January 18, 2011 | 4
For phosphor-based light-emitting diodes (LEDs) to continue their current trend of displacing incandescent and fluorescent bulbs, makers of these small semiconductor-based lights are looking to, among other things, continue improvements in energy efficiency, increase longevity and reduce glare.
LEDs are illuminated by the movement of electrons in a semiconductor material, but the common practice of coating the semiconductor with phosphors to produce white light is an impediment to potential improvements. With most phosphors, more than half of the photons generated divert back toward the semiconductor substrate called the die, where much of the light is absorbed and lost, says Nadarajah Narendran, director of research at Rensselaer Polytechnic Institute’s Lighting Research Center (LRC) in Troy, N.Y.
The avoid these problems, researchers have for several years experimented with ways of separating the phosphor from the underlying energy source. Such a layered configuration allows light traveling back toward the die to instead escape through the sides, generating 30 to 60 percent more light output (measured in lumens) and luminous efficacy (lumens per watt of electricity) than typical white LEDs, says Narendran, also a professor in Rensselaer’s School of Architecture, whose LRC research focus is LED performance, packaging and lighting application. "Phosphor is direction agnostic, so when you put the phosphor right on the chip, half the amount of light goes downward and half goes up," he adds.
In 2004, Narendran and his colleagues developed Scattered Photon Extraction (SPE), a method to improve the light output and efficacy of white LEDs by combining optimally shaped optics and placement of the phosphor away from the semiconductor die. "The two main benefits that we saw were the amount of light increased and the longevity of the light," he says.
Intematix, a Fremont, Calif.-based maker of LED phosphors, on Tuesday introduced an LED architecture called ChromaLit, which follows this remote-phosphor approach to produce what the company says is up to 30 percent higher system efficacy.
ChromaLit places a phosphor composite onto a separate substrate that emits white light when excited by a blue LED energy source. In separating the energy source from the light-generating phosphor, ChromaLit enables lighting system manufacturers to keep the phosphor performance stable over a lighting fixture’s lifetime, says Intematix CEO Mark Swoboda. In addition, he adds, lighting manufacturers also have more design flexibility because Intematix’s phosphors are turned from powder into pellets that can be injection-molded into different shapes or into inks that can be used to color plastic disks that cover the blue light source.
One of Intematix’s goals is to have ChromaLit used to make more diffuse LEDs that are more practical for high-volume lighting applications found in the hospitality industry and the residential market. "People in these settings don’t want to see hotspots or glare," says Swoboda, who adds that products based on ChromaLit should find their way to market by midyear.
Images of ChromaLit courtesy of Intematix; image of SPE courtesy of Rensselaer Polytechnic Institute Lighting Research Center
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