LED History
Discovery
The first known report of a light-emitting solid-state diode was made in 1907
by the British experimenter H. J. Round. However, no practical use was made of
the discovery for several decades.[18] Independently, Oleg Vladimirovich Losev
published "Luminous carborundum [[silicon carbide]] detector and detection with
crystals" in the Russian journal Telegrafiya i Telefoniya bez Provodov (Wireless
Telegraphy and Telephony).[19] Losev's work languished for decades.
The first practical LED was invented by Nick Holonyak, Jr., in 1962 while he was
at General Electric Company. The first LEDs became commercially available in
late 1960s, and were red. They were commonly used as replacements for
incandescent indicators, and in seven-segment displays, first in expensive
equipment such as laboratory and electronics test equipment, then later in such
appliances as TVs, radios, telephones, calculators, and even watches. These red
LEDs were bright enough only for use as indicators, as the light output was not
enough to illuminate an area. Later, other colors became widely available and
also appeared in appliances and equipment. As the LED materials technology
became more advanced, the light output was increased, and LEDs became bright
enough to be used for illumination.
Most LEDs were made in the very common 5 mm T1-3/4 and 3 mm T1 packages, but
with higher power, it has become increasingly necessary to get rid of the heat,
so the packages have become more complex and adapted for heat dissipation.
Packages for state-of-the-art high power LEDs bear little resemblance to early
LEDs (see, for example, Philips Lumileds).
LED panels
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 1,500 feet (457.2 m) long and is located in Las
Vegas, Nevada covering the Fremont Street Experience.
Most indoor screens on the market are built using SMD technology ¡ª a trend that
is now extending to the outdoor market. An SMD pixel consists of red, green, and
blue diodes mounted on a chipset, 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.
LED panels allow for smaller sets of interchangeable LEDs to be one large
display.Indoor use generally requires a screen that is based on SMD technology
and has a minimum brightness of 600 candelas per square meter (unofficially
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 led lighting that may require higher LED
brightness. Conversely, when a screen may appear in a shot on a television show,
the requirement will often be for lower brightness levels with lower color
temperatures (common displays have a white point of 6500 to 9000 K, which is
much bluer than the common led lighting on a television
production set).
For outdoor use, at least 2,000 nits are required for most situations, whereas
higher brightness types of up to 5,000 nits 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.
Early LED flat panel TV history
Perhaps the first recorded flat LED television screen prototype to be
developed was by James P. Mitchell in 1977. The modular, scalable display was
enabled by MV50 LEDs and newly available TTL (transistor transistor logic)
memory addressing circuit technology. The prototype and paper was displayed at
an Engineering Exposition in Anaheim May 1978, and organized by the Science
Service in Washington D.C. The LED flat panel TV display received special
recognition by NASA, General Motors Corporation, and area universities including
The University of California Irvine, Robert M. Saunders Prof. of Engineering and
IEEE President 1977. Additionally, technology business representatives from the
U.S. and overseas witnessed operation of the monochromatic LED flat panel
television display. The prototype remains operational. A LCD (liquid crystal
display) matrix design was also presented in the accompanying scientific paper,
as a future television display method using a similar scanning design method.
The early display prototype was red monochromatic. Low-cost efficient blue LEDs
did not emerge until the early 1990s, completing the RGB color triad.
High-brightness colors gradually emerged in the 1990s enabling new designs for
outdoor signage and huge video displays for billboards and stadiums.
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Multi-touch sensing
Since LEDs share some basic physical properties with photodiodes, which also use
p-n junctions with band gap energies in the visible light wavelengths, they can
also be used for photo detection. These properties have been known for some
time, but more recently so-called bidirectional LED matrices have been proposed
as a method of touch-sensing. In 2003, Dietz, Yerazunis, and Leigh published a
paper describing the use of LEDs as cheap sensor devices.
In this usage, various LEDs in the matrix are quickly switched on and off. LEDs
that are on shine light onto a user's fingers or a stylus. LEDs that are off
function as photodiodes to detect reflected light from the fingers or stylus.
The voltage thus induced in the reverse-biased LEDs can then be read by a
microprocessor, which interprets the voltage peaks and then uses them elsewhere.
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