Sources & Auxiliaries
Last Update: November 1, 2011
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Light-Emitting Diodes

This section covers the fundamental technical aspects of LED source technology. LED structure, LED array characteristics, and various aspects of LED performance are presented to give the reader a basic understanding of conditions that can affect overall efficiency and light output. Emphasis is given on exploring LED system performance, with descriptions of electronic drivers and the various relationships of system components. Thermal management techniques are reviewed, as well as LED binning structures and current standards efforts aimed at characterizing this emerging lighting technology. Finally, guidelines are presented to assist lighting designers and end users in the identification and selection of high performance LED lighting systems. 

Light-emitting diodes (LEDs) are solid-state electronic devices that generate light via the transformation of electric energy to radiant energy within the crystalline structure of a semiconductor chip. When used to describe LEDs and LED lighting systems, the term solid-state refers to the LED source component itself and implies that there are no gases or heated filaments; light is generated by a direct conversion of electrical energy into photons within a solid semiconductor die.
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Science of LEDs
As the name implies, an LED is a semiconductor diode that is configured to emit light when an electric current passes through the LED chip inside the package. Light is generated inside the chip in a region called the “junction.” When electrons pass through this chip junction, photons are generated that travel through the chip and escape the package to provide useful light.
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LED Colors
Blue LED and Phosphor (Single-Color White LEDs) - This approach uses a phosphor coating on a blue (possibly UV) LED chip. The resulting spectral light output shows a sharp blue spike (from the blue chip) and a wider distribution covering the green, yellow and red portions of the spectrum. Early attempts with this method produced bluish-white light, with a very high color temperature and uneven color in the light beam.
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LED Drivers
Several different terms can be used to describe the electronics that control power to the LED array. A power supply is a generic description of a device that converts an incoming AC supply voltage to a different, usually lower DC, voltage. This term has also been used to describe a device that additionally regulates the LED current, though this function is more properly left to the term LED driver.
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LED Systems
LED lighting systems, much like individual LEDs, are built from a series of subcomponents. Whether or not an LED luminaire contains a particular subcomponent depends on the overall design and application requirements for that luminaire.
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LED System Metrics
Most traditional light sources and luminaires are measured using relative photometry, which treats the lamp source and the luminaire separately. For LED systems, it is often impossible to separate the LED array or light engine from the luminaire or housing…
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LED Application Guidelines
Specifiers and designers are expressing a strong desire for guidance when they assess LED systems. Manufacturer product specifications often do not include important or relevant data regarding performance and lifetime.
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