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LED - Light-emitting Diodes

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Introduction

A light-emitting diode (LED) is a semiconductor light source.Introduced as a practical electronic component in 1962,early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.

The process by which LEDs emit light is called electroluminescence. The color of the light is determined by the energy gap of the semiconductor.

LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching.

Light-emitting diodes are used in applications as diverse as aviation lighting, automotive lighting, advertising, general lighting, and traffic signals. LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology. Infrared LEDs are also used in the remote control units of many commercial products including televisions, DVD players, and other domestic appliances.

Science behind LED

The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode.

The wavelength of the light emitted, and thus its color depends on the band gap energy of the materials forming the p-n junction.

LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well.

Most materials used for LED production have very high refractive indices. This means that much light will be reflected back into the material at the material/air surface interface. Thus, light extraction in LEDs is an important aspect of LED production, subject to much research and development.

Lifetime and failure

Solid-state devices such as LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. Typical lifetimes quoted are 25,000 to 100,000 hours, but heat and current settings can extend or shorten this time significantly.

Like other lighting devices, LED performance is temperature dependent. Most manufacturers' published ratings of LEDs are for an operating temperature of 25 °C. LEDs used outdoors, such as traffic signals or in-pavement signal lights, and that are utilized in climates where the temperature within the luminaire gets very hot, could result in low signal intensities or even failure.

Types of LEDs

The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-color.

Miniature

These are mostly single-die LEDs used as indicators, and they come in various sizes from 2 mm to 8 mm, through-hole and surface mount packages.

Mid-range

Medium-power LEDs are often through-hole-mounted and used when an output of a few lumen is needed. These LEDs are most commonly used in light panels, emergency lighting, and automotive tail-lights. Due to the larger amount of metal in the LED, they are able to handle higher currents (around 100 mA).

High-power

High-power LEDs (HPLED) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens.Since overheating is destructive, the HPLEDs must be mounted on a heat sink to allow for heat dissipation.

Advantages of LEDs

Efficiency: LEDs emit more light per watt than incandescent light bulbs.Their efficiency is not affected by shape and size, unlike fluorescent light bulbs or tubes.

Diversity of Colors: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.

Size: LEDs can be very small (smaller than 2 mm2) and are easily populated onto printed circuit boards.

Quick On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a microsecond.LEDs used in communications devices can have even faster response times.

Frequent On-Off Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail faster when cycled often.

Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current.

Cool Light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.

Slow Failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.

Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer.Compare these to CFLsthat are rated at about 10,000 to 15,000 hours, and incandescent light bulbs at 1,000 to 2,000 hours.

Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.

Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.

Disadvantages

High initial price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies.

Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment. Over-driving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life.

Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.

Light quality: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,red surfaces being rendered particularly badly by typical phosphor-based cool-white LEDs.

Droop: The efficiency of LEDs tends to decrease as the current increases.

Applications

In general, all the LED products can be divided into two major parts, the public lighting and indoor lighting. LED uses fall into four major categories:

  • Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning.
  • Illumination where light is reflected from objects to give visual response of these objects.
  • Measuring and interacting with processes involving no human vision.
  • Indicators and signs
  • Red and green traffic signals
  • The low energy consumption, low maintenance and small size of modern LEDs has led to uses as status indicators and displays on a variety of equipment and installations.
  • One-color light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships' navigation lights or lanterns
  • Because of their long life and fast switching times, LEDs have been used in brake lights for cars' high-mounted brake lights, trucks, and buses, and in turn signals for some time, but many vehicles now use LEDs for their rear light clusters.
  • Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as glowsticks, throwies, and the photonic textile Lumalive.

Lighting

 

LED streetlights 

With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. Replacement light bulbs have been made, as well as dedicated fixtures and LED lamps.

  • LEDs are used as street lights and in other architectural lighting where color changing is used. The mechanical robustness and long lifetime is used in automotive lighting on cars, motorcycles, and bicycle lights.
  • LED street lights are employed on poles and in parking garages.
  • LEDs are used in aviation lighting. Airbus has used LED lighting in their Airbus A320 Enhanced since 2007, and Boeing plans its use in the 787. LEDs are also being used now in airport and heliport lighting and also as runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.
  • LEDs are also suitable for backlighting for LCD televisions and lightweight laptop displays and light source for DLP projectors.
  • LEDs are used increasingly in aquarium lights. In particular for reef aquariums, LED lights provide an efficient light source with less heat output to help maintain optimal aquarium temperatures.
  • The lack of IR or heat radiation makes LEDs ideal for stage lights, as well as medical lighting where IR-radiation can be harmful.
  • LEDs are small, durable and need little power, so they are used in hand held devices such as flashlights.
  • LEDs are used for infrared illumination in night vision uses including security cameras.
  • LEDs are now used commonly in all market areas from commercial to home use: standard lighting, AV, stage, theatrical, architectural, and public installations, and wherever artificial light is used.

Smart Lighting

Light can be used to transmit broadband data, which is already implemented in IrDA standards using infrared LEDs. Because LEDs can cycle on and off millions of times per second, they can be wireless transmitters and access points for data transport.Lasers can also be modulated in this manner.

Sustainable Lighting

Efficient lighting that consumes less power and lasts long is needed for sustainable architecture. LEDs fit the bill perfectly.

Other applications

  • As the light from LEDs can be modulated quickly they are used extensively in optical fiber and Free Space Optics communications - remote controls, such as for TVs, VCRs, and LED Computers.
  • In many sensor systems rely on light as the signal source, LEDs are often ideal as a light source due to the requirements of the sensors. LEDs are used as movement sensors, for example in optical computer mice.
  • Plant growers are interested in LEDs because they are more energy-efficient, emit less heat (can damage plants close to hot lamps), and can provide the optimum light frequency for plant growth and bloom periods compared to currently used grow lights.
  • LEDs have also been used as a medium-quality voltage reference in electronic circuits.