Sandia researcher Jeff Tsao examines the set-up used to test diode lasers as an alternative to LED lighting. Skeptics felt laser light would be too harsh to be acceptable. Research by Tsao and colleagues suggests the skeptics were wrong. Photo: Randy Montoya |
The
human eye is as comfortable with white light generated by diode lasers
as with that produced by increasingly popular light-emitting diodes
(LEDs), according to tests conceived at Sandia National Laboratories,
Albuquerque, N.M.
Both
technologies pass electrical current through material to generate
light, but the simpler LED emits lights only through spontaneous
emission. Diode lasers bounce light back and forth internally before
releasing it.
The
finding is important because LEDs—widely accepted as more efficient and
hardier replacements for century-old tungsten incandescent bulb
technology—lose efficiency at electrical currents above 0.5 A. However,
the efficiency of a sister technology—the diode laser—improves at higher
currents, providing even more light than LEDs at higher amperages.
“What
we showed is that diode lasers are a worthy path to pursue for
lighting,” says Sandia researcher Jeff Tsao, who proposed the
comparative experiment. “Before these tests, our research in this
direction was stopped before it could get started. The typical response
was, ‘Are you kidding? The color rendering quality of white light
produced by diode lasers would be terrible.’ So finally it seemed like,
in order to go further, one really had to answer this very basic
question first.”
In the test setup, similar bowls of fruit were placed in a lightbox with a divider in the middle. In this photo, the bowl on one side was illuminated by a diode laser light and the other was lit by a standard incandescent bulb. The aesthetic quality of diode laser lighting (left bowl) compares favorably with standard incandescent lighting (right). Photo: Randy Montoya |
Little
research had been done on diode lasers for lighting because of a
widespread assumption that human eyes would find laser-based white light
unpleasant. It would comprise four extremely narrow-band
wavelengths—blue, red, green, and yellow—and would be very different
from sunlight, for example, which blends a wide spectrum of wavelengths
with no gaps in between. Diode laser light is also ten times narrower
than that emitted by LEDs.
The
tests—a kind of high-tech market research—took place at the University
of New Mexico’s Center for High Technology Materials. Forty volunteers
were seated, one by one, before two near-identical scenes of fruit in
bowls, housed in adjacent chambers. Each bowl was randomly illuminated
by warm, cool, or neutral white LEDs, by a tungsten-filament
incandescent light bulb, or by a combination of four lasers (blue, red,
green, yellow) tuned so their combination produced a white light.
The
experiment proceeded like an optometrist’s exam: the subjects were
asked: Do you prefer the left picture, or the right? All right, how
about now?
The
viewers were not told which source provided the illumination. They were
instructed merely to choose the lit scene with which they felt most
comfortable. The pairs were presented in random order to ensure that
neither sequence nor tester preconceptions played roles in subject
choices, but only the lighting itself. The computer program was written,
and the set created, by Alexander Neumann, a UNM doctoral student of
CHTM director Steve Brueck.
Each
participant, selected from a variety of age groups, was asked to choose
80 times between the two changing alternatives, a procedure that took
ten to twenty minutes, said Sandia scientist Jonathan Wierer, who helped
plan, calibrate and execute the experiments. Five results were excluded
when the participants proved to be color-blind. The result was that
there was a statistically significant preference for the
diode-laser-based white light over the warm and cool LED-based white
light, Wierer said, but no statistically significant preference between
the diode-laser-based and either the neutral LED-based or incandescent
white light.
Four laser beams—yellow, blue, green, and red—converge to produce a pleasantly warm white light. Results suggest that diode-based lighting could be an attractive alternative to increasingly popular LED lighting, themselves an alternative to compact-florescent lights and incandescent bulbs. Photo: Randy Montoya |
The
results probably won’t start a California gold rush of lighting
fabricators into diode lasers, says Tsao, but they may open a formerly
ignored line of research. Diode lasers are slightly more expensive to
fabricate than LEDs because their substrates must have fewer defects
than those used for LEDs. Still, he says, such substrates are likely to
become more available in the future because they improve LED performance
as well.
Also,
while blue diode lasers have good enough performance that the automaker
BMW is planning their use in its vehicles’ next-generation white
headlights, performance of red diode lasers is not as good, and yellow
and green have a ways to go before they are efficient enough for
commercial lighting opportunities.
Still,
says Tsao, a competition wouldn’t have to be all or nothing. Instead,
he said, a cooperative approach might use blue and red diode lasers with
yellow and green LEDs. Or blue diode lasers could be used to illuminate
phosphors—the technique currently used by fluorescent lights and the
current generation of LED-based white light—to create desirable shades
of light.
The
result makes possible still further efficiencies for the multibillion
dollar lighting industry. The so-called “smart beams” can be adjusted
on site for personalized color renderings for health reasons and,
because they are directional, also can provide illumination precisely
where it’s wanted.
Colorimetric and experimental guidance was provided by the National Institute of Standards and Technology.
The research was published in Optics Express.
This
work was conducted as part of the Solid-State Lighting Science Energy
Frontier Research Center, funded by the U.S. DOE Office of Science.
