news digest ♦ Lasers
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?”
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 by 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 laser diodes (LDs) were composed of the following materials :
Red : 635nm, 800mW maximum power, multiquantum well AlGaInP LD.
Yellow : 589 nm, 500 mW maximum power, sum frequency generation of 1064 nm and 1319 nm from 808 nm LD pumped Nd:YAG.
Green : 532 nm, 300 mW maximum power, frequency doubled 1064 nm from 808 nm LD pumped Nd:YVO4.
Blue : 457 nm, 300 mW maximum power, frequency doubled 914 nm from 808 nm LD pumped Nd:YVO4.
The tests, a kind of high-tech market research, took place at the University of New Mexico’s Centre for High Technology Materials.
Forty volunteers were seated, one by one, before two near-identical scenes of fruit in bowls, housed
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www.compoundsemiconductor.net November/December 2011
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 colour-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.
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
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