Alternate Packaging Improves White LEDs
Daniel S. Burgess
Various techniques for boosting the performance of white LEDs are being considered to reach the target of luminous efficacy of 150 lm/W by 2012. Investigators at Rensselaer Polytechnic Institute’s Lighting Research Center in Troy, N.Y., have reported that moving the phosphors used to down-convert the photons from a blue LED away from the die yields devices with overall performance 30 to 60 percent better than commercial products.
Most commercial LEDs produce white light by down-converting some blue photons to yellow and mixing them with other blue photons in the desired proportion. Typically, a GaN die is capped with an epoxy resin that contains YAG:Ce. The problem with this approach is that some percentage of the light is scattered back toward the die, where it is absorbed.
To quantify this effect, the researchers placed phosphor-coated glass plates between a pair of integrating spheres, excited the YAG:Ce using a blue LED and analyzed the output from the spheres. They found that approximately 60 percent of the light was backscattered at a phosphor density required to create suitable white light.
They therefore developed a package design that they call scattered photon extraction, in which they replaced the epoxy resin with an optical element that is designed to capture the backscattered light. Only the far side of the element is coated with YAG:Ce, so most of the white light that reflects from the phosphor layer still can escape around the die.
To assess the design, they compared the performance of six commercial 3-W white LEDs and six commercial 3-W blue LEDs of a similar type, which they converted into white emitters with the new package design. The commercial white LEDs displayed an average luminous flux of 56.5 lm and an average luminous efficacy of 22.6 lm/W. The converted devices, however, displayed an average flux of 90.7 lm and an average efficacy of 36.6 lm/W. At low currents, the efficacy of the new devices was more than 80 lm/W.
The researchers note that the design must be optimized to produce spatially uniform white light. The size of the scattered photon extraction package may make the design unsuitable for some purposes, but they do not consider this to affect its potential in general lighting applications.
Physica status solidi (a), May 2005, pp. R60-R62.
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