Tests show new kid on block – diode lasers – eventually
could challenge LEDs for home and industrial lighting supremacy.
 |
| 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 by Randy Montoya). |
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 amps. 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," said 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."
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.
 |
| 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 by Randy Montoya) |
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.
The results probably won't start a California gold rush of lighting
fabricators into diode lasers, said 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 said, 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.
back
top
