LED-backlit LCD

An Apple iPod Touch disassembled to show the array of white-edge LEDs powered on with the device

A LED-backlit LCD is a flat panel display which uses LED backlighting instead of the cold cathode fluorescent (CCFL) backlighting.[1] LED-backlit displays use the same TFT LCD (thin-film-transistor liquid-crystal display) technologies as CCFL-backlit displays, but offer reduced energy consumption, better contrast and brightness, greater color range (using more expensive RGB LEDs, blue LEDs with RG phosphors, or quantum dot enhancement film (QDEF)), more rapid response to changes in scene (with dynamic backlight dimming), and photorefractive effects.[2][3]

While not an LED display, a television using such a combination of an LED backlight with an LCD panel is called an LED TV by some manufacturers and suppliers[1][4].

Advantages

  • Offer a wider color gamut (with RGB-LED or QDEF)[5][6]
  • Allow a wider dimming range[7][8]
  • Can be extremely slim (some screens are less than 0.5 inch, or 1.27 cm) thin in edge-lit panels
  • Are significantly lighter, often as much as half the total chassis and system weight of a comparable CCFL
  • Run significantly cooler
  • Have (typically) 20–30% lower power consumption (and longer lifespans)
  • Are more reliable[9]

LED Arrangement

Direct LED full array of a smaller LCD

LED backlights replace CCFL (fluorescent) lamps with several dozen to several hundred white or blue LEDs. Two types of LED arrangement may be used:

Edge-lit LEDs
LEDs form a line around the rim of the screen, with a special diffusion panel (light guide) to spread the light evenly behind the screen
Direct LED full array
LEDs form an array directly behind the screen at equally spaced intervals

In Full array local dimming (FALD), LEDs are controlled individually to dynamically control the level of light intensity in a given part of the screen. This method of backlighting allows local dimming of specific areas of darkness on the screen, resulting in much higher dynamic-contrast ratios, though at the cost of less detail in small, bright objects on a dark background, such as star fields or shadow details.[10]

Technology

LED-backlit LCDs are not self-illuminating (unlike pure-LED systems). There are several methods of backlighting an LCD panel using LEDs, including the use of either white or RGB (Red, Green, and Blue) LED arrays behind the panel and edge-LED lighting (which uses white LEDs around the inside frame of the TV and a light-diffusion panel to spread the light evenly behind the LCD panel). Variations in LED backlighting offer different benefits. The first commercial full-array LED-backlit LCD TV was the Sony Qualia 005 (introduced in 2004), which used RGB LED arrays to produce a color gamut about twice that of a conventional CCFL LCD television. This was possible because red, green and blue LEDs have sharp spectral peaks which (combined with the LCD panel filters) result in significantly less bleed-through to adjacent color channels. Unwanted bleed-through channels do not "whiten" the desired color as much, resulting in a larger gamut. RGB LED technology continues to be used on Sony BRAVIA LCD models. LED backlighting using white LEDs produces a broader spectrum source feeding the individual LCD panel filters (similar to CCFL sources), resulting in a more limited display gamut than RGB LEDs at lower cost.

The commercially called "LED TV's" are LCDs-based television sets where the LED's are dynamically controlled using the video information[11] (dynamic backlight control or dynamic “local dimming” LED backlight, also marketed as HDR, high dynamic range television, invented by Philips researchers Douglas Stanton, Martinus Stroomer and Adrianus de Vaan [12][13][14]).

The evolution of energy standards and the increasing public expectations regarding power consumption have made it necessary for backlight systems to manage their power. As for other consumer electronics products (e.g., fridges or light bulbs), energy consumption categories are enforced for television sets[15]. Standards for power ratings for TV sets have been introduced, e.g., in the USA, EU, and Australia[16] as well as in China[17]. Moreover, a 2008 study[18] showed that among European countries, power consumption is one of the most important criteria for consumers when they choose a television, as important as the screen size[19].

Using PWM (pulse-width modulation), a technology where the intensity of the LEDs are kept constant but the brightness adjustment is achieved by varying a time interval of flashing these constant light intensity light sources[20], the backlight is dimmed to the brightest color that appears on the screen while simultaneously boosting the LCD contrast to the maximum achievable levels, drastically increasing the perceived contrast ratio, increasing the dynamic range, improving the viewing angle dependency of the LCD and drastically reducing the power consumption.

The combination of LED dynamic backlight control[12] in combination with reflective polarizers and prismatic films (invented by Philips researchers Adrianus de Vaan and Paulus Schaareman[21] make these "LED" (LCD) televisions far more efficient than the previous CRT-based sets leading to a worldwide energy saving of 600 TWh (2017), equal to 10% of the electricity consumption of all households worldwide or equal to 2 times the energy production of all solar cells in the world.[22][23].

The prismatic and reflective polarization films are generally achieved using so called DBEF films manufactured and supplied by 3M[24][25]. These reflective polarization films using uniaxial oriented polymerized liquid crystals (birefringent polymers or birefringent glue) are invented in 1989 by Philips researchers Dirk Broer, Adrianus de Vaan and Joerg Brambring[26].

A first dynamic “local dimming” LED backlight was public demonstrated by BrightSide Technologies in 2003,[27] and later commercially introduced for professional markets (such as video post-production).[28] Edge LED lighting was first introduced by Sony in September 2008 on the 40-inch (1,000 mm) BRAVIA KLV-40ZX1M (known as the ZX1 in Europe). Edge-LED lighting for LCDs allows thinner housing; the Sony BRAVIA KLV-40ZX1M is 1 cm thick, and others are also extremely thin.

LED-backlit LCDs have longer life and better energy efficiency than plasma and CCFL LCD TVs.[29] Unlike CCFL backlights, LEDs use no mercury (an environmental pollutant) in their manufacture. However, other elements (such as gallium and arsenic) are used in the manufacture of the LED emitters; there is debate over whether they are a better long-term solution to the problem of screen disposal.

Because LEDs can be switched on and off more quickly than CCFLs and can offer a higher light output, it is theoretically possible to offer very high contrast ratios. They can produce deep blacks (LEDs off) and high brightness (LEDs on). However, measurements made from pure-black and pure-white outputs are complicated by the fact that edge-LED lighting does not allow these outputs to be reproduced simultaneously on screen.

Full-array backlights that use mini-LED panels, consisting of several thousand WLEDs, are being researched for TVs and mobile devices.[30]

Quantum dot enhancement film

In September 2009 Nanoco Group announced a joint development agreement with a major Japanese electronics company, under which it will design and develop quantum dots (QDs) for LED backlights in LCDs.[31] Quantum dots are photoluminescent; they are useful in displays because they emit light in specific, narrow normal distributions of wavelengths. To generate white light best suited as an LCD backlight, parts of the light of a blue-emitting LED are transformed by quantum dots into small-bandwidth green and red light such that the combined white light allows for a nearly ideal color gamut generated by the RGB color filters of the LCD panel. In addition, efficiency is improved, as intermediate colors are not present anymore and don't have to be filtered out by the color filters of the LCD screen. This can result in a display that more accurately renders colors in the visible spectrum. Other companies are also developing quantum dot solutions for displays: Nanosys, 3M as a licensee of Nanosys and QD Vision of Lexington, Massachusetts.[32] This type of backlighting was demonstrated by various TV manufacturers at the Consumer Electronics Show 2015.[33] Samsung introduced their first 'QLED' quantum dot displays at CES 2017 and later formed the 'QLED Alliance' with Hisense and TCL to market the technology.[34][35]

Backlight-dimming flicker

LED backlights are often dimmed by applying pulse-width modulation to the supply current, switching the backlight off and on more quickly than the eye can perceive. If the dimming-pulse frequency is too low or the user is sensitive to flicker, this may cause discomfort and eyestrain (similar to the flicker of CRT displays at lower refresh rates).[36] This can be tested by a user simply by waving their hand in front of the screen; if it appears to have sharply-defined edges as it moves, the backlight is pulsing at a fairly low frequency. If the hand appears blurry, the display either has a continuously-illuminated backlight or is operating at a frequency too high to perceive. Flicker can be reduced (or eliminated) by setting the display to full brightness, although this degrades image quality and increases power consumption.

References

  1. 1 2 LED vs LCD TV Comparison
  2. ↑ Bong-Ryeol Park and Ho-Young Cha, Thermal consideration in LED array design for LCD backlight unit applications, IEICE Electron. Express, Vol. 7, No. 1, pp.40-46, (2010).
  3. ↑ H. Seetzen; et al. (2003). "SID03 Digest" (PDF).
  4. ↑ ASAAdjudication on Samsung Electronics UK Ltd
  5. ↑ Dell Studio XPS 16: Highest Color Gamut Ever?. Anandtech.com, 26 February 2009
  6. ↑ Competing display technologies for the best image performance; A.J.S.M. de Vaan; Journal of the society of information displays, Volume 15, Issue 9 September 2007 Pages 657–666; http://onlinelibrary.wiley.com/doi/10.1889/1.2785199/abstract?
  7. ↑ "Driving LEDs versus CCFLs for LCD backlighting". Retrieved 25 October 2011.
  8. ↑ Dimming options for LCD brightness; J. Moronski; Electronicproducts.com; 3 January 2004; http://www.electronicproducts.com/Optoelectronics/Dimming_options_for_LCD_brightness_control.aspx
  9. ↑ "Plasma Vs LCD vs LED TV". Retrieved 1 October 2011.
  10. ↑ Scott Wilkinson. "Ultimate Vizio Archived 26 August 2009 at the Wayback Machine.". UltimateAVmag.com. Posted Fri 29 May 2009. Retrieved 16 December 2009.
  11. ↑ LED TVs: 10 things you need to know; David Carnoy, David Katzmaier; CNET.com/news; 3 June 2010; https://www.cnet.com/news/led-tvs-10-things-you-need-to-know/
  12. 1 2 Method of and device for generating an image having a desired brightness; D.A. Stanton; M.V.C. Stroomer; A.J.S.M. de Vaan; US patent USRE42428E; 7 june 2011; https://worldwide.espacenet.com/publicationDetails/biblio?CC=US&NR=RE42428E
  13. ↑ LED local dimming explained; G. Morrison; CNET.com/news; 26 march 2016; https://www.cnet.com/news/led-local-dimming-explained/
  14. ↑ Pixel-by-pixel local dimming for high dynamic range liquid crystal displays; H. Chen; R. Zhu; M.C. Li; S.L. Lee and S.T. Wu; Vol. 25, No. 3; 6 Feb 2017; Optics Express 1973; https://www.osapublishing.org/oe/viewmedia.cfm?uri=oe-25-3-1973&seq=0
  15. ↑ "Implementing directive 2005/32/EC of the European Parliament and of the Council with regard to ecodesign requirements for televisions”, 2009; http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32009R0642
  16. ↑ “EU Australia and US regulation on energy consumption in TV sets”, 2008
  17. ↑ “China Regulation on Energy Consumption in TV Sets”, 2010
  18. ↑ “International survey on the importance of the energy efficiency of TV appliances”, 2008
  19. ↑ Controlling Power Consumption for Displays With Backlight Dimming; Claire Mantel et al; Journal of Display Technology; Volume: 9, Issue: 12, Dec. 2013; http://ieeexplore.ieee.org/document/6520956/?reload=true
  20. ↑ Dimming options for LCD brightness; J. Moronski; Electronicproducts.com; 3 Januari 2004; http://www.electronicproducts.com/Optoelectronics/Dimming_options_for_LCD_brightness_control.aspx
  21. ↑ Illumination system and display device including such a system; A.J.S.M. de Vaan; P.B. Schaareman; European patent EP0606939B1; https://worldwide.espacenet.com/publicationDetails/biblio?CC=EP&NR=0606939B1&KC=B1&FT=D&ND=5&date=19980506&DB=EPODOC&locale=en_EP#
  22. ↑ Energy Efficiency Success Story: TV Energy Consumption Shrinks as Screen Size and Performance Grow, Finds New CTA Study; Consumer Technology Association; press release 12 July 2017; https://cta.tech/News/Press-Releases/2017/July/Energy-Efficiency-Success-Story-TV-Energy-Consump.aspx
  23. ↑ LCD Television Power Draw Trends from 2003 to 2015; B. Urban and K. Roth; Fraunhofer USA Center for Sustainable Energy Systems; Final Report to the Consumer Technology Association; May 2017; http://www.cta.tech/cta/media/policyImages/policyPDFs/Fraunhofer-LCD-TV-Power-Draw-Trends-FINAL.pdf
  24. ↑ Brochure 3M Display Materials & Systems Division Solutions for Large Displays: The right look matters; http://multimedia.3m.com/mws/media/977332O/display-materials-systems-strategies-for-large-displays.pdf
  25. ↑ Broadband reflective polarizers based on form birefringence for ultra-thin liquid crystal displays; S.U. Pan; L. Tan and H.S. Kwok; Vol. 25, No. 15; 24 Jul 2017; Optics Express 17499; https://www.osapublishing.org/oe/viewmedia.cfm?uri=oe-25-15-17499&seq=0
  26. ↑ Polarisation-sensitive beam splitter; D.J. Broer; A.J.S.M. de Vaan; J. Brambring; European patent EP0428213B1; 27 July 1994; https://worldwide.espacenet.com/publicationDetails/biblio?CC=EP&NR=0428213B1&KC=B1&FT=D#
  27. ↑ H. Seetzen, et al.: "A High Dynamic Range Display System Using Low and High Resolution Modulators", SID03 Digest
  28. ↑ BrightSide DR37-P HDR display
  29. ↑ "Samsung.com". Samsung.com. Retrieved 17 May 2009.
  30. ↑ LEDInsight: Mini LED TV Backlight and Display to Be Rolled Out Soon
  31. ↑ "Nanoco PR: "Nanoco Signs Agreement with Major Japanese Electronics Company"".
  32. ↑ IEEE Spectrum, 2012, 8, p.11-12 Quantum Dots Are Behind New Displays
  33. ↑ CES 2015 - Placing bets on the New TV Technologies. IEEE Spectrum, January 7, 2015. Retrieved January 12, 2015
  34. ↑ http://www.flatpanelshd.com/news.php?subaction=showfull&id=1493202792
  35. ↑ http://www.nanosysinc.com/dot-color-archive/2017/4/18/welcome-to-the-qled-era
  36. ↑ http://www.tftcentral.co.uk/articles/content/pulse_width_modulation.htm#side_effects

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