Incandescent bulbs are already banned by many nations, but rumors of their death may, to paraphrase Mark Twain, be grossly exaggerated. An innovation from MIT could give them a new lease on life.
The bulbs, one of Thomas Edison’s most famous inventions, are a technology more than 130 years old. They work by heating a thin wire to around 2,700 °C, causing it to emit black body radiation, a broad spectrum of light that provides a warm look and a faithful rendering of all colors in a scene. But about 95 percent of the energy that goes into these bulbs is wasted, most of it as heat. The new research could finally change that.
Physics professor Marin Soljacic ’96, PhD ’00, postdoc Ognjen Ilic, PhD ’15, mechanical engineering department head Gang Chen, and three others published their solution early this year. The key, they say, is to create a two-stage process.
First, a conventional metal filament is heated. But the waste heat isn’t allowed to dissipate as infrared radiation, as usual. Instead, secondary structures surrounding the filament capture this radiation and reflect it back to the filament to be reabsorbed. These structures, a form of photonic crystal, are made of earth-abundant elements and can be made using conventional material deposition technology.
The desired visible wavelengths of light pass right through this material and out of the bulb, but the infrared wavelengths get reflected as if from a mirror. They then travel back to the filament, adding more heat that gets converted to more light. Since only the visible light ever gets out, the heat just keeps bouncing back toward the filament until it finally ends up as light.
That second step dramatically improves how efficiently electricity is converted into light. Measurements of luminous efficiency, which factor in the response of the human eye, are 2 to 3 percent for conventional incandescent bulbs, between 7 and 15 percent for fluorescents (including CFLs), and between 5 and 15 percent for most compact LEDs. The new two-stage incandescents could reach efficiencies as high as 40 percent.
The team’s first proof-of-concept units already reach about 6.6 percent luminous efficiency, a result that approaches the efficiency of some of today’s CFLs and compact LEDs and is a threefold improvement over today’s incandescents.
The researchers refer to their approach as “light recycling,” says Ilic, since their material in effect fishes “junk”—the unwanted, useless wavelengths of energy—out of the wastebasket and converts it into pleasing, useful visible light.