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The Fourth Source: Light-Emitting Diodes (LEDs) for General Illumination

September 2007
Advertorial course provided by Kim Lighting and Prescolite, Inc.

By C.C. Sullivan

Continuing Education

Use the following learning objectives to focus your study while reading this month’s Continuing Education article.

Learning Objectives - After reading this article, you will be able to:

  1. Discuss how LEDs work, and describe their relative merits for general illumination applications.
  2. List general criteria for evaluating LED light sources and fixtures.
  3. Evaluate LED fixtures based on specification criteria for downlight, outdoor and other general-illumination applications.

Credits: 1.00 HSW

This test is no longer available for credit

The light-emitting diode, or LED, is changing before our very eyes. The computer-age lighting source technology was, until recently, considered mainly for decorative and specialty applications. A common refrain was that "LEDs are meant to be seen, not seen by." Yet recent and significant leaps in illumination and operational performance have turned the lighting world upside-down. Over the last few years, lighting designers and architects have begun using LED lighting-also known as solid-state lighting (SSL)-fixtures for general illumination applications, in workplaces, schools, retail buildings and facilities.

LED and fixture manufacturers agree that the newest generation of products is ready for the "prime time" of general illumination. But even companies that have been actively involved in LEDs for many years have only recently released fixtures intended for general illumination: downlights, wall washers, flood lights, and the like. Manufacturers were waiting for the capability to produce fixtures that would be highly performance oriented, providing whiteness, brightness, good energy efficiency, and the power needed for indoor and outdoor use.

There's no question that LEDs are highly energy efficient. LED-illuminated exit signs consuming less than 5 watts (5W) per sign have been used for years now. And every traffic signal awarded the U.S. Environmental Protection Agency's Energy Star label has been designed to use LEDs.

 

 

 

LED Fixture Selection Criteria: General Illumination
Critical factor

Selection Criteria

1. Product useful life As with other types of light sources, an LED's light output diminishes over time. For a given application, the "useful life" of a specification-grade lamp or fixture is often defined in terms of lumen maintenance, such as the time it takes for the LED to reach 70 percent of its original lumen output.
2. Lumen output The light output of LEDs can vary by as much as 20 percent for LEDs in the same manufacturing run. (The LEDs are sorted following manufacture into groups of with like performance, in a process known as binning).
3. Heat dissipation LEDs are widely considered to be "cool" light sources, because they don't create infrared energy. But they do create heat, and fixture designs typically include a heat sink with metal fins to control fixture temperature-and LED performance.
4. color temperature LEDs produce colored light, and since most architectural lighting applications require white light, LEDs create white light by combining LED colors or by using blue or violet LEDs with phosphor coatings. All of the approaches suffer from a color shift over time, as the different components and materials degrade at varying rates, changing the light color.
5. Performance data Performance data for similar LED luminaires may be misleading or hard to compare. The U.S. Department of Energy's Solid-State Lighting Initiative recently tested four luminaires, and found that the efficacies of the luminaires was notably lower than the efficacy values for the LEDs used.

 

 

Originally published in the September 2007 issue of Architectural Record.
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