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Anders Hoveland

Are high efficiency indandescent lights possible?

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Photonic crystals do not obey the Planck Black body curve:

http://www.sciencene.../id/4290/title/

 

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.

http://www.rochester.edu/news/show.php?id=3385

 

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

 

 

41SchXfUAcL._SL500_AA300_.jpg

click link for diagram of how it works:

http://www.bulbs.com...n-IR-Bulbs.aspx

 

This commercially available IR halogen capsule, while currently more expensive, achieves an efficiency of 26 lumens per Watt.

 

 

I had a third 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.

 

The advantage of incandescent light is that it puts out a pleasing continuous full spectrum, which so far LEDs have not been able to match.

Edited by Anders Hoveland

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Hi Anders, nice to see you!

 

Interesting idea, if I got it: they create resonating small structures that increase towards 1 the emissivity of tungsten, and preferably only at the useful wavelengths. Well done.

 

About visible light produced by any other material, alas... Any decent efficiency for thermal emission in the visible spectrum demands a temperature such that only tungsten works. Any other material has a bigger vapour pressure (even refractive ceramics that melt at a higher temperature than tungsten, alas again) so it can't attain the temperature of tungsten, because of evaporation. Then, temperature determines very strongly if light is emitted in the visible spectrum or in the infra-red, and this influence tends to be stronger than any improvement we can add to the design of the emitting body.

 

Hence the trick with micro-resonators in tungsten is smart. I just wonder how long they last, because tungsten evaporates and a few µm are very little. Does the performance improvement allow to reduce the temperature, and this cooler operation still improve efficiency at identical lifespan? Low-voltage bulbs are better because the filament is thicker, and halogen because tungsten settles back on the filament.

 

I doubt you can put any material other than tungsten (let's forget ITO, this one was just an example) for visible light. But for IR maybe.

 

"Sort of" laser yes, because temperature won't achieve the population inversion needed to achieve gain at stimulated emission. But the resonator of the laser would improve light emission, yes. I wouldn't use metal for the mirrors since such thin layers evaporate so quickly, but dichroic mirrors. Then, at the E field antinodes of the cavity, emissivity increases - but it decreases at the nodes, cancelling out in a big part. So you would better have many small cavities, or a flat single one - and this begins to resemble quantum dots.

 

The other solution to efficiency is non-thermal emission like LED or laser diode, but as you said, their light is still unpleasant.

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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.

 

 

 

About visible light produced by any other material, alas... Any decent efficiency for thermal emission in the visible spectrum demands a temperature such that only tungsten works. Any other material has a bigger vapour pressure (even refractive ceramics that melt at a higher temperature than tungsten, alas again) so it can't attain the temperature of tungsten, because of evaporation.

This is not necessarily true. 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:

 

There are ways to deal with vapor pressure. Any metal that vapourises out could be allowed to condense back to a liquid and be passively reintroduced again, either through gravity or some other means. This would be similar to a refluxing flask in chemistry. We can see how a halogen bulb deals with vapour pressure problems also. Tungsten bromide decomposes back into its constituent elements at high temperatures, allowing the tungsten to crystallise back onto the filament. For more information about this type of chemistry: http://en.wikipedia....tal_bar_process

 

 

temperature won't achieve the population inversion needed to achieve gain at stimulated emission.

This is not necessarily true. If the conductor is transparent. 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.

 

 

I wouldn't use metal for the mirrors since such thin layers evaporate so quickly,

The mirrors do not have to actually be attached to the glowing transparent conductor. There could be some small space, so the mirrors would not get too hot. The mirrors would also probably be wider than the glowing material. Again, this does not have to have the precision of a laser, just some general directionality. Something like this:

 

 

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..ll....lllllllllllllllllllllll...ll

..ll....llllllllllllllllllllll...ll

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Edited by Anders Hoveland

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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.

Edited by Anders Hoveland

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A few ceramics do exist with a melting point that exceeds tungsten. Unfortunately, the limitig parameter is the vapour pressure at some 3300K, which defines the evaporation speed of the emitting part hence its lifetime, and tungsten is the best choice for that. Rhenium would be about as good but more exepensive, and other elements and compounds less good, especially graphite. Sadly, this doesn't suffice to produce Sun-like light (6000K).

 

Most methods trying to catch the evaporated tungsten (or replacement) for reuse have the drawback of catching the emitted light as well. Halogen is one method that works. Thicker filaments, resulting from a lower supply voltage, are one other, often combined with halogen.

 

I searched in this direction for a rocket engine that heats pure hydrogen using Sunlight, and couldn't find a better material than tungsten.

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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.

 

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?

 

 

I found this, that some of you may find interesting:

http://www.lamptech.co.uk/Spec%20Sheets/IN%20WC%20DuroTest%20120-65G30IRC-E26.htm

Edited by Anders Hoveland

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Alas! Ceramics do evaporate. Among all metals and ceramics, the material with the smallest vapour pressure is tungsten, that's why it's used.

 

What might perhaps be possible is use a quite lower temperature, combined with a material (just a coating perhaps) that emits little IR, and compensate the lower temperature with a bigger area. A little bird tells me that this straightforward idea is already considered and abandoned.

 

A different direction would use higher temperatures, but in a gas and for a short duration. Pressure pulses propagating in a (noble) gas can produce a huge temperature that makes them very bright. These lamps are powered by explosives, but maybe a different actuator could let them work continuously. Though, the indirect process shall stay efficient from beginning to end...

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My suggestion would be to first redefine efficiency. I live in Pennsylvania so in the wintertime when the heat of the lightbulb is not wasted it technically is much more efficient than what is rated at. I would also think that a coating that can convert infrared light to visible light would help greatly or possibly a one-way mirrored effect in which infrared light was reflected back to the tungsten filament, but visible light was able to transmit.

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If you know a coating that converts infrared to visible, please tell. Mankind would make good use of it.

 

The mirror for infrared only is one topic suggested here and has been experimented by the industry; it works less well than hoped, for instance because the mirror is directional and needs the filament to stay right at the center of a spherical bulb, so a lot of engineering is needed before this physics produces a better bulb.

 

Time is running out for filament bulbs because fluorecent bulbs are already here and LED improve quickly, so filament bulbs should improve on the short term to stay in the race.

 

Heat from a light bulb is sometimes useful... But not in Summer when one spends 3W electricity to remove 1W heat from a room, and anyway, electric heating is expensive. Have power-saving computers for the same reason.

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I would also think that a coating that can convert infrared light to visible light would help greatly or possibly a one-way mirrored effect in which infrared light was reflected back to the tungsten filament, but visible light was able to transmit.

This is actually possible. I have a book on non-linear optics. Frequency doubling is possible with high efficiencies (>50%). Normally this uses dichroic filters, so I am not sure how this would work with broad range frequencies. A common green laser pointer is an example of this.

 

The basic principle of frequency doubling is not really (very) complicated. In any optics system there will theoretically be some second harmonic generation, although normally this is negligable. However, by amplifying the desired second harmonic and suppressing the original frequency, conversion is possible. To preemptively answer your question, two photons do indeed condense into one, which is not that complicated to understand since the photons are already coupled with the transmitting medium. Indeed, transmittance is actually just absorbance over multiple dipoles (where the wavefuction has not collapsed) and synchronised emittance. (that is why light travels more slowly through glass)

 

For practical frequency doubling devices, it is almost always a mutliple pass optical system.

 

If such a lamp were possible, presumably just the filament unit would be replaceable, while the more expensive optics would be a permanent fixture.

 

Now to go off on a random tangent... spontaneous frequency doubling in a vacuum is not possible because of photon spin conservation, but spontaneous frequency tripling is... (although so negligible it cannot be detected)

Edited by Anders Hoveland

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"Transmittance is actually just absorbance ...and synchronised emittance (that is why light travels more slowly through glass)"

 

=> Beware this explanation, found everywhere because a stupid book teaches it, is false.

You may understand propagation as absorption and emission if you like, and vacuum does it just as good as matter, BUT emission occurs with zero delay.

 

Any delay at emission would imply an attenuation of light, as the delayed wave add less efficiently with the wave not absorbed.

 

Refractive index results from permittivity (and rarely permeability) which is the instantaneous polarisation of matter in the electric field.

 

-------------------------------------------------

 

Frequency doubling, tripling, mixing... is done commonly but only at a field intensity that lets matter behave nonlinearly - something inaccessible to thermal radiation.

 

It's the kind of peak power accessible to YAG lasers, preferably with a Q-switch, whose pulse is <1ns. Works great then.

 

With a continuous wave instead of pulses, it demands a laser to focus light tightly enough that matter (some very specific crystals in fact) behaves non linearly. Say, with much power in a monomode fibre, fine.

 

If using a CW laser outside a fibre, just with focussing optics, it's better to have a pair of resonators -preferably use the laser cavity itself - to increase the field at the fundamental frequency and let only the harmonic out.

Edited by Enthalpy

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