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How to Measure Long-Term Performance of LEDs?
Using Spectral Measurements to Calculate Junction Temperature
From Junction Temperature Measurements to System-Life Predictions

How to Measure Long-Term Performance of LEDs?

The lighting industry is witnessing great strides in the development of light-emitting diodes, (LEDs), particularly in the areas of general illumination and white light. Lighting systems employing LEDs are becoming quite common, especially for transportation and display-lighting applications, because of their efficiency and small size. However, with increased popularity and evolving technology come questions regarding the long-term performance of LED systems.

While individual LEDs may operate for up to 100,000 hours, several factors can potentially reduce the useful life of LEDs operating in a system. These factors are:

Drive current,
Packaging, and
Temperature.
Current methods for evaluating performance include life tests, which can take thousands of hours to conduct. With LED technology maturing rapidly, data from these tests may be outdated even before testing is finished.

Researchers with the Lighting Research Center at Rensselaer Polytechnic Institute (Troy, New York) have taken on the challenge of determining quickly how long LEDs in a system last and what factors affect LED-system life.

As the first step of this project, Dr. N. Narendran, director of research and leader of the LRC Solid-State Lighting program, and his graduate students have developed two new methods for determining the junction temperature of certain types of LEDs within a system. Earlier research has shown junction heat to be a major cause of light output degradation in LEDs.

"These new methods are particularly valuable because they allow junction temperature to be measured quickly and non-invasively," said Dr. Narendran. "This protects the stability of the lighting system." Previous methods for measuring junction temperature required contact with an LED's wiring, which meant taking apart a system and potentially altering its performance.

Using Spectral Measurements to Calculate Junction Temperature

The new methods connect junction temperature with emission spectra for AlGaInP red LEDs and phosphor-converted, GaN-based white LEDs.

For the AlGaInP red LEDs, the LRC researchers found that the peak wavelengths of these lamps shift as a function of junction temperature. Increasing the temperature, either through higher drive currents or higher ambient temperatures, causes the spectrum of these red LEDs to shift to longer wavelengths; this shift is proportional to changes in junction temperature. By measuring the peak wavelength shift and the initial ambient temperature, the LRC researchers were able to accurately predict the red-LED system's average junction temperature.

Gallium nitride-based white LEDs require a different type of spectral measurement in order to determine junction heat, said Dr. Narendran, because the wavelength shift is much smaller in these types of light sources. The LRC researchers were able to use measurements of blue and white radiant energies to effectively calculate junction temperature.

By measuring and analyzing the spectral power distribution and the junction temperature of the white LED, Dr. Narendran's team found a proportional relationship between LED junction temperature and the ratio of total radiant energy to blue radiant energy. The team tested white LEDs from several manufacturing batches with varying blue peak wavelengths and found this linear relationship to hold true, even when the spectral characteristics of the LEDs were not identical.

"Overall, these methods have the capacity to help manufacturers of lighting systems to provide useful life predictions," said Dr. Narendran. However, more research on junction temperature and light output degradation is needed before reaching this point.

From Junction Temperature Measurements to System-Life Predictions

In conjunction with researchers at the University of California at Santa Barbara, experiments are in progress to develop methods of predicting LED-system life. At the LRC, Dr. Narendran's team is working to determine how junction temperature relates to the degradation rate of LEDs by conducting life tests on several types of colored and white LEDs.

"When we understand how junction temperature affects the rate of light output degradation in LEDs, then we will have a good idea as to how long any system will run effectively," said Dr. Narendran. This data will eliminate the need for time-consuming life tests, allowing designers of LED-lighting systems to easily evaluate and turn out improved products.

These studies were presented at the SPIE Annual Meeting-Third International Conference on Solid State Lighting, held August 5 - 7 in San Diego. Detailed articles will be published this fall by the International Society for Optical Engineering: "A Method for Projecting Useful Life of LED Lighting Systems" by Eugene Hong and N. Narendran, and "A Non-contact Method for Determining Junction Temperature of Phosphor-Converted White LEDs" by Yimin Gu and N. Narendran, Proceedings of SPIE, Volume 5187 (2003).

For more information about LEDs and the Solid-State Lighting Program at the Lighting Research Center, visit http://www.lrc.rpi.edu/programs/solidstate/.