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LED
LEDs are semiconductor diodes, electronic devices that permit current to flow in only one direction. The diode is formed by bringing two slightly different materials together to form a PN junction . In a PN junction, the P side contains excess positive charge ("holes," indicating the absence of electrons) while the N side contains excess negative charge (electrons).y the LED. When a forward voltage is applied to the semiconducting element forming the PN junction (heretofore referred to as the junction), electrons move from the N area toward the P area and holes move toward the N area. Near the junction, the electrons and holes combine. As this occurs, energy is released in the form of light that is emitted by the LED.
LED Color
The material used in the semiconducting element of an LED determines its color. The two main types of LEDs presently used for lighting systems are aluminum gallium indium phosphide (AlGaInP, sometimes rearranged as AlInGaP) alloys for red, orange and yellow LEDs; and indium gallium nitride (InGaN) alloys for green, blue and white LEDs. Slight changes in the composition of these alloys changes the color of the emitted light.
White LEDs
There are two approaches to creating white light.
A: Mixed-color white light One approach is to mix the light from several colored LEDs to create a spectral power distribution that appears white. By locating red, green and blue LEDs adjacent to one another, and properly mixing the amount of their output, the resulting light is white in appearance.
B: Phosphor-converted white light Another approach to generating white light is by use of phosphors together with a short-wavelength LED. For example, when one phosphor material used in LEDs is illuminated by blue light, it emits yellow light having a fairly broad spectral power distribution. By incorporating the phosphor in the body of a blue LED with a peak wavelength around 450 to 470 nanometers, some of the blue light will be converted to yellow light by the phosphor. The remaining blue light, when mixed with the yellow light, results in white light.
LED Lighting System
Lighting systems using LEDs can be thought of as having a light source (typically, the individual LED sources), a ballast (for LEDs, often called a driver), and a luminaire (the surrounding materials for optical control of the emitted light and thermal control of the overall system). Unlike traditional lighting systems with few (typically, one to four) light sources, LED systems contain arrays of many individual light sources.
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