Light Emitting Diode (LED) is a semiconductor (solid state) device that when a voltage is applied across its electrodes (electrically biased in the forward direction) it converts the electric power into light. The emitted light is incoherent monochromatic within a very narrow wavelength and is very much unidirectional, i.e. very narrow viewing angle. The wavelength of the light, or colour depends on the silicon material used in the construction of the LEDs. The emitted light can be near-ultraviolet, visible or infrared. The difference between a LED light and laser is that in a laser all waves are all of the same frequency (monochromatic) and all the waves are in phase.
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Incandescent lamps convert the electrical energy to light and heat. The electricity passing through the lamp filament causes the filament to heat up and become red-hot and hence producing light that is evenly distributed in all directions. About 60% of the energy is wasted through heat. Also a portion of the energy is wasted through Ultraviolet and Infrared lights that are not receptive to human eyes. In the contrary in an LED only a very small amount of power is dissipated through heat. The majority of the energy is converted to light. The LEDs are in general in excess of 10 times more efficient than a normal incandescent light source.
LEDs are the ideal choice to replace many applications that use incandescent, neon, and fluorescent lamps. Some examples of these applications are: emergency signs, fringe lighting, house and shop lighting, automotive lights, advertising displays, traffic signs, garden lights, pathway and street lighting, pond and pool lighting and feature lighting are only a few of the categories where LEDs could be effectively used.
Unlike incandescent bulbs, which can operate with either AC or DC, LEDs require a DC supply of the correct polarity. When the voltage across its electrods is in the correct direction, a significant current flows and the device is said to be forward biased. The voltage across the LED in this case is fixed for a given LED and is proportional to the energy of the emitted photons. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted.
There are numerous advantages LED can offer:
- Longer life lasting up to 100,000 hours or about 10 years
- LED lighting will degrade over time rather than suddenly burn out and be rendered ineffective.
- Reduces maintenance and service cost
- Energy efficient
- Very low heat dissipation and UV light
- Increased efficiency due to directional light emission
- Easy to use and design with
- 100% dimmable without loss of colour and any noise
- Long lasting and durable technology
- Superior and brilliant colour range
- Easily controllable both colour and intensity (dimmable)
- Fast response even in cold start condition
- Shock and vibration resistant
- No toxic materials such as Mercury
- Safe low voltage operation - 2.0 to 4.0VDC
Incandescent bulbs are low cost but very inefficient. They generate from about 16 lumen per watt for a domestic tungsten bulb to 22 lm/W for a halogen bulb. Fluorescent tubes are more efficient, from 50 to 100 lm/W for domestic tubes, allowing large energy savings, but are bulky and fragile and require starter circuits. LEDs are robust and moderately efficient, up to 32 lumen per watt, but are more expensive, although their cost is falling (about 25% per year). The technologies for LED production are developing rapidly and brighter and more efficient LEDs are being developed evry year. For example the intensity of white LED has incrased from about 3000mcd (mili-candelas) in 2002 to over 10000mcd in 2004 with fall in cost. It is estimated that by year 2008 the cost and efficiency of the LEDs could reach of the fluorescent tubes.
This is best demonstrated by an example. In a typical application where the light source is a 60W incandescent bulb and it is used for 12 hours per day, 365 days per year and for the argument’s sake the cost of 10c cents per KWh, the average cost per year will be:
[(60 x 12 x 365days)/1000] x $0.16 = $42.00 per year
Using LED light source that produces the same luminescence the cost will be:
[(10 x 12 x 365days)/1000] x $0.16 = $7.00 per year
This amounts to 6 times cost reduction in the electricity bill. You will recover the initial investment cost within the first 12 months and then after you will keep saving electricity and $$$$. With the cost of electricity increasing every year the LED lighting system makes sense.
Further saving in energy is gained by using less power to cool the hot air created by the incandescent lamps. Remember that major portion of energy in an incandescent lamp is converted to heat.
It is also notable that typical lifetime of a LED light system in ideal conditions is about 100,000 hours, with a half life of about 50,000 hours(70% lumen maintenance) from initial luminous output and without any catastrophic failures (sudden burn out).