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

An 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 this display is called an LED TV by some manufacturers and suppliers.[1][4]

AdvantagesEdit

  • Offer a wider color gamut (with RGB-LED or QDEF)[5]
  • Produce images with greater dynamic range and contrast ratio
  • Allow a wider dimming range[6]
  • 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[7]
  • Produce less environmental pollution on disposal

Contents

LED ArrangementEdit

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.[8]

TechnologyEdit

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. A dynamic “local dimming” LED backlight was first demonstrated by BrightSide Technologies in 2003,[9] and later commercially introduced for professional markets (such as video post-production).[10] 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.[11] 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.[clarification needed]

Full-array backlights that use mini-LED panels, consisting of several thousand WLEDs, are being researched for TVs and mobile devices.[12]

Quantum dot enhancement filmEdit

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.[13] 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.[14] This type of backlighting was demonstrated by various TV manufacturers at the Consumer Electronics Show 2015.[15] 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.[16][17]

Backlight-dimming flickerEdit

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).[18] 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.

ReferencesEdit

  1. ^ a b 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. ^ "Driving LEDs versus CCFLs for LCD backlighting". Retrieved 25 October 2011. 
  7. ^ "Plasma Vs LCD vs LED TV". Retrieved 1 October 2011. 
  8. ^ Scott Wilkinson. "Ultimate Vizio". UltimateAVmag.com. Posted Fri 29 May 2009. Retrieved 16 December 2009.
  9. ^ H. Seetzen, et al.: "A High Dynamic Range Display System Using Low and High Resolution Modulators", SID03 Digest
  10. ^ BrightSide DR37-P HDR display
  11. ^ "Samsung.com". Samsung.com. Retrieved 17 May 2009. 
  12. ^ LEDInsight: Mini LED TV Backlight and Display to Be Rolled Out Soon
  13. ^ "Nanoco PR: "Nanoco Signs Agreement with Major Japanese Electronics Company"". 
  14. ^ IEEE Spectrum, 2012, 8, p.11-12 Quantum Dots Are Behind New Displays
  15. ^ CES 2015 - Placing bets on the New TV Technologies. IEEE Spectrum, January 7, 2015. Retrieved January 12, 2015
  16. ^ http://www.flatpanelshd.com/news.php?subaction=showfull&id=1493202792
  17. ^ http://www.nanosysinc.com/dot-color-archive/2017/4/18/welcome-to-the-qled-era
  18. ^ http://www.tftcentral.co.uk/articles/content/pulse_width_modulation.htm#side_effects

External linksEdit

  Media related to LED-backlit LCD television sets at Wikimedia Commons