Will new tech revive incandescent bulbs?

A technological breakthrough might just usher traditional lightbulbs to enter into a revival.

Due to regulations that aim at improving energy efficiency, more efficient compact fluorescent bulbs (CFLs) and newer light-emitting diode bulbs (LEDs) are penetrating the market, while the incandescent bulbs are being phased out.

Incandescent bulbs work by heating a thin tungsten wire to temperatures of around 2,700°C. That hot wire emits what is known as black body radiation, a very broad spectrum of light that provides a warm look and a faithful rendering of all colours in a scene.

But these bulbs have always suffered from one major problem: More than 95% of the energy that goes into them is wasted, most of it as heat. That's why country after country has banned or is phasing out the inefficient technology. Now, researchers at MIT and Purdue University may have found a way to change all that.

Figure 1: Light recycling is the recycling of energy by taking in the unwanted, useless wavelengths of energy produced by the metal filament in the bulb to convert them into the visible light wavelengths. (Source: MIT)

Light recycling

The key is to create a two-stage process, the researchers report. The first stage involves a conventional heated metal filament, with all its attendant losses. But instead of allowing the waste heat to dissipate in the form of infrared radiation, secondary structures surrounding the filament capture this radiation and reflect it back to the filament to be re-absorbed and re-emitted as visible light. These structures, a form of photonic crystal, are made of Earth-abundant elements and can be made using conventional material-deposition technology.

That second step makes a dramatic difference in how efficiently the system converts electricity into light. One quantity that characterizes a lighting source is the so-called luminous efficiency, which takes into account the response of the human eye. Whereas the luminous efficiency of conventional incandescent lights is between 2 and 3%, that of fluorescents (including CFLs) is between 7 and 15%, and that of most commercial LEDs between 5 and 20%, the new two-stage incandescents could reach efficiencies as high as 40%, the team says.

The first proof-of-concept units made by the team do not yet reach that level, achieving about 6.6% efficiency. But even that preliminary result matches the efficiency of some of today's CFLs and LEDs, they point out. And it is already a threefold improvement over the efficiency of today's incandescents.

The team refers to their approach as "light recycling," says Ilic, since their material takes in the unwanted, useless wavelengths of energy and converts them into the visible light wavelengths that are desired. "It recycles the energy that would otherwise be wasted," says Soljacic.

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