Diode Lasers Produce High-Quality White Light
ALBUQUERQUE, N.M., Oct. 26, 2011 — While LED lighting is widely accepted as the more efficient alternative to the century-old tungsten incandescent bulb, it may soon be dethroned by the diode laser.
Researchers at the US Department of Energy’s Sandia National Laboratories (SNL) were able to produce a high-quality white light using nothing more than a four-color laser source.
The team was curious as to whether or not white light from a diode laser influences the human eye differently than white light generated by an LED. To test the issue, the researchers developed a series of tests, which took place at the University of New Mexico’s Center for High Technology Materials.
Sandia researcher Jeff Tsao examines the setup 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. (Images: Randy Montoya)
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 lightbulb, or by a combination of four lasers (blue, red, green, yellow) tuned so their combination produced a white light.
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 a role in subject choices, but only the lighting itself.
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).
Each participant, selected from a variety of age groups, was asked to choose 80 times between the two changing alternatives. The result was a statistically significant preference for the diode-laser-based white light over the warm and cool LED-based white light, but no statistically significant preference between the diode-laser-based and either the neutral LED-based or incandescent white light.
One of the reasons why these results are so significant is that LED lasers become less efficient when the electrical current feeding them exceeds 0.5 amps. These devices were created to replace light produced by tungsten incandescent bulb technology.
Interestingly enough, the efficiency of diode lasers tends to improve as more amperage is applied to the circuit, producing even more light in situations where their LED-based counterparts falter.
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. Both of these offer an alternative to compact-fluorescent lights and incandescent bulbs.
“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.”
Diode lasers are slightly more expensive to fabricate than LEDs because their substrates must have fewer defects than those used for LEDs. Still, Tsao 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 (See: BMW to Replace LED Headlights with Lasers), 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.
The research was published in Optics Express.
For more information, visit: www.sandia.gov
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