Lens boosts LED efficiency

June 15/22, 2005

One-fifth of the electricity consumed in the U.S. is used for lighting. Several research efforts are aimed at improve lighting efficiency. One promising area is light-emitting diodes.

Researchers from the Rensselaer Polytechnic Institute have come up with a way to boost light emitted from white light-emitting diodes by as much as 60 percent.

White light-emitting diodes are made from blue light-emitting diodes covered by a phosphor that absorbs blue photons and emits them in the broad spectrum of wavelengths that makes up white light. Usually 60 percent of the photons emitted by the phosphor or scattered back to the diode, where they are reabsorbed. The researchers showed that it is possible to recover many of the back-scattered photons if the phosphor is placed away from the diode portion of the device.

The researchers' scattered photon extraction method involves placing a lens between the diode and phosphor. Because the phosphor layer covers a larger area than the diode, the lens is shaped like an upside-down pyramid. This allows much of the back-scattered light to escape out the sides of the lens.

This increases the amount of useful light the device produces without increasing the power needed; the method increased the luminous efficiency of light-emitting diodes by 30 to 60 percent over commercial white light-emitting diodes, according to the researchers.

The one drawback to the method is that the lens makes the light-emitting diode larger.

The method could be ready for use in commercial products within a few years, according to the researchers. The work appeared in the April 6, 2005 issue of Physica Status Solidi (Extracting Phosphor-Scattered Photons to Improve White LED Efficiency).

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