From Wikipedia, the free encyclopedia - View original article
|This article needs additional citations for verification. (June 2009)|
An LED display is a flat panel display, which uses an array of 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.
|This section does not cite any references or sources. (August 2008)|
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 ft × 72 ft (49 m × 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.[clarification needed]
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.
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 Texas Instruments. The ¼-inch 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. Additional recognition was provided by Westinghouse Educational Foundation "Honors Group" and the concept prototype was also a selected scientific paper at the Iowa Academy of Science of the University of Northern Iowa. 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 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 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.