April 15, 2005, Troy, NY--Scientists at the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute have developed a method known as scattered photon extraction or "SPE" to get significantly more light from white light emitting diodes (LEDs) without requiring more energy.
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.
A research group, led by Nadarajah Narendran, director of research at the LRC, 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. According to Narendran, his group is the first to use the SPE method to improve white LED performance.
"Demonstration of this new 'remote phosphor' concept by Rensselaer's Lighting Research Center is an exciting development for solid-state lighting," said Jeffrey Tsao, principal member of the technical staff at Sandia National Laboratories. "This advance has a number of significant implications, including higher-efficiency extraction of photons."
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. Further research into the SPE technology could result in even higher levels of light output and greater luminous efficacy, according to Narendran.
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.