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New development could lead to more effective lightbulbs

New development could lead to more effective lightbulbs

Due to a technological breakthrough, traditional light bulbs may get a reprieve.

Incandescent lighting and its warm, familiar glow is well over a century old yet can be seen around the world. This is changing, however, as regulations aimed at improving energy efficiency are phasing out, the old bulbs in favor of more efficient compact fluorescent bulbs (CFLs) and newer light-emitting diode bulbs (LEDs), are coming.

The incandescent bulb is an example of a high temperature thermal emitter. It is very useful, but only a small fraction of the emitted light is used: most of the light is emitted in the infrared, invisible to the human eye, and in this context wasted.

Now, in a study published in Nature Nanotechnology on January 11th 2016 (online), a team of MIT researchers describes another way to recycle light emitted at unwanted infrared wavelengths while optimizing the emission at useful visible wavelengths.

While as a proof-of-concept the research group built a more energy-efficient incandescent light bulb, the same approach could also be used to improve the performance of other hot thermal emitters, including thermo-photovoltaic devices.

To showcase this idea, the team picked one of the highest temperature thermal emitters available, an incandescent light bulb. The authors designed nanofilters to recycle the infrared light, while allowing the visible light to go through. The key advance was to design a photonic structure that transmits visible light and reflects infrared light for a very wide range of angles, explains Ilic. Conventional photonic filters usually operate for a single incidence angle. The challenge for us was to extend the desired optical properties across all directions, a feat the authors achieved using special numerical optimization techniques.

However, for this scheme to work, the authors had to redesign the incandescent filament from scratch. In a regular light bulb, the filament is a long and curly piece of tungsten wire. Here, the filament is laser-machined out of a flat sheet of tungsten: it is completely planar, says Bermel. A planar filament has a large area, and is therefore very efficient in re-absorbing the light that was reflected by the filter.

In describing how the new device differs from previously suggested concepts, Marin Soljacic, the project lead, emphasizes that it is the combination of the exceptional properties of the filter and the shape of the filament that enabled substantial recycling of unwanted radiated light.

In the new-concept light bulb prototype built by the authors, the efficiency approaches some fluorescent and LED bulbs. Nonetheless, the theoretical model predicts plenty of room for improvement.

This experimental device is a proof-of-concept, at the low end of performance that could be ultimately achieved by this approach, argues principal research scientist Ivan Celanovic. There are other advantages of this approach: An important feature is that our demonstrated device achieves near-ideal rendering of colors, notes postdoc Ognjen Ilic, referring to the requirement of light sources to faithfully reproduce surrounding colors. That is precisely the reason why incandescent lights remained dominant for so long: their warm light has remained preferable to drab fluorescent lighting for decades.

New development could lead to more effective lightbulbs