Researchers at MIT have shown that by surrounding the filament with a special crystal structure in the glass they can bounce back the energy which is usually lost in heat, while still allowing the light through.
They refer to the technique as ‘recycling light’ because the energy which would usually escape into the air is redirected back to the filament where it can create new light.
“It recycles the energy that would otherwise be wasted,” said Professor Marin Soljacic.
Usually traditional light bulbs are only about five per cent efficient, with 95 per cent of the energy being lost to the atmosphere. In comparison LED or florescent bulbs manage around 14 per cent efficiency. But the scientists believe that the new bulb could reach efficiency levels of 40 per cent.
And it shows colors far more naturally than modern energy-efficient bulbs. Traditional incandescent bulbs have a ‘colour rendering index’ rating of 100, because they match the hue of objects seen in natural daylight. However even ‘warm’ finish LED or florescent bulbs can only manage an index rating of 80 and most are far less.
To design a reflector suitable to the task, the team relied on an idea similar to that used in the anti-reflection coatings applied to spectacle lenses. These coatings are made of thin layers of materials of slightly different refractive indices (that is, light moves at slightly different speeds within them). The layers’ thicknesses and compositions are chosen so as to force light waves to add together or subtract from one another as they are reflected from the various layers. This process of interference is governed by the wavelength of the light in question, so it can affect light of different wavelengths differently. And visible light and infrared light do have different wavelengths (infrared waves are longer).
Engineering a stack of layers that can pass visible light unchanged and reflect infrared—and do so from all of the angles from which the filament radiates—was no easy task. For that, the team used a computer program which was able to learn from its mistakes to work out what material each layer should be made from, and how thick it should be. Their first attempt involved stacks of 90 alternating layers of silica and tantalum oxide, two common coating materials, of varying thicknesses, from 17 to 426 nanometres (billionths of a metre). As they report in Nature Nanotechnology, a pair of such stacks, arranged on either side of the flat filament, led to a bulb that converted about 6.6% of the electrical energy running through it into visible light. That is far better than the 2% or so of a conventional incandescent bulb, and is comparable with low-efficiency LEDs. Their calculations show that adding layers of aluminium oxide to the mix and increasing the number of layers to 300 should increase that efficiency to 40%, far better than even the most efficient fluorescent lights.
SOURCES – MIT, Economist, Nature Nanotechnology