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===Flicker===
===Flicker===
At low [[refresh rate]]s (below 60 [[Hz]]), the periodic scanning nature of the display in most CRTs (particularly [[Raster graphics|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.<ref> {{cite web|url=http://www.pcmech.com/article/crt-monitor-flickering/|title=CRT Monitor Flickering?|accessdate=2009-10-04 }}</ref>
At low [[refresh rate]]s (below 60 [[Hz]]), the periodic scanning nature of the display in most CRTs (particularly [[Raster graphics|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.<ref> {{cite web|url=http://www.pcmech.com/article/crt-monitor-flickering/|title=CRT Monitor Flickering?|accessdate=2009-10-04 }}</ref> Non-computer CRTs or CRT for Sonar or Radio may have longer persistence phosphor and thus be flicker free. If the persistence is too long on a video display, the movement will have ghost trails.


===High-frequency noise===
===High-frequency noise===

Revision as of 11:52, 2 December 2009

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.

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] 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.[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 more power per display area for larger than 12" displays, assuming the same per sq. cm brightness and a modern design aperture grill. This is because up to 2/3rds of the backlight power is lost by the R, G & B stripe filter. Many LCDs are poorer colour rendition and can change colour with view angle. Monochrome CRT are even more efficient. Smaller LCD displays may be more efficient than CRT due to overhead of cathode heaters.

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.[30][31]

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. [32]

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.[33]

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.[34]

Health concerns

See also: 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.[35] However, studies that examined this possibility showed no signs that CRT radiation had any effect on health.[36] 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 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 from any external surface; since 2007, most CRTs have emissions that fall well below this limit.[37]

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.[38] 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.[39]

In Europe, disposal of CRT televisions and monitors is covered by the WEEE Directive.[40]

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.[41] Non-computer CRTs or CRT for Sonar or Radio may have longer persistence phosphor and thus be flicker free. If the persistence is too long on a video display, the movement will have ghost trails.

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)).[42] 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.[43] The sound is due to vibration of the magnetic components in Scan coils or LOPT generating the EHT. It's possible to design a set such that these components vibrate less and are mechanically damped.

Implosion

A high vacuum exists within all cathode ray tubes. If the outer glass envelope is damaged, a dangerous implosion may occur. Due to the power of the implosion, glass pieces may explode outwards at dangerous 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.[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

References

  1. ^ "Cathode Ray Tube". Medical Discoveries. Advameg, Inc. 2007. Retrieved 2008-04-27.
  2. ^ a b "History of the Cathode Ray Tube". About.com. Retrieved 2009-10-04.
  3. ^ a b "'How Computer Monitors Work'". Retrieved 2009-10-04.
  4. ^ "Oscilloscope CRT Clock". Retrieved 2009-10-04.
  5. ^ Bristol, Lloyd R. "US Patent 4667135 - Z-axis orthogonality compensation system for an oscilloscope". Retrieved 2009-10-04.
  6. ^ "The Cathode-Ray Tube" (PDF). Pearson Education. Retrieved 2009-10-05.
  7. ^ Doebelin, Ernest (2003). Measurement Systems. McGraw Hill Professional. p. 972. ISBN 007292201X, 9780072922011. {{cite book}}: Check |isbn= value: invalid character (help)
  8. ^ Shionoya, Shigeo (1999). Phosphor handbook. CRC Press. p. 499. ISBN 0849375606, 9780849375606. {{cite book}}: Check |isbn= value: invalid character (help); Cite has empty unknown parameter: |coauthors= (help)
  9. ^ Williams, Jim (1991). Analog circuit design: art, science, and personalities. Newnes. pp. 115–116. ISBN 0750696400, 9780750696401. {{cite book}}: Check |isbn= value: invalid character (help)
  10. ^ Yen, William M. (2006). Practical Applications of Phosphors. CRC Press. p. 211. ISBN 1420043692, 9781420043693. {{cite book}}: Check |isbn= value: invalid character (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ Bakshi, U.A. (2008). Electronic Devices And Circuits. Technical Publications. p. 38. ISBN 8184313322, 9788184313321. {{cite book}}: Check |isbn= value: invalid character (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ Hickman, Ian (2001). Oscilloscopes: how to use them, how they work. Newnes. p. 47. ISBN 0750647574, 9780750647571. {{cite book}}: Check |isbn= value: invalid character (help)
  13. ^ a b "How CRT and LCD monitors work". bit-tech.net. Retrieved 2009-10-04.
  14. ^ a b "The Shadow Mask and Aperture Grill". Retrieved 2009-10-04.
  15. ^ "Picture This". Retrieved 2009-10-04.
  16. ^ "Magnetization and Degaussing". Retrieved 2009-10-04.
  17. ^ Van Burnham (2001). Supercade: A Visual History of the Videogame Age, 1971-1984. MIT Press. ISBN 0262524201.
  18. ^ "Moiré Interference Patterns". DisplayMate. Retrieved 2009-10-04.
  19. ^ "What causes the faint horizontal lines on my monitor?". HowStuffWorks. Retrieved 2009-10-04.
  20. ^ Robin, Michael (2005-01-01). "Gamma correction". BroadcastEngineering. Retrieved 2009-10-04.
  21. ^ "Tuning-Eye Tubes". vacuumtube.com. Retrieved 2009-12-01.
  22. ^ "CATHODE RAY APPARATUS". Retrieved 2009-10-04.
  23. ^ "INPUT". Retrieved 2009-10-04.
  24. ^ "IEE Nimo CRT 10 gun readout tube datasheet" (PDF). Retrieved 2009-12-01.
  25. ^ Wong, May (October 22, 2006). "Flat Panels Drive Old TVs From Market". AP via USA Today. Retrieved 2008-10-08.
  26. ^ "End of an era". The San Diego Union-Tribune. 2006-01-20. Retrieved 2008-06-12.
  27. ^ "Matsushita says good-bye to CRTs". engadgetHD. 2005-12-01. Retrieved 2008-06-12.
  28. ^ "SlimFit HDTV". Samsung. Retrieved 2008-06-12.
  29. ^ "The future is flat as Dixons withdraws sale of 'big box' televisions". London Evening Standard. November 26, 2006. Retrieved 2006-12-03.
  30. ^ "Worldwide LCD TV shipments surpass CRTs for first time ever". engadgetHD. 2008-02-19. Retrieved 2008-06-12.
  31. ^ "LCD outsells plasma 8-to-1 in Q1 2008". engadgetHD. 2008-05-22. Retrieved 2008-06-12.
  32. ^ LCD televisions outsell plasma 8 to 1 worldwide 21 May 2008 - Digital Home
  33. ^ "X-bit's Guide: Contemporary LCD Monitor Parameters and Characteristics (page 11)". Retrieved 2009-10-04.
  34. ^ "14 Gaming Myths Exposed". GamePro.com. 2007-02-15. Retrieved 2007-08-15.
  35. ^ VDU work and hazards to health Retrieved on 2008-02-29
  36. ^ Computer/VDT Screens
  37. ^ "SUBCHAPTER J--RADIOLOGICAL HEALTH (21CFR1020.10)". U.S. Food and Drug Administration. April 12006. Retrieved 2007-08-13. {{cite web}}: Check date values in: |date= (help)
  38. ^ "CHARACTERIZATION OF LEAD LEACHABILITY FROM CATHODE RAY TUBES USING THE TOXICITY CHARACTERISTIC LEACHING PROCEDURE" (PDF). Retrieved 2009-10-04.
  39. ^ "Final Rules on Cathode Ray Tubes and Discarded Mercury-Containing Equipment". Retrieved 2009-10-04.
  40. ^ "WEEE and CRT Processing". Retrieved 2009-10-04.
  41. ^ "CRT Monitor Flickering?". Retrieved 2009-10-04.
  42. ^ "Re: How to convert Composite video (AV) to VGA monitor". Retrieved 2009-10-04.
  43. ^ "The monitor is producing a high-pitched whine". Retrieved 2009-10-04.
  44. ^ Bali, S.P. (1994-06-01). Colour Television: Theory and Practice. Tata McGraw-Hill. p. 129. ISBN 0074600249, 9780074600245. {{cite book}}: Check |isbn= value: invalid character (help); Cite has empty unknown parameter: |coauthors= (help)
  45. ^ "Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?" (PDF). Retrieved 2009-10-04.

Selected patents

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