Published on Dec 17, 2015
Once limited to simple status indicators, light emitting diodes (LEDs) are now widely used in backlighting, panel indication, decorative illumination, emergency lighting, animated signage, etc. LED lights for commercial and residential buildings are also emerging.It was the low light output and a lack of colour options that had limited the use of earlier LEDs.
However, new LED materials and improved production processes have resulted in brighter LEDs in colours throughout the visible spectrum with efficacies greater than incandescent lamps. These brighter, more efficient and colourful LEDs are finding more and more lighting applications.
LED stands for Light Emitting Diode. An LED is a semiconductor chip that converts electrical energy into light. The conversion of energy into light happens on the quantum level within the molecular makeup of the semiconductor chip. The process begins with the chip acting as a diode with two terminals, a P (Positive hole carrier) and N (Negative electron) region in its basic structure, which allow the chip to conduct in one direction for operation. In addition, there are added chemical layers called epitaxy layers that enhance the ability of the device to emit light (Photons).
As electrical energy passes through the P and N regions of the LED, electrons move to higher energy levels called band gap potentials. To meet the conservation of energy law, the electron's excess energy, gained while moving energy levels, will then produce a photon that our eye will perceive as light. At this point, the band gap potentials equal the energy of the photon created when the electron that was moving energy levels comes back to the ground state.
The colour of the light emitted directly relates to the size of the band gap potentials or the amount of energy the photons produce. Since different colours occur at different band gap potentials, or energy levels, this explains why different colour LEDs exhibit different forward voltages to operate. Recent advances in LED technology have led to brighter LEDs due to higher quantum efficiencies and higher chip extraction efficiencies. Another recent development of a blue color LED has led to RGB (Red Green Blue) white lighting as well as Phosphor on Blue to form white LEDs. The technique of Phosphor coating on Blue has shown that in the near future, white lighting from solid-state sources is a possibility, which has led to a lot of excitement.
Bright LEDs For Outdoor Applications
The first LEDs bright enough for use in outdoor applications were made of aluminium-gallium arsenide (AlGaAs). These red LEDs appeared as high mount-stop lights on automobiles and in a limited number of traffic lights. The recent advent of efficient green, blue and white LEDs may lead to more applications. Aluminium-gallium-indium phosphide (AlGaInP) and indium-gallium-nitride (InGaN) LEDs have succeeded AlGaAs as the brightest available LEDs. AlGaInP LEDs range in color from red to amber and produce about 3 lumens with efficacies greater than 20 lumens per electrical watt, although green and yellow AlGaInP LEDs have much lower efficacies.Hewlett-Packard plans to release AlGaInP LEDs with a light output of more than 10 lumens per LED.
The Nichia Chemical Company in 1993 introduced InGaN LEDs with efficacies more than 100 times that of earlier blue and green LEDs.Other companies, including Hewlett-Packard and Panasonic, offer similar InGaN LED products. Green InGaN LEDs have efficacies exceeding 30 lumens per watt, and blue InGaN LEDs have efficacies of 10 lumens per watt. InGaN technology also makes possible the first white LEDs.
LEDs Vs Incandescent Lamps
For many applications, LED lamps are superior to incandescent lamps. Their efficiency is the most apparent in applications requiring colour. Unlike incandescent bulbs that give off the full spectrum of light in a spherical pattern, LEDs emit a focused beam of a single wavelength (colour) in only one direction, in a variety of angles. The composition of the materials in the semiconductor chip determines the wavelength and, therefore, the specific colour of the light. Lenses, reflectors and diffusers can be integrated into the package to achieve the desired spatial radiation characteristics.
The beam patterns on the lamps change when we select different diffusers. Since incandescent lamps emit the full spectrum of light, these require a filtering system to produce light of a specific colour. This, in turn, reduces their light output. For example, a red-filtered incandescent lamp is as much as ten times less visible than a red LED. The filtering of the light greatly reduces the efficiency of the lamp to only a few lumens per watt.
Colored LEDs do not have the same problem since they produce light in a single colour and at higher efficiencies than white LEDs. To get around this issue with white LEDs the lamps are blue LEDs with a yellow phosphor coating to produce white light, making them much more efficient and therefore requiring less LEDs to do the job.
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