A light-emitting diode (LED) is a semiconductor light source. First appearing in 1962 as practical electronic components, the early version of LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.
Getting straight to the physics: When this form of diode is turned on, electrons are able to recombine with holes manufactured within the device – thus releasing energy in the form of photons. This effect is called electroluminescence, and the colour of the light (corresponding to the energy of the photon) is determined by the energy band-gap of the semiconductor. An LED is often small in area (less than 1 mm), and other (optical) mechanisms may be used to form its radiation pattern.
With a huge range of applications, LEDs can be found in aviation and motor-industry lighting, as well as traffic signals, torches, and even your alarm clock.
The LED’s ability to switch on and off with immediate lighting effect have allowed for new text, video displays, and sensors to be developed, and are also useful in advanced communications technology. Infrared LEDs are also used in the remote control of many commercial products including televisions, DVD players and other domestic appliances.
Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round. He used a crystal of silicon carbide and a cat’s-whisker detector – sometimes called a crystal detector – which is an antique electronic component consisting of a thin wire that lightly touches a crystal of semiconducting mineral to make a crude point-contact rectifier.
Russian inventor and engineer, Oleg Losev reported creation of the first LED in 1927 and his research was distributed in Russian, German and British scientific journals. However, no practical use was made of the discovery until 1962 when the first practical visual spectrum (red) LED was developed by General Electric’s Nick Holonyak Jr. (who is now known as the “father of the LED”). Ten years later, Holonyak’s student, Craford, invented the first yellow LED and improved the brightness of LED lights tenfold. However, it was a further four years until the first high-brightness; high-efficiency LED light was ready. In 1976 TP Pearsall invented several new semiconductor materials that were specifically adapted to optical fibre wavelengths, and finally the LED was ready for public use.
Many LED semiconductor chips are encapsulated or potted in clear or coloured moulded plastic or glass shells. This makes it easier to mount the semiconductor chip, it physically supports and protects the fragile electrical wiring and acts as a refractor to ‘boost’ the amount of light seen by the human eye. This boost works because the potting (whether made from plastic, glass or aluminium) acts as a diffusing lens that allows the produced light to emit at a much higher angle of incidence than the chip alone is able to create.
LEDs belong to a group of devices known in the science world to be ‘solid state’, which means that they are a substance of very limited wear and tear if operated at low currents and low temperatures. In fact, many of the LEDs that were made in the 70’s and 80’s are still in service today!
Although sudden failures do occur (rarely), the biggest problem the LED faces is the lowering of light emission and loss of efficiency over time. This is mostly related to the temperature at which the LED is needed to perform. For example, LEDs in a traffic light that are subjected to high day-time temperatures could underperform with regards to how much light is emitted or to how much electricity is saved. Low temperatures could also have a negative impact on the performance of the LEDs. If the same traffic light was operating in climates where ice and snow are common, the LED’s low heat output could be the cause of its downfall as ice can form around the light, restricting or even stopping the emission of light. In response to this problem, some LED lighting systems have been designed with an added heating circuit at the expense of reduced overall electrical efficiency of the system; additionally, research has been done to develop heat sink technologies that will transfer heat produced within the junction to appropriate areas of the light fixture.
The vast majority of devices containing LEDs are “safe under all conditions of normal use”, and so are classified as “Class 1 LED product”/”LED Klasse 1”. At present, only a few LEDs—extremely bright LEDs that also have a tightly focused viewing angle of 8° or less—could, in theory, cause temporary blindness, and so are classified as “Class 2”.
Advantages and Application
The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
Colour: LEDs can emit light of an intended colour without using any colour 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 attached to printed circuit boards.
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.
Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current. This pulse-width modulation is why LED lights viewed on camera, particularly headlights on cars, appear to be flashing or flickering. This is a type of stroboscopic effect.
Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of Infrared 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. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.
Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.
The low energy consumption, low maintenance and small size of LEDs have led to uses as status indicators and displays on a variety of equipment and installations. Large-area LED displays are used as stadium displays and as dynamic decorative displays. Thin, lightweight message displays are used at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.
One-color light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships’ navigation lights or lanterns (chromacity and luminance standards being set under the Convention on the International Regulations for Preventing Collisions at Sea 1972, Annex I and the CIE) and LED-based Christmas lights. In cold climates, LED traffic lights may remain snow covered. 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.
Because of their long life, fast switching times, and their ability to be seen in broad daylight due to their high output and focus, 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. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, up to 0.5 second faster than an incandescent bulb. This gives drivers behind more time to react. It is reported that at normal highway speeds, this equals one car length equivalent in increased time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED will appear if the eyes quickly scan across the array. White LED headlamps are starting to be used. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps with parabolic reflectors (s a reflective surface used to collect or project energy such as light, sound, or radio waves).
Weather and all-hazards radio receivers with Specific Area Message Encoding (SAME) have three LEDs: red for warnings, orange for watches, and yellow for advisories and statements whenever issued.
LEDs are used as street lights and in other architectural lighting where colour changing is used. The mechanical robustness and long lifetime is used in automotive lighting on cars, motorcycles, and bicycle lights.
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. LEDs are also being used now in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.
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. LED-based aquarium fixtures also have the advantage of being manually adjustable to emit a specific colour-spectrum for ideal coloration of corals, fish, and invertebrates while optimizing photosynthetically active radiation (PAR), which raises growth and sustainability of photosynthetic life such as corals, anemones, clams, and macroalgae. These fixtures can be electronically programmed to simulate various lighting conditions throughout the day, reflecting phases of the sun and moon for a dynamic reef experience. LED fixtures typically cost up to five times as much as similarly rated fluorescent or high-intensity discharge lighting designed for reef aquariums and is not as high output.
The lack of IR or heat radiation makes LEDs ideal for stage lights using banks of RGB LEDs that can easily change colour and decrease heating from traditional stage lighting, as well as medical lighting where IR-radiation can be harmful. In energy conservation, the lower heat output of LEDs also means air conditioning (cooling) systems have less heat to dispose of, reducing carbon dioxide emissions.
LEDs are small, durable and need little power, so they are used in hand held devices such as flashlights. LED strobe lights or camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in xenon flash lamp-based lighting. This is especially useful in cameras on mobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable.
LEDs are used for infrared illumination in night vision uses including security cameras. A ring of LEDs around a video camera, aimed forward into a retro reflective background, allows chroma keying in video productions (a special effects / post-production technique for layering two images or video streams together based on colour hues or chroma range).
LEDs are used in mining operations, as cap lamps to provide light for miners. Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury for the miners.
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.
Efficient lighting is needed for sustainable architecture. In 2009, a typical 13-watt LED lamp emitted 450 to 650 lumens, which is equivalent to a standard 40-watt incandescent bulb. In 2011, LEDs have become more efficient, so that a 6-watt LED can easily achieve the same results. A standard 40-watt incandescent bulb has an expected lifespan of 1,000 hours, whereas an LED can continue to operate with reduced efficiency for more than 50,000 hours, 50 times longer than the incandescent bulb.
LED light bulbs could be a cost-effective option for lighting a home or office space because of their very long lifetimes. Consumer use of LEDs as a replacement for conventional lighting system is currently hampered by the high cost and low efficiency of available products. 2009 DOE testing results showed an average efficacy of 35 lm/W, below that of typical CFLs, and as low as 9 lm/W, worse than standard incandescent.
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: HPS (high-pressure sodium), metal-halide (MH) or CFL/low-energy. However, LEDs have not replaced these grow lights due to higher price. As mass production and LED kits develop, the LED products will become cheaper.
LEDs are an essential part of the illumination system in digital microscopes, such as those which can be directly attached to the USB port of a computer. Such USB microscopes and endoscopes enable images to be recorded and stored directly on the computer without use of a separate camera.
To end off this explanation of how LEDs work, the most interesting development in the application of these lights is the switch the Empire State Building in NYC has made to LEDs. The old lighting system only had ten colours that took several hours to change (by hand). The new LED system has a palette of sixteen MILLION colours and ranges with real time effects such as ‘ripple’, ‘cross fade’ and ‘burst’ to name a few. With light pollution, carbon emissions and landfills to take into account – this new ‘greener’ light is the beginning of a brighter more beautiful New York skyline.
