Its figures for other sources are: 2-3% for conventional incandescent lights, 7-15% for fluorescent sources including CCFLs, and 5-15% for what it calls “compact LEDs”. That said, there are more efficient LED and fluorescent sources.
At issue is the amount of heat that is emitted by incandescent light sources – around 95% of electrical energy entering an incandescent light bulb is wasted as non-illuminating infra-red radiation and surface heat. However, that that does leave as light is of exceedingly good quality with a sunlight-like continuous spectrum and high colour rendering index.
If the infra-red can be bounced back to the filament to keep it hot, then less incoming energy is required to keep the bulb emitting the same amount of visible light – a more efficient light bulb.
Attempts have been made to bounce infra-red back before – generally with spherical IR reflective structures around more conventional light bulb filaments. Photonic crystal structures have also been built into the surface of incandescent filaments, although these are at the mercy of tungsten migration which alters the surface of hot filaments over time.
How is MIT doing it?
Firstly, this is no longer a light ‘bulb’. Instead it is a flat structure – a planar filament between two sheets of glass, separated by a vacuum. The glass layers are coated with multiple transparent coatings of different refractive index, forming Bragg or Bragg-like reflectors.
Different ‘stacks’ of coatings have been tried and modelled.
It looks like quarter-wavelength stacks will be less effective than stacks of material that give a sinusoidal variation of refractive index through the reflector – so-called rugate filters. Some of the reflectors are also ‘chirped’, suggesting layer thickness varies at different depths in the coating to reflect different wavelengths. Some reflectors use three or four different materials in their stacks.
The 40% efficiency figure is achieved with 300 layers of an optimised reflective stack, according to MIT, while the bare filament comes out at less than 5% efficient and a simple chirped quarter-wavelength stack reflector still fails to beat 10%.
“These structures, a form of photonic crystal, are made of Earth-abundant elements and can be made using conventional material-deposition technology,” said MIT, which also pointed out that similar technology might improve the efficiency of thermo-voltaic converters – IR analogues of solar cells.
The work, to which Purdue University contributed, is reported in Nature Nanotechnology.