EGEE 102
Energy Conservation for Environmental Protection

Types of Lighting: LED


As we mentioned before, LED stands for light emitting diode. This type of lighting is completely different than the other types of lighting we have discussed so far. They do not need specific gases, filaments or moving parts because they are made from semiconducting materials, which makes and LED a semi-conducting device that produces light. The underlying principles of an LED light are the opposite function of a photovoltaic systems. An LED semiconductor chip form an junction between and n-type and p-type semiconductor materials. N-type materials pass charge using electrons and p-types pass charge using holes. See the figure below for a description of the circuit.

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Basic LED circuit

LEDs do not directly produce white light. Due to this quirk, LEDs were originally used for colored light applications such as traffic lights and exit signs.

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LED Traffic Light

Because of their extremely high efficiencies (150 lumens per watt!!, and up to 90 % more efficient than incandescent light bulbs), researchers found ways to convert their outputs to white light. As such, they are one of the highest efficiency lighting options available.

Here are three examples:

  • Phosphor conversion, in which a phosphor is used on or near the LED to convert the colored light to white light
  • Color-mixed systems, in which light from multiple monochromatic LEDs (e.g., red, green, and blue) is mixed, resulting in white light
  • A hybrid method, which uses both phosphor-converted (PC) and monochromatic LEDs.
A low pressure sodium lamp.
Methods of making white light from LEDs. Credit DOE.

 These innovations have allowed these bulbs to be suitable for general lighting in residential applications. These bulbs last for 5-10 years depending on their usage.  Now, you can find them in almost every store, and they look something like this.

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General purpose whitelight LED lights.