a better incandescent light bulb

Metal content Tungsten.
http://www.pddnet.com/news-regular-light-bulbs-turned-into-power_sippers-060209 /
Thank You, Randy
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Neat. Sounds like the process would work well for a lot of things. Solar Collectors, stealth airplanes.
Dan
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Randy wrote:

http://www.pddnet.com/news-regular-light-bulbs-turned-into-power_sippers-060209 /
You can increase the intensity and shift the color of any incandescent bulb into the blue region while reducing relative power just by using a thinner filament. That process reduces bulb lifetime however. The following cites indicate that there is a downside to using a laser to thin a filament.
http://en.wikipedia.org/wiki/Incandescent_light_bulb#cite_note-24
"Small variations in resistivity along the filament cause "hot spots" to form at points of higher resistivity; a variation of diameter of only 1% will cause a 25% reduction in service life. The hot spots evaporate faster than the rest of the filament, increasing resistance at that point—a positive feedback which ends in the familiar tiny gap in an otherwise healthy-looking filament."
"In flood lamps used for photographic lighting, the tradeoff is made in the other direction. Compared to general-service bulbs, for the same power, these bulbs produce far more light, and (more importantly) light at a higher color temperature, at the expense of greatly reduced life (which may be as short as 2 hours for a type P1 lamp)."
--Winston <-- Goofed around with light bulbs as a kid.
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wrote:

I understood the article to say not that the filament is thinned, but that its surface is modified to increase emissivity.
I've got a prototype on my desk of some rebuildable lamps I designed and built a couple years ago for a customer who was doing similar research. They disassemble easily so new filaments can be mounted, and have cute plumbing and electrical feedthroughs for thorough purging. The most surprising part of the project, which I think I've mentioned here before, was obtaining a small quantity of getter. One of the ingredients in the getter for incandescent lamps is red phosphorous, which is apparently strictly controlled these days because it's used in cooking meth.
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Ned Simmons wrote:
(...)

Chunlei Guo:
"We fired the laser beam right through the glass of the bulb and altered a small area on the filament. When we lit the bulb, we could actually see this one patch was clearly brighter than the rest of the filament."
Sounds to me as if the modification was just redistribution of filament metal. What would happen when you increase the resistance of a portion of a filament by thinning it? It dissipates more power because it's resistance is higher (P=I^2R) It glows more brightly than the rest of the filament but will cause the bulb to fail much earlier than it would have without the modification, I think. Would Occam be pleased with this guess? :)

That'll be the thing I learned today. :)
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I got the impression that the object was to "roughen" the surface. This would make the tungsten a better emitter. As they said, if you have good control of the size and spacing of the asperities, you will be able to selectively change the emissivity for different colours. If they merely wanted to selectively thin the filament, it would be orders of magnitude cheaper to do it when drawing the wire. Conventional bulb can (and are, for some types) be made more efficient by using a dichroic reflector so that much of the IR gets bounced back to the filament rather than heating the rest of the world.
Mark Rand RTFM
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Mark Rand wrote:
(...)

"Though Guo cannot yet make a simple bulb shine pure blue, for instance, he can change the overall radiated spectrum so that the tungsten, which normally radiates a yellowish light, could radiate a more purely white light."
You can do the same thing without a laser. Just push more power into the bulb and the color temperature will shift upward. Bulb life suffers, though.

OK, but how do I justify this nifty femtosecond pulsed laser then?
Gotta stay focused on the goal here. <VBG>
Creation of that roughened surface means redistributing filament tungsten, yes? Subtract some here, add some there?
I'll bet you a dollar that we don't hear anything from Chunlei Guo or University of Rochester regarding bulb life tests. :)
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wrote:

If the filament has a thin section the power density at that spot will indeed be higher, but the overall consumption of the lamp will be lower. (P=V^2/R) The article says, "... we could actually see this one patch was clearly brighter than the rest of the filament, but there was no change in the bulb's energy usage."
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Ned Simmons wrote:
(...)

Yes, but bulb life is significantly reduced.
Can we really call it a "better light bulb" if we have to replace it ~twice as often? (I don't think so.)
http://en.wikipedia.org/wiki/Incandescent_light_bulb#cite_note-24
"Small variations in resistivity along the filament cause "hot spots" to form at points of higher resistivity; a variation of diameter of only 1% will cause a 25% reduction in service life. The hot spots evaporate faster than the rest of the filament, increasing resistance at that point—a positive feedback which ends in the familiar tiny gap in an otherwise healthy-looking filament."
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wrote:

As I said above, if the experiment created a hot spot by thinning the filament the resistance of the filament, and the power consumed, would change. The experimenters reported there was *no change* in the power consumption.
The relationship between power density, temperature, efficiency and life is well understood -- you can find it in old texts that date back to the early days of electric lighting. It's hard to believe that these guys, whose previous work involved fiddling with the emissivity of metals, are unaware of that relationship and have misinterpreted the results of their experiment.
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Ned Simmons wrote:

In *net* consumption, no change was detected. I get that. I hypothesize that the local power consumption at the thinned area of the filament did increase, because it's increased resistance dropped more voltage across the thinned area for a given amount of current. The increased resistance of the filament as a whole would have decreased the filament current a tiny amount, cancelling the effect of the local power consumption increase.
Model it as two PTC resistors in series, one of which is about a percent of the value of the second one. Triple the value of the smaller resistance and it's power consumption triples. The net resistance of the network as a whole is decreased by a couple percent, causing the second resistor to decrease in value. As a result, current increases once again and a new equilbrium is established which is very close, if not identical to the pre-modification current. The bulb lifetime is significantly decreased but the study did not encompass bulb lifetime, just hotspots and spectral shift.

They didn't misinterpret, as far as I can see. They didn't say a word about the post-modification reliability of the bulb. You could argue that they committed an error of omission about that, if you were feeling very charitable. :)
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wrote:

That's a pretty remarkable claim -- that, at least in the range where a normal lamp operates, the current flowing thru a filament is completely independent of the voltage applied to it. It's also very easy to test, so I did, and it just ain't so. Even using a 3-1/2 digit meter it was easy to observe the current thru a 75W bulb change with <1% changes in applied voltage. The effects of the temperature on the resistance were clearly apparent, but did not entirely compensate for increasing voltage.
V    I    calculated R 119.9    588    203.9 120.8    590    204.7 121.8    592    205.7 122.4    594    206.1

Then I'm not sure what you're claiming. That the researchers have convinced their peers at a major research university, the Air Force Office of Scientific Research, and the referees at Physical Review Letters, but not you, that blasting material off a filament with a laser is a significant achievement?
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Ned Simmons wrote:
(...)

Your model shows a net reduction of voltage available to the bulb. In the experimenter's case, the amount of voltage to the bulb remained the same but it was distributed differently in the filament eg. a 'hot spot' area dissipating significantly more power than it had before it was thinned.
I said that the positive temperature coefficient of the filament would tend to limit the change of the current through the bulb as the power distribution in the bulb was changed by thinning some portion of the filament.
Your numbers show that the variation in current through your bulb was about half the variation in voltage across the bulb. A one volt change across the bulb caused a 2 milliamp change in current. This is the nonlinear positive temperature coefficient variation I was on about.
Had that PTC effect not been in place, we would expect to see a 5 milliamp change in current for a 1 volt change. A new current equilibrium was established that was within 0.35% of the pre-modification current, under a voltage change of 0.82%.
(...)

I'm not claiming much of anything.
Their claim, IIRC is that one can make a light bulb universally 'better' by exposing it's filament to laser light, because the resulting roughened surface somehow causes it to convert electrical power into visible - frequency photon emission more efficiently than does the smoother un-modified filament, without changing the cross sectional area of the filament anywhere along it's length.
I don't deny that you can shift the average color temperature of the bulb upwards towards blue without increased bulb power by thinning the filament, but I do deny that the effect is produced by anything other than merely thinning the filament.
I also deny that laser beam exposure 'improves' the bulb because it will significantly decrease the amount of time that the lamp remains functional as compared to it's lifetime had it not been exposed.
A bulb design that fails significantly more often than average is not a better design. It is a worse design even if it is more efficient during it's short stay in the socket.

Blasting material off a filament is a parlor trick, not a significant achievement.
I believe Mr. Guo and Mr. Vorobeyv are mistaken at best.
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wrote:

Not my model. It's the one you proposed in your previous post ("Model it as two PTC resistors in series, one of which is about a percent of the value of the second one"), except I moved one resistance outside the lamp's envelope and turned a knob rather than tweaking it with a laser.

And in my experiment, the voltage across the two resistors was also constant.

What you said (in order to explain the experimenters' observation that no change in power accompanied the increase in light output) was that the PTC of the untreated section of the filament would cause the current to restabilize at a point "very close, if not identical to the pre-modification current." For that to happen, the current passing thru the untreated filament must be independent of the applied voltage, and that's clearly not the case.

Exactly. If treating a spot on the filament changed its resistance (as I changed the resistance in series with my filament), the resistance of the adjacent untreated filament would change in the opposite direction, but the magnitude of change would be smaller. In other words, there'd be a net change in the lamp's resistance, which would show up as a change in power.
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Ned Simmons wrote:

But first, you changed R1 to a linear resistance. :) After thinning, it dissipates much more power than it had before.
As power across R1 increases, it's resistance increases nonlinearly. As power across R2 decreases, it's resistance decreases nonlinearly.
I'm not going to do the arithmetic, but it's a PTC series circuit with a constant voltage across it. Is it plausible that net power dissipated by this network remains fairly constant despite the fact that the ratio of power dissipated by R1 is inversely proportional to the power dissipated by R2 over a range of say 3% of P2? I think so.
(...)

I ain't so sure. My cheapo SPICE simulator does not have a PTC thermistor component, else I would ask the computer for an answer.
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wrote:

But the voltmeter was connected only across the lamp, the point being to determine how an unmodified filament behaves when the voltage across it varies in small increments. And for that purpose, as long as the measurements are taken at equilibrium, it doesn't matter whether R1 is temperature dependent or not.
So we know that small changes in the voltage across a lamp result in proportionally smaller, but measurable, changes in current. Now replace R1 (my variable resistor) with a second lamp. Apply the appropriate voltage to the string and note the current. Thin the filament in the second lamp with a laser, or a genie with an angle grinder. As we agreed before, the network will reach a new equilibrium, with the voltage divided according to the new ratio of the filament resistances, such that the voltage across the unmolested lamp is slightly higher than it was before. Consequently, the current in the circuit will have increased a small, but detectable amount, and so has the sum of the power consumed by the two lamps.
Which is contrary to what was reported in the article. In other words, a localized thinning of the filament can't explain an increase in brightness without an increase in power.
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Ned Simmons wrote:

The point remains that:
1) Mr. Guo and Mr. Vorobeyv's claim that they 'improved' a light bulb by subjecting it to laser light does not pass muster.
2) Their explanation that the laser improved lamp efficiency by creating surface features (independent of a reduction in filament cross sectional area) is far less likely than a simple reduction in the cross sectional area of the filament, causing a hot spot.
3) Thinning of the filament will produce: A) Some aggregate spectral shift towards the blue in the hot spot. B) Increased power dissipation in the thinned area. C) Decreased power dissipation in the unmolested area. D) A reproducible decrease in the life of any lamp thus modified.
We can worry this subject to death, but let's please not lose sight of the fact that Mr. Guo and Mr. Vorobeyv have apparently made a technical boo - boo.

It matters bigtime. We should not draw a conclusion based on a circuit in a given state of equilibrium and apply it to a different circuit in a different state of equilibrium.

Slightly lower, yes? The resistance of the thinned portion of the filament will increase, causing more voltage to be dropped across it rather than the unmolested length of the filament. Power shifts.

I disagree. Power shifts away from the unmolested portion of the filament to the thinned portion of the filament.
Is this fun or what? :)
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none of listed places have any relevance to light bulbs in any way at all.
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True, but the article was about changing the emissivity of the filament. Note the article started with how they changed the adsorption of aluminum so that it reflected no light by modifying the surface so that it had grooves with their size in the order of the light waves. Then went on to how they were able to make metals have different colors. So if one modified the surface of the filament so that it would radiate less IR and more visible light, the light bulb be more efficient.
Dan
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snipped-for-privacy@krl.org wrote:

I respectfully disagree. The article was about changing the power distribution ratio and spectral shift in various parts of the filament. Our researchers cleverly implied that changing the thickness of a metal oxide layer (and the resulting emissivity changes) had something to do with the changes in the light bulb but the two effects are quite different and have nothing to do with each other, IMHO.
The filament is in a vacuum. Heating it does not cause it's surface to mix more readily with oxygen because there is no oxygen to speak of, in the vicinity of the filament.

Sure! Anyone who has held a torch to steel has seen how the surface changes the color of reflected light.

Theoretically, I agree. This isn't what our researchers were doing, however.
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