LED-backlit LCD

A LED-backlit LCD is a liquid-crystal display that uses LED backlighting instead of traditional cold cathode fluorescent (CCFL) backlighting.[1] LED-backlit displays use the same TFT LCD (thin-film-transistor liquid-crystal display) technologies as CCFL-backlit LCDs, but offer a variety of advantages over them.

"LED TV" redirects here. For true LED displays, see LED display.
An Apple iPod Touch disassembled to show the array of white-edge LEDs powered on with the device

While not an LED display, a television using such a combination of an LED backlight with an LCD panel is advertised as an LED TV by some manufacturers and suppliers.[1][2]

Advantages

When compared with earlier CCFL backlights, using LEDs for backlighting offers:

  • Wider color gamut (with RGB-LED or QDEF)[3][4] and dimming range[5][6]
  • Greater contrast ratio
  • Very slim (some screens are less than 0.5 inches (13 mm) thin in edge-lit panels)
  • Significantly lighter and cooler, as much as half the total chassis and system weight of a comparable CCFL
  • Typically 20–30% lower power consumption and longer lifespan
  • More reliable[7]

LED Arrangement
a single direct LED cluster of a LCD

LED backlights replace CCFL (fluorescent) lamps with a few to several hundred white, RGB or blue LEDs. Four types of LED arrangement may be used:

'edge-lit'
LEDs form a line around the rim of the screen
'direct-lit'
LEDs form an array directly behind the screen at equally spaced intervals
'local dimming'
'direct-lit' LED clusters (rectangles, rows or columns) are individually controlled
'full array local dimming (FALD)'
'direct-lit' LEDs are individually controlled

Additionally a special diffusion panel (Light guide plate, LGP) is often to be used to spread the light evenly behind the screen.

The local dimming method of backlighting allows to dynamically control the level of light intensity 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]

A study by the University of California (Berkeley) from January 2016 suggests that the subjectively perceived visual enhancement with common contrast source material levels off at about 60 LCD local dimming zones.[9]

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 TVs" are LCDs-based television sets where the LEDs are dynamically controlled using the video information[10] (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 [11][12][13]).

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.[14] Standards for power ratings for TV sets have been introduced, e.g., in the US, EU, and Australia[15] as well as in China.[16] Moreover, a 2008 study[17] 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.[18]

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,[19] 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[11] in combination with reflective polarizers and prismatic films (invented by Philips researchers Adrianus de Vaan and Paulus Schaareman[20] 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.[21][22]

The prismatic and reflective polarization films are generally achieved using so called DBEF films manufactured and supplied by 3M.[23][24] These reflective polarization films using uniaxial oriented polymerized liquid crystals (birefringent polymers or birefringent glue) were invented in 1989 by Philips researchers Dirk Broer, Adrianus de Vaan and Joerg Brambring.[25]

A first dynamic “local dimming” LED backlight was public demonstrated by BrightSide Technologies in 2003,[26] and later commercially introduced for professional markets (such as video post-production).[27] 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.[28] 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.[29]

The white LEDs in LED backlights may use special silicate phosphors as they are brighter but degrade faster.[30] The size of the LEDs is one of the factors that determines the size of the bezel of LED-backlit LCDs.[31]

Quantum dot enhancement film (QDEF)
Main article: Quantum dot display

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, QD Vision of Lexington, Massachusetts and Avantama of Switzerland.[32][33] This type of backlighting was demonstrated by various TV manufacturers at the Consumer Electronics Show 2015.[34] 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.[35][36]

Mini LED

Mini LED displays are LED-backlit LCD with Mini LED–based backlighting supporting over a thousand of Full array local dimming (FALD) zones. This allows deeper blacks and higher contrast ratio.[37] Not to be confused with MicroLED.

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).[38] 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
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