LED-backlit LCD display

An Apple iPod Touch music player 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 used by most other LCDs.^[1] LED-backlit LCD TVs use the same TFT LCD (thin film transistor liquid crystal display) technologies as CCFL-backlit LCD TVs. Picture quality is primarily based on TFT LCD technology, independent of backlight type. While not an LED display, a television using this display is called an “LED TV” by some manufacturers and suppliers.^[1] In the UK, the Advertising Standards Authority has made it clear in correspondence that it does not object to the use of the term “LED TV”, but requires it to be explained in advertising.^[2]

Three types of LED may be used:

• Edge-lit LEDs - in which the LEDs are formed around the rim of the screen, using a special diffusion panel to spread the light evenly behind the screen (the most common use)
• LED backlighting (Full array)- behind the screen, whose brightness is not controlled individually
• Dynamic “local dimming” backlight - LEDs controlled individually (or in clusters) to control the level of light/color intensity in a given part of the screen.

The iPhone 5 has an LED-backlit TFT IPS LCD, while the Sony Xperia S is an example of an LED-backlit TFT TN LCD (also referred as TFT LCD).

Techniques

Full-array LEDs

Many brands use LED backlighting technology, which offer the advantages over CCFL LCDs of reduced energy consumption, better contrast and brightness, greater color range, more rapid response to changes in scene and more accurate image rendering.^[3]

Dynamic “local dimming” LEDs

This method of backlighting allows local dimming of specific areas of darkness on the screen. This type of display comes in white or the more expensive RGB LED configuration. This can show truer blacks, whites, proper color saturation (on RGB LEDs), and photorefractive effects^[4] at much higher dynamic-contrast ratios by dimming (or brightening) the backlight locally (at the cost of less detail in small, bright objects on a dark background, such as star fields or shadow details).^[5]

Comparison with CCFL backlit displays

Comparison of LCD, edge-lit LED and LED TVs

Compared to CCFL-backlit LCDs, LED-backlit LCDs:^[1]

• Produce images with greater dynamic contrast
• Can be extremely slim (some screens are less than 0.5 inch (0.92 cm)) thin in edge-lit panels^[6]
• Offer a wider color gamut (when RGB-LED backlighting is used)^[7]
• Produce less environmental pollution on disposal
• Have (typically) 20- to 30-percent lower power consumption (and longer lifespans)
• Run significantly cooler
• Are more reliable^[8]
• Allow a wider dimming range^[9]
• Are significantly lighter, often as much as half the total chassis and system weight of a comparable CCFL.

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. A dynamic “local dimming” LED backlight was first demonstrated by BrightSide Technologies in 2003,^[10] and later commercially introduced for professional markets (such as video post-production).^[11] 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.^[12] 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.

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 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] The importance of this type of backlighting was demonstrated by various TV manufacturers at the Consumer Electronics Show 2015.^[15]

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