An LED-backlit LCD display is a flat panel display which uses LED backlighting instead of the cold cathode fluorescent (CCFL) backlighting used by most other LCDs. LED-backlit LCD TVs use the same TFT LCD (thin film transistor liquid crystal display) technologys 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. 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.
Three types 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 common use)
- LED array behind the screen, whose brightness is not controlled individually
- Dynamic “local dimming” array of LEDs, controlled individually (or in clusters) to achieve a modulated backlight pattern
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.
Dynamic “local dimming” LEDs
This method of backlighting allows local dimming of specific areas of darkness on the screen. This can show truer blacks, whites and photorefractive effects 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).
Comparison with CCFL backlit displays
Compared to CCFL-backlit LCDs, LED-backlit LCDs:
- Produce images with greater dynamic contrast
- Can be extremely slim (some screens are less than .5 inch (0.92 cm) thin in edge-lit panels
- 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-percent lower power consumption
- Are more reliable
- Allow a wider dimming range
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, 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 faster 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 for LED backlights in LCDs. Quantum dots are useful in displays, because they emit light in specific, normal distributions. This can result in a display that more accurately renders colors in the visible spectrum. Other companies are also developing quantum dots for displays: Nanosys, 3M, QD Vision of Lexington, Massachusetts and LG Display of South Korea.
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 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.
An LED display is a flat panel display, which uses light-emitting diodes as a video display. An LED panel is a small display, or a component of a larger display. They are typically used outdoors in store signs and billboards, and in recent years have also become commonly used in destination signs on public transport vehicles or even as part of transparent glass area. LED panels are sometimes used as form of lighting, for the purpose of general illumination, task lighting, or even stage lighting rather than display.
There are two types of LED panels: conventional (using discrete LEDs) and surface-mounted device (SMD) panels. Most outdoor screens and some indoor screens are built around discrete LEDs, also known as individually mounted LEDs. A cluster of red, green, and blue diodes is driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution. The largest LED display in the world is over 500 meters long and is located in Suzhou, China, covering the Yuanrong Times Square. The largest LED television in the world is the Center Hung Video Display at Cowboys Stadium, which is 160 × 72 ft (49 × 22 m), 11,520 square feet (1,070 m2).
Most indoor screens on the market are built using SMD technology — a trend that is now extending to the outdoor market. An SMD pixel consists of red, green, and blue diodes mounted in a single package, which is then mounted on the driver PC board. The individual diodes are smaller than a pinhead and are set very close together. The difference is that the maximum viewing distance is reduced by 25% from the discrete diode screen with the same resolution.
Indoor use generally requires a screen that is based on SMD technology and has a minimum brightness of 600 candelas per square meter (cd/m², sometimes informally called nits). This will usually be more than sufficient for corporate and retail applications, but under high ambient-brightness conditions, higher brightness may be required for visibility. Fashion and auto shows are two examples of high-brightness stage lighting that may require higher LED brightness. Conversely, when a screen may appear in a shot on a television studio set, the requirement will often be for lower brightness levels with lower color temperatures; common displays have a white point of 6500–9000 K, which is much bluer than the common lighting on a television production set.
For outdoor use, at least 2,000 cd/m² is required for most situations, whereas higher-brightness types of up to 5,000 cd/m² cope even better with direct sunlight on the screen. (The brightness of LED panels can be reduced from the designed maximum, if required.)
Suitable locations for large display panels are identified by factors such as line of sight, local authority planning requirements (if the installation is to become semi-permanent), vehicular access (trucks carrying the screen, truck-mounted screens, or cranes), cable runs for power and video (accounting for both distance and health and safety requirements), power, suitability of the ground for the location of the screen (if there are no pipes, shallow drains, caves, or tunnels that may not be able to support heavy loads), and overhead obstructions.
Flat panel LED television display
The first true all-LED flat panel television screen was possibly developed, demonstrated and documented by James P. Mitchell in 1977. The modular, scalable display was initially designed with hundreds of MV50 LEDs and a newly available transistor-transistor logic memory addressing circuit from National Semiconductor. The ¼ in thin flat panel prototype and the scientific paper were displayed at the 29th ISEF expo in Washington D.C. in May 1978. It received awards by NASA, and General Motors Corporation. A liquid crystal display (LCD) matrix design was also cited in the LED paper as an alternative x-y scan technology and as a future alternate television display method. The replacement of the 70 year+ high-voltage analog system (cathode-ray tube technology) with a digital x-y scan system has been a significant achievement. Displacement of the electromagnetic scan systems included the removal of inductive deflection, electron beam and color convergence circuits. The digital x-y scan system has helped the modern television to “collapse” into its current thin form factor.
The 1977 model was monochromatic by design. Efficient blue LEDs did not arrive for another decade. Large displays now use high-brightness diodes to generate a wide spectrum of colors. It took three decades and organic light-emitting diodes for Sony to introduce an LED TV: the Sony XEL-1 OLED screen which was marketed in 2009.
The largest 3D LED television display
The 2011 UEFA Champions League Final match between Manchester United and Barcelona was broadcast live in 3D format in Gothenburg (Sweden), on an EKTA screen. It had a refresh rate of 100 Hz, a diagonal of 7.11 m (23 ft 3.92 in) and a display area of 6.192×3.483 m, and was listed in the Guinness Book of Records as the largest LED 3D TV.
LED text displays
LED text displays refer to types that are specialized and limited to display of alpha-numeric characters. Most types display either one character or a group of characters. One character is generally displayed by a matrix of LEDs, or by a matrix of segments.