A redesigned incandescent bulb recycles heat radiation to emit visible light with an efficiency comparable to that of some fluorescent and LED bulbs. The ubiquitous incandescent light bulb is a high-temperature thermal emitter, producing mostly invisible and, in the context of illumination, unwanted IR light. Now researchers from the Massachusetts Institute of Technology (MIT) have designed a bulb that mitigates wasted energy. The proof-of-concept incandescent bulbs are more energy efficient, and elements of their design could also improve the performance of other hot thermal emitters such as thermophotovoltaic devices, wherein external heat causes the material to glow, emitting light that is converted into an electric current by an absorbing photovoltaic element. A nanophotonic incandescent light bulb demonstrates the ability to tailor light radiated by a hot object. Courtesy of Ognjen Ilic. "Light radiated from a hot object can be quite useful, whether that object is an incandescent filament or the sun," said postdoctoral researcher Ognjen Ilic. Thermal emitters operating at moderate temperatures often feature nanopatterned surfaces that alter emission wavelengths, the researchers said. But at high temperatures — above 3000 K — nanostructures on the surface of the emitter deteriorate. "A 3000° filament is one of the hottest and the most challenging sources to work with," said Ilic. "It's also what makes it a crucial test of our approach." To address the challenge, the team surrounded the hot element with nanophotonic structures that spectrally filter the emitted light, allowing it to reflect or pass through according to wavelength. Because the filters were not in direct physical contact with the emitter, the nanostructures kept their integrity at high temperatures. "The key advance was to design a photonic structure that transmits visible light and reflects infrared light for a very wide range of angles," said 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 they achieved using numerical optimization techniques. The research also involved redesigning the incandescent filament from scratch using a planar filament with a large area, which allowed it to efficiently reabsorb light reflected by the filter. "In a regular light bulb, the filament is a long and curly piece of tungsten wire," said professor Peter Bermel, now at Purdue University. "Here, the filament is laser-machined out of a flat sheet of tungsten: it is completely planar." In addition to energy efficiency, the design achieves near-ideal color rendering. In fact, one reason incandescent bulbs have remained the dominant light source over the years, the researchers said, is because their warm light reproduces surrounding colors in a more appealing way than, for example, fluorescent lights. Ongoing research will examine issues of thermal stability and lifetime. The materials involved, however, are abundant and inexpensive, said professor John Joannopoulos, and the filters are amenable to large-scale deposition. The study was published in Nature Nanotechnology (doi: 10.1038/nnano.2015.309).