Photo courtesy of the Lighting Research Center

Scientists at Rensselaer's Lighting Research Center (LRC) have developed a method known as "SPE" (scattered photon extraction) to get significantly more light from white LEDs (light-emitting diodes) without requiring more energy.

"We have developed a technology based on a new scattered photon extraction (SPE) method that will speed up the progress of solid-state lighting and help secure our nation's energy future," said Nadarajah Narendran, Ph.D., director of research at the LRC. "The new technology dramatically increases light output and efficacy of white LEDs, and could play a fundamental role in the evolution of white LEDs for lighting in homes and offices."
 
Commercially available white LEDs combine a light-emitting semiconductor with a phosphor, a rare earth compound, to produce visible white light. However, more than half of the light, or photons, produced by the phosphor is diverted back toward the LED where much of it is lost due to absorption. This reduces the LED's overall light output.

Narendran's research group developed a method to extract the backscattered photons by moving the phosphor away from the semiconductor and shaping the LED lens geometry. When combined, these changes allow the photons that would typically be absorbed inside the LED to escape as visible light. The new technology is patent pending.

Compared to commercial white LEDs, prototypes of the new SPE LED technology produced 30-60 percent more light output and luminous efficacy — light output (lumens) per watt of electricity. This means more visible light is produced without increasing energy consumption.

The industry has set a target for white LEDs to reach 150 lumens per watt (lm/W) by the year 2012. The new SPE LEDs, under certain operating conditions, are able to achieve more than 80 lm/W, compared to today's typical compact fluorescent lamp at 60 lm/W and a typical incandescent lamp at 14 lm/W.

"As LED components improve in efficiency, SPE will further multiply those improvements and help catapult the industry toward its goal," said Narendran.

According to Narendran, his group is the first to use the SPE method to improve white LED performance. The research was funded by the U.S. Department of Energy and is a collaborative effort with the University of California, Santa Barbara.

The SPE research is published online in the journal physica status solidi (a) and will be published in an upcoming print edition of the journal.

Published April 11, 2005