Recycling light

Conventional incandescent lightbulbs reach the 3000 K needed to emit in the visible range thanks to ohmic heating of a resistive tungsten filament. That process is extremely inefficient at producing light, though, because most of the spectrum of a 3000 K blackbody lies in the IR. Tailoring the spectrum by suppressing all but the wavelengths of interest can avoid the wasted energy. To that end, MIT researcher Ognjen Ilic, his postdoctoral adviser Marin Soljačić, and their colleagues surrounded the filament with interference filters that transmit visible light but reflect the IR. The idea of recycling unwanted IR emission to provide an additional source of heat to the filament isn’t new. Two features distinguish the team’s implementation: First, unlike earlier filters, which were designed for a single incidence angle, the MIT filters are made of thin layers of common oxides—ranging from the low-refractive-index silica to the high-index titania—that are effective over a wide range of wavelengths and angles. Second, the filament is planar, so it efficiently reabsorbs and reemits the reflected radiation; that shape also makes the filament interchangeable with advanced thermal emitters such as photonic crystals. The team’s prototype, shown here, achieved a luminous efficacy—essentially the ratio of visible-light flux to consumed power—of 45 lum/W, roughly triple that of a conventional tungsten filament and approaching some commercially available compact fluorescent bulbs and LEDs. Numerical simulations predict that an optimized stack of 300 layers of four oxides can reach efficacies as high as 270 lum/W, far surpassing that of the best commercial lights. (O. Ilic et al., Nat. Nanotechnol., in press, doi:10.1038/nnano.2015.309 .)