A LED-backlit LCD is a flat panel display which uses LED backlighting instead of the cold cathode fluorescent (CCFL) backlighting. 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.
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.
Video LED-backlit LCD
Advantages
- Offer a wider color gamut (with RGB-LED or QDEF)
- Allow a wider dimming range
- 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
- Produce less environmental pollution on disposal
Maps LED-backlit LCD
LED Arrangement
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.
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 (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 ).
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. Standards for power ratings for TV sets have been introduced, e.g., in the USA, EU, and Australia as well as in China. Moreover, a 2008 study 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.
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, 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 in combination with reflective polarizers and prismatic films (invented by Philips researchers Adrianus de Vaan and Paulus Schaareman 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..
The prismatic and reflective polarization films are generally achieved using so called DBEF films manufactured and supplied by 3M. 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.
A first dynamic "local dimming" LED backlight was public demonstrated by BrightSide Technologies in 2003, and later commercially introduced for professional markets (such as video post-production). 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. 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.
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. 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. This type of backlighting was demonstrated by various TV manufacturers at the Consumer Electronics Show 2015. 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.
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). 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
External links
Media related to LED-backlit LCD television sets at Wikimedia Commons
Source of article : Wikipedia
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