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Author Topic: Are high efficiency incandescent lights possible?  (Read 20851 times)

Offline Anders Hoveland

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Are high efficiency incandescent lights possible?
« on: February 02, 2013, 04:13:57 pm »
The advantage of incandescent light is that it puts out a pleasing continuous full spectrum of color frequencies, which so far have been very difficult for LEDs to match. But so far incandescent lighting has been less efficient at converting electricity into light than other types of lighting. Is it possible to increase the efficiency of incandescent technology? Incandescent technology has changed little over the last 100 years. If there were any simple way to improve the efficiency, it would have already been done.

Infrared Reflective Coatings
Halogen IR (or halogen infrared) bulbs increase efficiency by using a coating applied to the inside of the halogen capsule. This coating is designed to redirect some of the infrared energy back to the filament, which results in less additional energy being required to keep the filament hot and producing visible light

click link for diagram of how it works:
This commercially available IR halogen capsule, while currently more expensive, achieves an efficiency of 26 lumens per Watt.
Duro-Test was the first company to offer IR bulbs,,1432584
One of the disadvantages though is that these infrared reflective coatings are usually ultrathin coatings of gold or silver.

One company is already manufacturing IR replacement bulbs. Supposedly these new 50 watt IR replacement bulbs will put out as much light as the old 100 watt bulbs. However, these bulbs still are unnable to meet the 45 lumen per watt mandate, meaning that they too will probably be banned in the USA when the final provision of the current law comes into effect in 2020.

Photonic Crystals
Photonic crystals do not obey the Planck Black body curve:
Guo Chunlei, associate professor of optics at the University of Rochester and his assistant, Anatoliy Vorobyev, used high powered lasers to create nano- and micro-scale structures on the surface of a regular tungsten filament. The tungsten filament is the small thin wire inside the light bulb. In doing this scientists can make the incandescent radiator 40 percent more efficient. The laser can also be used to make the light bulbs brighter and possibly even change their colors.
My understanding of those photonic crystals is that, because of the spacing of the gaps in the lattice, only light that has a shorter wavelength than the gap length can escape. Most of the energy will be radiated from within as light before it has a chance to migrate to the surface and radiate as longer wavelength infrared. The problem, of course, is actually constructing these photonic crystals, since the spacing must be at such a small scale.

What about candoluminescence? Like the thorium mantle used in camping laterns?
Candoluminescence is the light given off by certain materials that, when heated to incandescence, emit a larger proportion of their radiation in the shorter-wavelength visible spectrum rather than infrared, compared to a blackbody at the same temperature. Before electric lighting, "limelight" stage lighting was quite common. It used an oxygen-hydrogen flame to heat calcium oxide to give off a glowing white light. Could tungsten filaments coated with a thorium and cerium oxide coating to increase their efficiency?
 Could coating the tungsten filament with a thorium dioxide coating prevent evaportation of the tungsten? Since thorium dioxide is a ceramic, it is not vulnerable to evaporation at high temperatures close to its melting point.

 With the photonic crystal filaments, would it not be impossible to somehow fill the tungsten lattice structure with translucent thorium dioxide ceramic to prevent evaporation and degredation of the vulnerable fine structure?

Other Materials and Increasing the Temperature of the Filament
I was thinking about the idea of using molten tungsten as the incandescent source, contained within some translucent ceramic.
 Thorium dioxide is a translucent white ceramic with a melting point of 3390 C (3663 K).
 Tantalum nitride is a dark brown colored ceramic with which melts at approximately 3360 C. It is insoluble in water.
 Unfortunately nothing seems to quite match tungsten's 3422 C melting point.
or perhaps someting like the Nernst lamp.

If the filament was immersed in a molten ceramic, it would probably prevent evaporation of the filament so that it could me operated much closer to its melting point.

Who said the incandescent conductor has to be a solid ? Of course, this would create technical challenges. Magnetic confinement might not be out of the question, since Lorentz forces induced by a strong current could keep the molten metal in the shape of a narrow filament. Or magnetic induction could be used to both simultaneously heat the molten metal to incandescence and to levitate it. Such a design would not be too complicated. Here is video of a homemade induction coil levitating a piece of aluminum until it becomes molten:

Tunable Amplification
I had another idea. What about using a transparent conductor (such as indium tin oxide) and heat it to indandescence. It could then act as a sort of laser. By putting semi-reflective coatings on both ends to reflect back the vissible light, rather than the infrared, it would act as an amplifier, increasing the gain in the vissible light radiation, and shifting the Planck black body curve. The transparent conductor would still have to be very hot, but it might not have to be as hot as a tungsten filament because of this shift. The concept would be similar to a tunable laser. By putting reflective coatings on just two sides of the transparent conductor, the light source could even be made directional like LEDs.
I am fairly sure population inversion would not be an issue here. Have you never seen the picture of a heat-insulating ceramic material that has been heated to white hot in a furnace, then allowed to cool. A researcher is holding the cube of this material by two corners with his unprotected fingers, while light is coming out of the sides of the cube. The inside of the ceramic cube is still glowing hot, and the light is making its way out the sides.
 If the conductor is transparent, and the light generated from incandescence, then the conductor will not absorb at any particular frequency. Also, not all types of lasers need to achieve population inversion. In some cases, the absorbing atom immediately decays to a lower excited state, and it is this lower excited state that undergoes the stimulated emission. An example of such a laser is the Nd:YAG that is used in common green laser pointers to create an infrared beam before it is frequency doubled to green.
« Last Edit: February 02, 2013, 04:31:47 pm by Anders Hoveland »