[edit] List of LED applications

Some of these applications are further elaborated upon in the following text.

[edit] Devices, medical applications, clothing, toys

• Remote controls, such as for TVs and VCRs, often use infrared LEDs.
• Glowlights, as a more expensive but longer lasting and reusable alternative to Glowsticks.
• Movement sensors, for example in optical computer mice
• The Nintendo Wii's sensor bar uses infrared LEDs.
• In optical fiber and Free Space Optics communications.
• Toys and recreational sporting goods, such as the Flashflight
• Lumalive, a photonic textile
• In pulse oximeters for measuring oxygen saturation
• LED phototherapy for acne using blue or red LEDs has been proven to significantly reduce acne over a three-month period.^{[citation needed]}
• Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up.
• Computers, for hard drive activity, power on, to draw attention to a given component or show the state of the device, as the modern Macs that "breathe" slowly in sleep mode.
• Sterilization of water and other substances using UV light.^[4]

[edit] Lighting

• Grow lights using LEDs to increase photosynthesis in plants^[38]
• Light bulbs
• Lanterns
• Streetlights
• Large scale video displays
• Architectural lighting
• Light source for machine vision systems, requiring bright, focused, homogeneous and possibly strobed illumination.
• Motorcycle and Bicycle lights
• Flashlights, including some mechanically powered models.
• Emergency vehicle lighting
• Backlighting for LCD televisions and displays. using RGB LEDs increase the color gamut by as much as 45%.
• Stage lights using banks of RGB LEDs to easily change color and decrease heating from traditional stage lighting.
• LED-based Christmas lights available in different colors and with low energy consumption.

[edit] LED panels

The 1,500 foot long LED display on the Fremont Street Experience is currently the largest in the world.

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 foot (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 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.

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 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 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.

[edit] Flat-panel LED TV history

The first known recorded flat panel LED television screen prototype was developed 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 were displayed at an Engineering Exposition in Anaheim May 1978, organized by the Science Service in Washington D.C. The LED TV display received special awards and recognition from NASA, General Motors Corporation and area universities including Robert M. Saunders of The University of California Irvine, Professor of Engineering and National IEEE President 1977. Additionally, technology and business representatives from the U.S. and overseas witnessed operation of the monochromatic LED television display. The prototype remains operational. An LCD (liquid crystal display) matrix design was also presented in the accompanying scientific paper as a future alternate television display method using a similar array scanning design.

The early display prototype was red monochromatic. Low-cost efficient blue LEDs did not emerge until the early 1990s, completing the desired 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.

[edit] Indicators and signs

• Status indicators on a variety of equipment
• Traffic lights and signals
• Exit signs
• Railroad crossing signals
• Continuity indicators
• Elevator push-button Lighting
• Thin, lightweight message displays at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.
• Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use.
• Red, yellow, green, and blue LEDs can be used for model railroading applications
• In dot matrix arrangements for displaying messages.
• Because of their long life and fast switching times, LEDs have been used for automotive high-mounted brake lights and truck and bus brake lights and turn signals for some time, but many high-end vehicles are now starting to use LEDs for their entire rear light clusters. Besides the gain in reliability, this has styling advantages because LEDs are capable of forming much thinner lights than incandescent lamps with parabolic reflectors. The significant improvement in the time taken to light up (perhaps 0.5s faster than an incandescent bulb) improves safety by giving drivers more time to react. It has been reported that at normal highway speeds this equals one car length increased reaction time for the car behind. White LED headlamps are beginning to make an appearance.
• As a medium quality voltage reference in electronic circuits. The forward voltage drop (e.g., about 1.7 V for a normal red LED) can be used instead of a Zener diode in low-voltage regulators. Although LED forward voltage is much more current-dependent than a good Zener, Zener diodes are not available below voltages of about 3 V.

[edit] Optoisolators and optocouplers

Main article: Opto-isolator

The LED may be combined with a photodiode or phototransistor in a single electronic device to provide a signal path with electrical isolation between two circuits. An optoisolator will have typical breakdown voltages between the input and output circuits of typically 500–3000 V. This is especially useful in medical equipment where the signals from a low voltage sensor circuit (usually battery powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also allows information to be transferred between circuits not sharing a common ground potential. An optocoupler may not have such high breakdown voltages and may even share a ground between input and output, but both types are useful in preventing electrical noise, particularly common mode electrical noise, on a sensor circuit from being transferred to the receiving circuit (where it may adversely affect the operation or durability of various components) and/or transferring a noisy signal. Optoisolators are also used in the feedback circuit of a DC to DC converter, allowing feedback information to be transferred while retaining electrical isolation between the input and output.

[edit] Light sources for machine vision systems

Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used to this purpose, and this field of application is likely to remain one of the major application areas until price drops low enough to make signaling and illumination applications more widespread. LEDs constitute a nearly ideal light source for machine vision systems for several main reasons:

• Size of illuminated field is usually comparatively small and Vision systems or smart camera are quite expensive, so cost of LEDs is usually a minor concern, compared to signaling applications.
• LED elements tend to be small and can be placed with high density over flat or even shaped substrates (PCBs etc) so that bright and homogeneous sources can be designed which direct light from tightly controlled directions on inspected parts.
• LEDs often have or can be used with small, inexpensive lenses and diffusers, helping to achieve high light densities and very good lighting control and homogeneity.
• LEDs can be easily strobed (in the microsecond range and below) and synchronized; their power also has reached high enough levels that sufficiently high intensity can be obtained, allowing well lit images even with very short light pulses: this is often used in order to obtain crisp and sharp “still” images of quickly-moving parts.
• LEDs come in several different colors and wavelengths, easily allowing to use the best color for each application, where different color may provide better visibility of features of interest. Having a precisely known spectrum allows tightly matched filters to be used to separate informative bandwidth or to reduce disturbing effect of ambient light.
• LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation, therefore allowing plastic lenses, filters and diffusers to be used. Waterproof units can also easily be designed, allowing for use in harsh or wet environments (food, beverage, oil industries).
• LED sources can be shaped in several main configurations (spot lights for reflective illumination; ring lights for coaxial illumination; back lights for contour illumination; linear assemblies; flat, large format panels; dome sources for diffused, omnidirectional illumination).
• Very compact designs are possible, allowing for small LED illuminators to be integrated within smart cameras and vision sensors.

[edit] 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 also uses them elsewhere.