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In today’s green-centric world, designers and manufacturers are constantly looking for ways to improve energy efficiency in their products. From cell phone manufacturers to PC makers, new industry standards are emerging that will significantly impact the rate of power consumption in electronic devices. In the lighting industry, tremendous energy savings can also be realized with a change from incandescent or fluorescent technologies to LED lighting.
A light-emitting diode (LED) s a semiconductor light source. LEDs are used as indicator lamps in many devices, and are increasingly used for lighting. 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.
With the development of high efficiency and high power LEDs it has become possible to incorporate 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 on bicycle lights.
Basic Principle
The LED is based on the semiconductor diode. When a diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is usually small in area (less than 1 mm2), and integrated optical components are used to shape its radiation pattern and assist in reflection. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. However, they are relatively expensive and require more precise current and heat management than traditional light sources. Current LED products for general lighting are more expensive to buy than fluorescent lamp sources of comparable output.
They also enjoy use in applications as diverse as replacements for traditional light sources in automotive lighting (particularly indicators) and in traffic signals. Airbus uses LED lightning in their A320 Enhanced since 2007, and Boeing plans its use in the 787. The compact size of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are useful in advanced communications technology.
Considerations for use
LED power sources
The current/voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (see Shockley diode equation). This means that a small change in voltage can lead to a large change in current. If the maximum voltage rating is exceeded by a small amount the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is therefore to use constant current power supplies, or driving the LED at a voltage much below the maximum rating. Since most household power sources (batteries, mains) are not constant current sources, most LED fixtures must include a power converter. However, the I/V curve of nitride-based LEDs is quite steep above the knee and gives an If of a few m.A. at a Vf of 3V, making it possible to power a nitride-based LED from a 3V battery such as a coin cell without the need for a current limiting resistor.
Electrical polarity
As with all diodes, current flows easily from p-type to n-type material. However, no current flows and no light is produced if a small voltage is applied in the reverse direction. If the reverse voltage becomes large enough to exceed the breakdown voltage, a large current flows and the LED may be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful noise diode.
LEDs offer numerous benefits due to their mode of operation:
Energy Efficiency : LEDs are highly efficient. In traffic signal lights, a strong market for LEDs, a red traffic signal head that contains 196 LEDs draws 10W versus its incandescent counterpart that draws 150W. Various estimates of potential energy savings range from 82% to 93%. With the red signal operating about 50% of the day, the complete traffic signal unit is estimated to save 35-40%.
Long Life : Some LEDs are projected to produce a long service life of about 100,000 hours. For this reason, LEDs are ideal for hard-to-reach/maintain fixtures such as exit sign lighting and, combined with its durability, pathway lighting. This service life can be affected by the application and environmental factors, including heat and if being overdriven by the power supply.
No UV Emissions/Little Infrared : LEDs produce no UV radiation and little heat, making them ideal for illuminating objects, such as works of art, which are sensitive to UV light.
Durable : LEDs are highly rugged. They feature no filament that can be damaged due to shock and vibrations. They are subject to heat, however, and being overdriven by the power supply.
Color : LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs. LEDs are available in a range of colors, including white light. White light can also be produced through color mixing of red, blue and green LEDs. In addition, through the innovative combination of various-colored LEDs, dramatic color-changing effects can be produced from a single fixture through dynamic activation of various sets of LEDs.
Size : LEDs can be very small (smaller than 2 mm) and are easily populated onto printed circuit boards. A single LED is very small and produces little light overall. However, this weakness is actually its strength. LEDs can be combined in any shape to produce desired lumen packages as the design goals and economics permit. In addition, LEDs can be considered miniature light fixtures; distribution of light can be controlled by the LEDs’ epoxy lens, simplifying the construction of architectural fixtures designed to utilize LEDs. A controller can be connected to an LED fixture to selectively dim individual LEDs, resulting in the dynamic control of distribution, light output and color. Finally, DC power enables the unit to be easily adaptable to different power supplies.
On/Off time : LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds. LEDs used in communications devices can have even faster response times and lights instantaneously.
Cycling: LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
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 burn-out 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 1,000–2,000 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.
Toxicity: LEDs do not contain mercury, unlike fluorescent lamps.
Advantages over Conventional Light lamps
LEDs have several advantages over conventional incandescent lamps. For one thing, they do not have a filament that will burn out, so they last much longer. Additionally, their small plastic bulb makes them a lot more durable. They also fit more easily into modern electronic circuits.
However, the main advantage is efficiency. In conventional incandescent bulbs, the light-production process involves generating a lot of heat (the filament must be warmed). This is completely wasted energy, unless you’re using the lamp as a heater, because a huge portion of the available electricity isn’t going toward producing visible light. LEDs generate very little heat, relatively speaking. A much higher percentage of the electrical power is going directly to generating light, which cuts down on the electricity demands considerably.
Up until recently, LEDs were too expensive to use for most lighting applications because they are built around advanced semiconductor material. The price of semiconductor devices has plummeted over the past decade, however, making LEDs a more cost-effective lighting option for a wide range of situations. While they may be more expensive than incandescent lights up front, their lower cost in the long run can make them a better buy. In the future, they will play an even bigger role in the world of technology.
High initial price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed.
Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate.
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. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.
Area light source: LEDs do not approximate a “point source” of light, but rather a lambertian distribution. Therefore, LEDs are difficult to use in applications requiring a spherical light field. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less.
Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.
Blue pollution: Because cool-white LEDs (i.e., LEDs with high color temperature) emit proportionally more blue light than conventional outdoor light sources such as high-pressure sodium lamps, the strong wavelength dependence of Rayleigh scattering means that cool-white LEDs can cause more light pollution than other light sources. The International Dark-Sky Association discourages the use of white light sources with Correlated Color Temperature above 3,000 K.
Applications
Application of LEDs falls into three major categories :
Visual signal application where the light goes more or less directly from the LED to the human eye, to convey a message or meaning.
Illumination where LED light is reflected from object to give visual response of these objects.
Generate light for measuring and interacting with processes that do not involve the human visual system.
