatively accumulated, or left unprotected from weather and other damage.

Leaded CRT glass is sold to be remelted into other CRTs, or even broken down and used in road construction.

Advantages and disadvantages

Further information: Comparison CRT, LCD, Plasma

Pros

• High contrast ratio (over 15,000:1), excellent color, fairly wide color gamut and low black level.
• No native resolution; the only current display technology capable of true multisyncing (displaying many different resolutions and refresh rates without the need for scaling).
• No input lag.
• No ghosting and smearing artifacts during fast motion due to sub-millisecond response time, and impulse-based operation.
• Higher refresh rates, resolution, deeper colors, phosphor glow
• Long life (a few decades or even several decades, depending on use and quality of model)
• Newer CRTs from the 2000s have similar weight and power consumption (NEC thin profile models and similar), though higher cost of production prevented these from going into mass production against LCD screens.
• Near zero color distortion, saturation distortion, contrast distortion, or brightness distortion.
• Excellent viewing angle, which enables viewers to see a good image from many angles.
• Allows the use of light guns/pens.
• Can be used or stored in both extreme hot and cold temperature conditions without harm to the system.
• Even decades older quality CRTs have far better viewing quality and size than LCDs put into laptops well into the 2000s or still.
• The heavier CRTs (not NEC thin) have a very low theft rate, due to their heavy weight and bulk, but they offer an engineering quality picture.
• An adjusted CRT will appear new for decades, while an LCD with a replaced back-light will often show artifacts such as dust spots, bright spots, dark corners, improper filter order issues, tube light showing through, etc.
• CRTs are not damaged by a child pushing a finger onto the picture, while even quality LCDs can be damaged (none seem to come with a front filter to protect them, possibly to keep weight competitive).

Cons

• Cost per square foot to produce an acceptably viewed screen is far higher for CRT (especially if the LCD manufacturer did not pay patents, most or all are continually in patent disputes). Much of an LCD is done with plastic layers.
• CRT equivalents of LCD are simply unavailable in the 2000-era market, few high definition CRTs were ever made for the home TV market (they looked great, but were very heavy and limited in size to about 38", whereas 50", 60" and even larger LCD screens are widely available); computer CRTs were excluded from support of the market.
• CRTs may lack newer 2000s-era standards (i.e., internal computer, HDMI inputs, digital tuner): one important one is the "very long cord" ability some LCDs have (to send data to a monitor rather than attempt to bus a high fidelity picture signal over short cable). However this was by fashion not needed: older CRTs and Computer Terminals accepted data rather than analog signal up into the 1980s, but were left on the shelf for the push on the better image viewing consumers demanded; which for the price only analog short wires could provide.
• Large size and weight of older technology CRTs (preceding NEC thin technologies, which are about the same weight as LCDs but cannot be produced at the larger screen sizes LCDs can be); however that is comparing CRTs to LCDs before LCDs were even used for image viewing; not quite equitable.
• CRTs are less suitable to be installed in outdoor locations, although many were (with weather protection features).
• Higher power consumption of lower cost CRTs.
• Dust electrostatically accumulates inside non-sealed units due to the high voltages of 10-50kV, which may cause short circuits. In a house with smokers, particles accumulate on the tube and electronics.
• Higher weight and box dimension of lower cost CRTs (though CRTs may be more repairable than LCD screens).
• Geometric distortion on lower cost CRTs (not very poor LCD viewing on lower cost LCDs).
• Older technology CRTs suffer from screen burn-in when the same screen image is used on the screen for a long time (e.g., a stock ticker image), though not as quickly as Plasma displays. CRTs used in commercial applications where a standard table appears onscreen for many hours, or when used in video games with a static image (e.g., the maze on Pac-Man), burn-in this image.
• For lower cost CRTs, refresh rates lower than 85 Hz are poorer for viewing (wrong persistence of phosphor used in some low cost CRTs) compared to old broadcast TV 24 Hz having a long persistence phosphor. However LCDs had blurring issues and were terrible in comparison (some still) to even 72 Hz.
• Both CRTs and LCDs are hazardous to repair/service without proper training and precautions taken, both have invisible dangers due to the high voltage.
• Maximum size for direct-view displays is limited to about 38 or 40 inches due to practical and manufacturing restrictions (a CRT display of this size can weigh about 300 pounds), though the sizing can be increased with an array of separate displays, such as the original Jumbotron used at sports arenas.
• The glass envelopes contain toxic lead and barium as X-ray radiation shielding. The phosphors can also contain toxic elements such as cadmium. Many countries treat CRTs as toxic waste and prohibit their disposal in landfills or by incineration, and charge a fee for deposit of the 50-75 lb of well recyclable material (copper, aluminum, plastic, rare earths).
• A damaged CRT could be a radiation hazard, though anything from about the 1980s and beyond have radiation auto-shutoff built in by law.
• Purity and convergence in color tubes requires extra personal equipment to set but can be eyed for presentable viewing if lost (convergence would sway and need to be set in older TVs, but in new CRTs automatic degaussing solves focus and purity for most; on modern CRTs having this set is one of the least likely end of life scenarios).
• Sensitive to magnetic interference more than LCDs, a slight extra chance a TV would be moved during installation where an LCD would not (note such a disturbance is caused by faulty home wiring, the wiring should be tended to if the TV shows EMF disturbance, i.e. polarity reversal and misinstallation of ground wires are possible).
• Image edges of most CRTs viewed very close are slightly diffuse (blurred). However CRT light waves are designed to blend to be sharper at viewing distance (viewing them up close is amateurish). However many CRT manufacturers like Sony used perfect lines. Packard Bell in the mid-1980s had 0 blur using 3 dots in a square, which showed perfect square text but the sharp edges were a little disappointing for those interested in still or moving picture imagery using low resolutions (and many were).

See also

Basics of Cathode ray and discharge in low pressure gas

• Cathode ray
• Vacuum tube

Cathode ray causing light

• Crookes tube
• Phosphor
• Cathodoluminescence
• Scintillation (physics)

Manipulating the electron beam

• Electrostatic deflection
• Electrostatic lens
• Magnetic lens
• Magnetic deflection
• Electron beam processing
• Deflection yoke

Applying CRT in different display-purpose

• Oscilloscope
• Oscilloscope#Cathode-ray oscilloscope (CRO)
• Image dissector
• Raster scan
• Scan line
• Analog television
• Analog television#Displaying an image
• Horizontal blanking interval
• Vertical blanking interval
• Overscan in Television
• Monoscope
• Computer
• Monochrome monitor
• Computer monitor
• Comparison of display technology
• Comparison of CRT, LCD, Plasma, and OLED
• CRT projector

Miscellaneous phenomena

• Noise (video)

Historical aspect

• History of display technology
• Image dissector
• Direct-view bistable storage tube
• Trinitron
• Penetron
• Surface-conduction electron-emitter display
• Flat panel display
• LCD television / LED-backlit LCD

Safety and precautions

• Monitor filter
• Photosensitive epilepsy
• TCO Certification

Selected patents

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

External links

• Electromagnetic Deflection in a Cathode Ray Tube, Part 1 Interactive tutorial from the National High Magnetic Field Laboratory
• Electromagnetic Deflection in a Cathode Ray Tube, Part 2 Interactive tutorial from the National High Magnetic Field Laboratory
• 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