Published on Dec 06, 2015
Until recently, though, the price of an LED lighting system was too high for most residential use. With sales rising and prices steadily decreasing, it's been said that whoever makes the best white LED will open a goldmine.
White LED lighting has been used for years by the RV and boating crowd, running off direct current (DC) battery systems. It then got popular in off-the-grid houses, powered by photovoltaic cells. It used to be that white LED was possible only by "rainbow" groups of three LEDs -- red, green, and blue -- and controlling the current to each to yield an overall white light. Now a blue indium gallium chip with a phosphor coating is used to create the wave shift necessary to emit white light from a single diode. This process is much less expensive for the amount of light generated.
Each diode is about 1/4 inch and consumes about ten milliamps (a tenth of a watt). Lamps come in various arrangements of diodes on a circuit board. Standard arrays are three, six, 12, or 18 diodes, or custom sizes -- factories can incorporate these into custom-built down lights, sconces and surface-mounted fixtures. With an inexpensive transformer, they run on standard 120-volt alternating current (AC), albeit with a slight (about 15% to 20%) power loss. They are also available as screw-in lamps to replace incandescent. A 1.2 watt white LED light cluster is as bright as a 20-watt incandescent lamp.
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.
2.2 BRIGHT LEDS FOR OUTDOORS 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 (AlGalnP) and indium-gallium-nitride (InGaN) LEDs have succeeded AlGaAs as the brightest available LEDs. AlGalnP LEDs range in colour from red to amber and produce about 3 lumens with efficacies greater than 20 lumens per electrical watt, although green and yellow AlGalnP LEDs have much lower efficacies. Hewlett-Packard plans to release AlGalnP 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.
3. 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. The job in this case would be decorative lighting, landscape lighting, accent lighting, wall washing and special effects. These type applications tend to be the most suitable for LEDs in the lighting world today.
The blue-white light from an LED array looks 'cold' compared to the yellow glow from a filament lamp. For example, the yellow bias of a halogen bulb flatters mud, sand and 'stal' in caves whereas the green/blue bias of a white LED is better for the outdoors. Experiments are going on with introducing some yellow or orange LEDs to a white LED array to shift its colour balance to better suit caves, and this may also reduce the overall cost.
We need to talk about the light efficiency of a conventional incandescent lamp. These range from around 12-17 lumens per watt (depending on the rated life). LED's on the other hand, have efficiencies that vary with the color of the light. If we are talking white only, the efficiency of the LED that we use is between 29-42 lumens per watt. We can increase this efficiency even more by designing a range of products to be spot lamps that concentrate the light in a smaller area without any significant losses in lumens per watt.
New 'doping' technologies have increased the LED light output by as much as 20 times over earlier generations and allowed the production of daylight-visible LEDs in virtually any colour of the spectrum. In addition to red, yellow and amber/orange, LEDs are now available in many colours from leaf green to ultra blue. LEDs produce sharp, vibrant colours. Even pure white light, long thought to be an impossibility, is now available in three different shades. Since lamps are LED based, they provide saturated colors.
Now about the power requirement. Typical pin spot fixtures use 30-50 watt lamps with colored filters to project a colored spot of light. With LED lamps, the power requirements would be only 2.5 Watts of power! LED lamps also have the power supply built in so they screw directly into a standard lighting socket.
While LEDs deliver 100 per cent of their energy as colored light, incandescent bulbs waste 90 per cent or more of their energy in light blocked by the colored lens or filter. Incandescent bulbs also waste 80 to 90 per cent of their energy on heat generation to reach the temperature (in Kelvins) for which they are designed. LED lamps produce very little heat. Less heat is important for a couple of reasons; firstly, we can reduce air-con loading and electric bills and secondly, we don't have to worry about the fire hazards involved with flammable material touching the surfaces of the lamps.
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