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LEDs in a traffic signal system

LEDs in a traffic signal system

TECHNICAL PAPER: pdf logo
A Method for Projecting Useful Life of LED Lighting Systems

Projecting the Useful Life of LED Lighting Systems (2003)

Lighting systems employing light-emitting diodes (LEDs) have become increasingly popular during the last five years. LEDs are now common is systems such as exit signs, traffic signals, and other display-lighting applications. Manufacturers typically quote life estimates for their systems based on the average life of a single LED measured under specific laboratory conditions (commonly 100,000 hours). In reality, rates of LED degradation are much different when these lamps are placed in a system because of packaging and environmental conditions. Current practices for evaluating system life involve time-consuming life tests, which can take thousands of hours to complete. With LED technology evolving quickly, the data from these tests may be outdated before testing is even finished.

Previous research has shown that heat at the p-n junction is the primary cause of light output degradation in LEDs. Therefore, the goal of this study was to determine a non-invasive, quick method of measuring junction temperature—a method that leaves systems operating intact. Based on prior studies showing that peak wavelengths of red LEDs shift with junction temperature changes, LRC researchers chose to investigate whether spectral shift could be used as a non-invasive method to determine junction temperature and, ultimately, degradation rates of LED systems.

EXPERIMENT

This study tested 5 mm epoxy-encapsulated AlGaInP LEDs, a type of red LED, which were operated in a specially designed thermal chamber. During the experiment, LRC researchers raised the junction temperatures of the test LEDs and analyzed the resulting LED spectrum changes. The junction temperature increased by changing the drive current while holding the surrounding ambient temperature steady, and by changing the ambient temperature while holding the current at a constant rate. The peak wavelength shift was measured and compared to the measured junction temperature.

CONCLUSIONS
  • The peak wavelength of a red LED shifts proportionally to changes in junction temperature, regardless of how the heat is generated.
  • This linear relationship can be used as a direct measure of junction temperature in commercial red AlGaInP LEDs. The average junction temperature can be found by measuring the peak wavelength shift and the initial room temperature.
  • Once the relationship between junction temperature and light output degradation rate is understood, the degradation rate of these LEDs can be easily predicted by non-invasive spectral shift measurements.
Peak wavelength as a function of LED junction temperature for a 5 mm red AlGaInP LED.

Peak wavelength as a function of LED junction
temperature for a 5 mm red AlGaInP LED.



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