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Tuesday, November 22, 2011

Lasers could offer alternative to LED light sources

BMW has demonstrated laser-based headlights as an alternative to LED-based systems, while Sandia has shown that lasers might become a viable light source for general illumination.

Two recent announcements, from Sandia Labs and car maker BMW, have suggested that lasers could ultimately provide a more-efficient alternative to LEDs in certain lighting applications. Sandia conducted experiments to evaluate whether a four-color laser system could provide a viable white-light illumination source. Meanwhile, BMW has demonstrated a prototype laser-based headlight system on its i8 Concept car (Fig. 1) that uses blue lasers and phosphors.

The Sandia experiment combined the output of four laser colors – blue, red, green and yellow – to create a white light source that was compared with other light sources, including incandescent and LED-based sources. The results were reported in the OSA’s Optics Express journal (see details below).

Intuitively, one would not expect that lasers would be an ideal light source. One reason is the extremely narrow spot size: imagine trying to illuminate a kitchen surface with a white version of a laser pointer. Obviously, this can be addressed with a suitable optical system (see Fig. 2, which shows Sandia’s experimental setup). The same applies to optical safety issues caused by the intense laser beams.

Another factor is the extremely narrow linewidth of the laser sources. Four very narrow peaks at red, blue, green and yellow wavelengths would not be expected to result in good color rendering, compared to much broader peaks from LED sources, or the continuous spectrum of sunlight.

“Before these tests, our research in this direction was stopped before it could get started,” said Sandia researcher Jeff Tsao, who proposed the comparative experiment. “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.”

Sandia’s experiment asked volunteers to state their preference for objects illuminated by different light sources. The laser-based approach fared quite well, and was certainly not terrible. “What we showed is that diode lasers are a worthy path to pursue for lighting,” said Tsao.

More specifically, there 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.

The results don’t go much beyond suggesting that lasers shouldn’t be ruled out as a potential illumination source. There is no immediate threat to LED-based lighting from a four-laser system, since the performance of different-colored laser diodes varies widely. In a Sandia press release, Tsao suggested that blue and red diode lasers might be used in combination with yellow and green LEDs.

Lasers in cars
Another approach, which is the one taken by BMW, is to use blue lasers to illuminate a remote phosphor system, generating white light (Fig. 3). The idea of laser beams shooting out from car headlights could create some concerns with optical safety, but of course the optical system prevents direct viewing of the laser source. “Safety is a key consideration in the development of laser lighting for use in passenger cars. For BMW, the complete eye safety of this technology for all road users and its complete reliability in day-to-day use have top priority,” said a BMW press release.

Efficiency is a key factor for BMW, which quotes figures of around 100 lm/W for LED lighting compared with 170 lm/W for laser lighting (there is no indication whether these figures refer to the system or component level).

Size is also a factor, since laser diodes have a length of just ten microns while LEDs typically measure 1x1 mm. Even so, BMW says that its engineers have no plans to radically reduce the size of the headlights. Instead, the thinking is that the headlights would retain their conventional surface-area dimensions and so continue to play an important role in the styling of a BMW, while the size advantages could be used to reduce the depth of the headlight unit, and so open up new possibilities for headlight positioning and body styling.

LEDs and laser diodes have similar light-generating structures, but while the LED relies on spontaneous emission of light, a laser diode contains mirror structures that stimulate emission within the structure and amplify the amount of light (hence the acronym laser, which stands for Light Amplification by Stimulated Emission of Radiation).

According to Sandia, one advantage of lasers over LEDs is that the efficiency of lasers actually increases at higher currents, compared with the efficiency “droop” that is a common feature of LEDs.


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