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
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
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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