LED-backlit LCD

"LED TV" redirects here. For true LED displays, see LED display.

Comparison of LCD, edge lit LED and LED TVs

An LED-backlit LCD display is a flat panel display that uses LED backlighting instead of the cold cathode (CCFL) backlighting used in other LCD displays. While not an LED display, televisions using this display are called "LED TV" by some manufacturers and suppliers. The use of LED backlighting allows a thinner panel, lower power consumption, better heat dissipation, a brighter display, and better contrast levels. LED backlighted LCD TVs are using the same TFT LCD technologies as a CCFL backlighted LCD TVs. Picture quality is primary based on TFT LCD technology independent on backlight type.

Three forms of LED may be used:

• White edge-LEDs around the rim of the screen, using a special diffusion panel to spread the light evenly behind the screen (the most usual form)
• Dynamic RGB LEDs behind the panel
• A full array of LEDs arranged behind the screen whose brightness cannot be controlled individually

Techniques

An LED-backlit TV

Edge-LEDs

This method of back-lighting allows LED-backlit displays to be extremely thin. The light is diffused uniformly across the screen by a special panel.

RGB dynamic LEDs

This method of backlighting allows local dimming of specific areas of darkness on the screen. This can show truer blacks, whites and PRs at much higher dynamic contrast ratios, at the cost of less detail in small, bright objects on a dark background, such as star fields as well as shadow detail.

Full-array LEDs

Many brands use LED backlighting technology, which can offer benefits over CCFL LCD TVs of reduced energy consumption, better contrast and brightness, greater colour range, more rapid response to changes in scene and more accurate image rendering.

Differences with CCFL-backlit LCD displays

Compared to CCFL-backlit LCD displays, LED-backlit LCD TVs and computer displays:

• produce images with greater dynamic contrast;
• with edge-LED lighting can be extremely slim, some screens are about one inch (2.5 cm) thick;
• offer a wider color gamut when RGB-LED backlighting is used;
• produce less environmental pollution on disposal;
• are more expensive;
• have typically 20 to 30% lower power consumption;
• are more reliable;
• can allow a wider dimming range.

Technology

LCD-based televisions, often described as 'LED TVs', are not self-illuminating Organic light-emitting diode (OLED), OEL or AMOLED display technologies. In the UK the Advertising Standards Authority (ASA) has made it clear in correspondence that it does not object to the use of the term 'LED TV', but does require it to be clarified in any advertising. 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 arranged around the inside frame of the TV and a light diffusion panel to spread the light evenly behind the LCD panel.

The variations of LED backlighting offer different benefits. The first commercial LED backlit LCD TV was the Sony Qualia 005 introduced in 2004, which used RGB LED arrays to produce a color gamut around twice that of a conventional CCFL LCD television, possible because red, green and blue LEDs have very sharp spectral peaks which, combined with the LCD panel filters, results in significantly less bleed-though to adjacent color channels. In this way the 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 selected Sony BRAVIA LCD models, with the addition of 'local dimming' which enables excellent on-screen contrast through selectively turning off the LEDs behind dark parts of a picture frame.

LED backlighting employing so-called "white" LEDs produces a broader spectrum source feeding the individual LCD panel filters that is more similar to CCFL sources, and hence results in a more limited display gamut than RGB LEDs, but at lower cost.

Edge-LED lighting was first introduced by Sony in September 2008 on the 40 inch BRAVIA KLV-40ZX1M (referred to as the ZX1 in Europe). Edge-LED lighting for LCD televisions allows thinner housings; the Sony BRAVIA KLV-40ZX1M is 1 cm thick; others are also extremely thin. The 55" Samsung LED8000 Smart TV has a 0.2" (5mm) bezel.

LED-backlit LCD TVs are claimed to 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 themselves; there is some debate over whether they are a better long-term solution to the problem of TV disposal.

Because LEDs can be switched on and off faster than CCFL displays 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 that it has signed a joint development agreement with a major Japanese electronics company under which it will design and develop quantum dots for LED Backlights in LCD televisions. Quantum dots are valued for displays, because they emit light in very specific gaussian distributions. This can result in a display that more accurately renders colors in the visible spectrum.

Flicker due to backlight dimming

LED backlights are often dimmed by applying pulse-width modulation to the supply current, switching the backlight off and on faster than the eye can perceive. If the dimming pulse frequency is too low or the user is very sensitive to flicker, this may cause discomfort and eye-strain, similar to the flicker of CRT displays at lower refresh rates. This can be tested by a user simply by waving a 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 object 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, though this gives worse image quality and increased power consumption.