Advancing the effective use of light for society and the environment.

Tuesday, April 6, 2004

LED study seeks to benchmark performance of high-power devices
LRC research specialist Yimin Gu examining one of the LED life test chambers

LRC research specialist Yimin Gu examining one of the LED life test chambers

The evolution of light-emitting diode technology has brought to market LED products with greater light output, efficiency and reliability than ever before. Improved performance, experts say, has propelled these devices into an increasing number of niche applications requiring colored light. These experts also agree that we may soon see white LEDs employed in some types of general lighting. Despite a basic understanding of current LED performance, specific data and life predictions have not been established, particularly for the long-term performance of these devices. For the past year, LRC solid-state lighting researchers have been conducting an ongoing, long-term study to better understand the changing characteristics of different LEDs over time and how a variety of operating conditions affect device performance.

“It’s really a Catch-22 situation,” says Dr. N. Narendran, director of research at the LRC and head of its Solid-State Lighting program. “Long-term performance testing takes a great deal of time—years, in fact—and is especially challenging when products change so quickly. Yet without the data, it’s difficult to build light fixtures that will be reliable in the long run.” Dr. Narendran notes that most manufacturers’ catalogs do not provide long-term data because of the testing challenges involved.

The LED life test lab, located at the LRC

The LED life test lab, located at the LRC

The Solid-State Lighting program at the LRC is presently working on a number of projects involving LED device packaging, reliability analysis, system integration, and field evaluation of lighting systems. The long-term performance research currently in process, however, will provide some of the most desirable data at this time because it will ultimately influence the design and set the expectation for applications and fixtures to come, says Narendran. “A technology that has raised end users’ expectations must deliver products that live up to promises if it is to be successful down the road,” he adds.

7,500 hours and counting

As part of a larger study sponsored by the U.S. Department of Energy and the University of California at Santa Barbara, LRC researchers are investigating the performance characteristics—light output depreciation and color shift—of five types of high-power LEDs: single-die red, green, blue, and white, and multiple-die white, with each package containing multiple emitters. Three LED arrays of each type were mounted in specially designed life-test chambers and are currently operating at two drive currents and two ambient temperatures. Over the last year, the team has gathered more than 7,500 hours of light output and chromaticity data.

The results to date show a large variation in LED product performance, says Yimin Gu, an LRC research specialist working on the project. “Generally, blue, green and white high-power LEDs based on indium-gallium-nitride (InGaN) are performing much better than the corresponding traditional packages used for indicator lights,” says Gu. However, aluminum-indium-gallium-phosphorus (AlInGaP) red LEDs in their high power configuration are performing much worse than the indicator-style packages, she says. High-power, single-die white LEDs have degraded only 6 percent during the first 7,500 hours. Data extrapolation indicates that these devices will maintain 70 percent of their lumen output at 45,000 hours, a significant improvement over earlier types of white LEDs, say Narendran and Gu.

In terms of color, the individual white LEDs in each package show significant color variations, which creates an aesthetic challenge for general illumination with LEDs. The good news, though, is that the color shift over time has remained quite small, says Narendran.

The team expects to continue studies to approximately 12,000 hours for the colored LEDs and to 15,000 hours for the white LEDs. Narendran says that although benchmarking product performance is the immediate objective, the long-range goal is to develop metrics and life predictors that can be used by the industry to quantify product performance.

For more information, visit Benchmarking High-power LEDs: A Life Test.

About the LRC

The Lighting Research Center (LRC) is part of Rensselaer Polytechnic Institute and is the leading university-based research center devoted to lighting. Founded in 1988, the Lighting Research Center has built an international reputation as a trusted and reliable source for objective information about lighting technologies, applications, and products. Its mission is to advance the effective use of light and create a positive legacy of change for society and the environment.

2004 Rensselaer Polytechnic Institute, Troy, NY 12180 USA.

Rennselear Polytechnic Institute