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A diode is a specialized electronic component with two electrodes called the anode and the cathode. Most diodes are made with semiconductor materials such as silicon, germanium, or selenium. The fundamental property of a diode is its tendency to conduct electric current in only one direction.

A semiconductor is a substance, usually a solid chemical element or compound that can conduct electricity under some conditions but not others, making it a good medium for the control of electrical current.

A Light Emitting Diode (LED) is a solid-state semiconductor device that converts electrical energy directly into light. In short it is a small tube which glows when energized. The semiconductor comprises of two regions viz. p & n. The p-region contains positive electrical charges while the n-region contains negative electrical charges. When voltage is applied and current begins to flow, the electrons move across the n-region into the p-region releasing energy (light) in the process. Unlike the incandescent light bulb, LED's have no filament to burn out resulting in much longer life. They also produce the same amount of light with approximately 10% of the electricity making them much less expensive to operate.

High Power LEDs (HPLED) can be driven at hundreds of mA (vs. tens of mA for other LEDs), some with more than one ampere of current, and give out large amounts of light. Since overheating is destructive, the HPLEDs must be highly efficient to minimize excess heat; furthermore, they are often mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will burn out in seconds.

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The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. Round. However, no practical use was made of the discovery for several decades. The LED was first invented in Russia in the 1920s, and introduced in America as a practical electronic component in 1962 by Nick Holonyak, Jr. The first LEDs became commercially available in late 1960s, and were red. They were commonly used as replacements for incandescent indicators, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in appliances such as TVs, radios, telephones, calculators, and even watches. These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, and LEDs became bright enough to be used for illumination.

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LED technology is growing by leaps and bounds. Extensive research has made it possible to use LEDs in all walks of life. From cars to space crafts, from mobiles to display monitors, from home lights to industrial lights, LEDs have made their presence everywhere.

Organic Light Emitting Diodes (OLED) represent another emerging technology. If it can be made practical, it may make even more of a dramatic impact on how an area is illuminated as compared to HPLEDs. In fact, it may one day replace LEDs as an energy-efficient alternative for general lighting.

OLEDs are similar to electroluminescent lighting, in which a sheet of material is excited so that it emits light. An OLED light source is a thin, flexible sheet of material consisting of three layers, a polymer or sublimed molecular film sandwiched between two layers of electrodes, one of them transparent.

Lighting engineers and designers often try to integrate lighting hardware and architecture in a cohesive manner; with OLEDs, the architecture may be the lighting hardware itself! Sheets of material can be cut and placed like “lighting wallpaper” or integrated with building materials such as wood, glass and other materials, converting them into luminous surfaces.

LED TVs incorporate this technology and have already arrived in the market.

A : Almost Anything!

• Automotive Applications with LEDs: Instrument Panels & Switches, Courtesy Lighting, Rear Stop/Turn/Tai, Retrofits, New Turn / Tail / Marker Lights, etc.

• Consumer Electronics & General Indication with LEDs: Household appliances, VCR/ DVD/ Stereo/Audio/Video devices, Toys/Games Instrumentation, Security Equipment, Switches, etc.

• Illumination with LEDs: Street Lights, Neon and bulb Replacement, Architectural Lighting, Signage (Channel Letters), Machine Vision, Retail Displays, Emergency Lighting (Exit Signs), Flashlights, Accent Lighting - Pathways, Marker Lights, etc.

• Sign Applications with LEDs: Full Color Video, Monochrome Message Boards, Traffic/VMS, Transportation - Passenger Information, etc.

• Signal Application with LEDs: Traffic, Rail, Aviation, Tower Lights, Runway Lights, Emergency/Police Vehicle Lighting, etc.

• Mobile Applications with LEDs: Mobile Phone, PDA's, Digital Cameras, Laptops, General Backlighting, etc.

• Photo Sensor Applications with LEDs: Medical Instrumentation, Bar Code Readers, Color & Money Sensors, Encoders, Optical Switches, Fiber Optic Communication, etc.

With all of the media attention given to environmental issues these days, it is widely known that the electricity we use every day is contributing towards harmful greenhouse gas emissions in the atmosphere. While some people lead their lives not worried about these issues, there are many people who are making the small contributions that they can toward alleviating this problem. Using LED lighting is one of these methods.

The recent advent of White LEDs as well as advancements in technology, have begun to replace conventional type lighting in a variety of illumination applications. LEDs not only consume far less electricity than traditional forms of illumination, resulting in reduced energy costs, but require less maintenance and repair.

On/Off time: Faster turn-on times than incandescent bulbs. LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds.

Efficiency: Increased efficiency and less power consumption than incandescent bulbs. LEDs produce more light per watt than incandescent bulbs.

Typical lumens per watt are as follows:

Lamp type	Lumens per watt
Incandescent	12 - 18
Halogen	18 - 24
Compact Fluorescent	40 - 70
LED	80 - 140

• Shock resistance: Vibration and shock resistant. LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile and can break easily

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Correlated Color Temperature (CCT): The measure used to describe the relative color appearance of a white light source. CCT indicates whether a light source appears more yellow/gold/orange or more blue, in terms of the range of available shades of white. CCT is given in Kelvin.

Color Rendering Index (CRI): It indicates how well a light source renders colors of people and objects, compared to a reference source. The higher the CRI (based upon a 0-100 scale), the more natural the colors appear. Natural outdoor light has a CRI of 100.

Candela: A unit of measurement of the intensity of light. Another way of putting it is that an ordinary wax candle generates approximately one candela.

Flux: The sum of all the lumens (lm) emitted by a source.

Lumen: A unit of measurement of the amount of brightness that comes from a light source. Lumens define "luminous flux," which is energy within the range of frequencies we perceive as light. For example, a wax candle generates 13lumens; a 100 watt bulb generates 1,200 lumens and a 100watt LED generates 15,000 lumens.

Luminous Efficacy: It is the most commonly used measure of the energy efficiency of a light source. It is stated in lumens per watt (lm/w), indicating the amount of light, a light source produces for each watt of electricity consumed.

Lux: A unit of measurement of the intensity of light. It is equal to the illumination of a surface one meter away from a single candle.

Luminaire: A lighting fixture complete with installed lamps and other accessories.

P-N Junction: Area on an LED chip where the positively and negatively charged regions meet. In short, the area on a chip where light is produced.

RGB: This stands for red, green and blue, the three primary colors of light. When the primaries are mixed, the resulting light appears white to the human eye.

Solid-state lighting: A description of the devices that do not contain moving parts or parts that can break, rupture, shatter, leak or contaminate the environment.

SI: International System of Units. A system of standard units of measurement. Reactive Power: Reactive Power is a term used to describe the loss of energy in a system arising from the production of electric and magnetic fields. It defines the efficiency of the fixture. Reactive power is the power which is not consumed by the fixture itself but the energy that is wasted during the consumption required for activating and running the fixture. It is also termed as the power factor of the fixture. Higher the power factor (i.e. close to 1) higher will be the efficiency of the fixture. Reactive power is significant because it must be provided and maintained to ensure the supply of continuous and steady voltage.

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