low-cost LED-based "one sun" light source

Hi,

Ok what about this: if you have a 4000K black body and filter out all emitted frequencies below 0.5um, then illuminate a 5000K black body with the filtered 4000K black body light (composed of wavelengths shorter than 0.5um) this will increase the 5000K black body temperature! :)

info based on this graph of black body radiation:

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cheers, Jamie

Reply to
Jamie M
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Hi,

I think this is the most useful page:

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The graph at the top of the page shows the thermal equilibrium for different black body temperatures, if you disrupt that equilibrium by illuminating a black body with a light source that doesn't match this exact thermal equilibrium (apparently universal to all matter) then you are pumping in extra energy at whatever frequencies have intensities greater than a thermal equilibrium source would have. In that case it is theoretically possible to raise the temperature to infinity using a non equilibrium source I think, as long as there is excess light at any frequency, it will rebalance to a new thermal equilibrium based on how much excess light there is at a given frequency.

ie even an infrared laser can be used to illuminate a black body and create a UV source from the black body if the infrared laser intensity is high enough, it will create a thermal equilibrium in the black body to match the infra red intensity as shown on that graph, but the infra-red laser intensity would have to be high.

cheers, Jamie

Reply to
Jamie M

Hi,

If you have a black body that has light from a 1000K black body focused onto its surface, at arbitrary intensities 1x, 100x, 1000x, etc, eventually the black body temperature will reach 1000K (ie say at

1000x), and then according to you guys if 1001x focused light is used, the temperature will not increase further. Do you also think that the black body will emit the same intensity of radiation at 1000x and 1001x? I think the black body temperature will remain the same but it will still give off more radiation at 1001x compared to 1000x.

At some point, say 10000x focusing on the black body, the black body should eventually not be able to emit/reflect radiation anymore, as the emitted intensity should go up linearly with the focused light on the black body. No mirror is 100% perfect, so the black body will absorb some extra energy even if it becomes reflective.

Basically I think if you focus enough radiation from a thermal equilibrium source of any temperature above absolute zero on an object you can vaporize it, as long as the black body boiling point is below infinity :D

cheers, Jamie

Reply to
Jamie M

huh? if you do that the 4000k black body isn't black any more.

Likely the filter will get hotter with heat contributed by the two black bodies.

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Reply to
Jasen Betts

from wiki:

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"A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic radiation called black-body radiation"

So my idea is to filter out the portion of that emitted spectrum from the 4000K black body that is longer than 0.5um wavelength, and then illuminate the 5000K with the resultant light.

The filter could be right at the 4000K black body source, and then a lens could focus the output from the filter onto a microscopic

5000K black body (which wouldn't emit significant heat to effect the filter or 4000K black body)

cheers, Jamie

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Reply to
Jamie M

Pay attention to what Phil Hobbs said: there's heat flow BOTH DIRECTIONS to be considered, you cannot just 'illuminate ...a black body with' something, the black body radiates heat back. The outflow kills the 'increase' you describe.

Reply to
whit3rd

Hi,

The outflow is heat, so how much heat can a finite size black body made of matter radiate before something has to give?

cheers, Jamie

Reply to
Jamie M

No. It will just make it slightly easier to maintain the 5000K body at that temperature. The two bodies will each end up with a slightly hotter spot on the sides facing each other. Everywhere else they are losing heat to infinity and a 4K background radiation.

This configuration is relatively common in close binary stars. Things get interesting when one ages to the point where it becomes a red giant and mass gets transferred on to its smaller companion. Then it really does end up with a hot spot where the mass crashes down onto the stellar surface and they are referred to as cataclysmic variables.

You can play games with modern designer techniques to make something which is black in the thermal IR but a good visible and near IR mirror.

Such a device can be made good enough to actually cool when placed outdoors in full sunlight. It reflects away most of the incident light in the 5000K radiation but is an efficient black body radiator at 300K.

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Whether or not they can be made suitable for building is debatable.

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Regards, 
Martin Brown
Reply to
Martin Brown

Depends on what is holding it together. The standard stability equation for stars balances the out going radiation pressure at the surface of the star against the gravitational force on a proton. Since the proton would be accelerated away from the surface otherwise.

Pretty much like applying the fixed wing aircraft equations to a bee you can show that based on the stellar stability equilibrium equation that humans should fly apart due to radiation pressure!

That is our self gravity is insufficient to hold us together at 310K - luckily for us we are actually held together by the much much stronger force of electromagnetism and chemical bonds rather than feeble gravity!

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Regards, 
Martin Brown
Reply to
Martin Brown

You seem to have a serious misconception here.

The best you can ever hope for with a 1000K thermal source is to immerse the object inside it when it will see 1000K thermal radiation in every direction and heat up until it matches its environment. It doesn't matter if you do this by physically putting it in there or configuring perfect mirrors around it to reflect the light onto it.

We actually had a slightly insane device for turning a wet mist into a drier one to avoid cooling a plasma unduly. It was a carefully constructed elliptical tube mirror with the borosilicate pipe containing the atomised spray at one focus and a fire bar at the other.

You can't do any better than surrounding the black body entirely with radiation at 1000K - you only have 4pi steradians to play with!

If you could you would have a perpetual motion machine of the first kind. Basically you have not understood thermodynamics at all.

If things worked as you believe then with two such passive amplifier devices you could make something that got hotter and hotter forever.

The best you can hope for is to immerse the target object inside the source object so that in every direction it is exposed to thermal radiation at that temperature. Heat transfer balances out at when they reach the same temperature with exactly as much leaving as arrives.

The only get out a jail free card that I know of uses total internal reflection inside a shaped diamond stylus to get a ridiculous flux out of the very fine tip. But even then it is just about equivalent to 2pi steradians of the radiation being used to illuminate it.

If you allow yourself the luxury of high power non-thermal coherent laser sources then it is quite easy to focus them to make a plasma.

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Regards, 
Martin Brown
Reply to
Martin Brown

the best "sun-like" (highest CRI) LEDs I've used are from Soraa. They use the 405nm GaN from Shuji Nakamura (co-founder of Soraa).

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We relamped our viewing room with them, replacing all the 50W "4700K" quartz-halogen+dichroic-filter bulbs. The LEDs are much more consistent and uniform, even without any lenses.

Regards, Rich S.

Reply to
Rich S

Hi,

That is constructed very similarly to the thermal metamaterials used for thermophotovoltaic (to restrict the emission to a narrower spectrum)

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quote from that page: "nanoscale layers of tungsten and hafnium oxide"

And a quote from the page you linked: "It is made of seven layers of silicon dioxide and hafnium oxide on top of a thin layer of silver. These layers are not a uniform thickness, but are instead engineered to create a new material. Its internal structure is tuned to radiate infrared rays at a frequency that lets them pass into space without warming the air near the building."

cheers, Jamie

Reply to
Jamie M

Hi,

You can increase the amount of radiation at 1000K on the central black body, just by increasing the size of the surrounding sphere and focusing the larger surface area onto the central black body.

The central black body might remain at 1000K unless it suffers a mechanical breakdown from the excess radiation pressure as you mentioned before.

What if you have a 1000K source as large as the sun and focus it all down to the size of a 1cm black body? The 1cm black body will have to radiate the full heat of the 1000K source or else eventually will build up excess energy and vaporize right?

I find it hard to believe there is an arbitrary limit to temperature of

1000K in that case, most likely for a black body made of matter I think the black body would quickly vaporize and turn into an expanding gas, giving off X-rays etc, equivalent to a much higher temperature, if not undergo fusion from the radiation pressure.

cheers, Jamie

Reply to
Jamie M

No the size of the outer sphere has no effect. if you make it bigger more photons miss the inner sphere, that's all.

focusing? impossible to do without blocking some of the outer sphere, so, no net gain.

at reasonable size scales Van Der Waals forces should be enough to keep things together.

how would you do that? (that seems to be impossible)

Learn, and understand, the three laws of thermodynamics.

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Reply to
Jasen Betts

Trying just one more time for luck. You can't focus a thermal source the size of the Sun down to a centimetre. Did you make any effort to understand the thought experiment I talked about? Here's my post from 2004 in sci.optics:

general flavour.

Instead of trying to think up complicated scenarios, how about analyzing the above actual argument? I'll stay with you if you're actually willing to make an effort.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

to a centimetre. Did you make any effort to understand the thought experiment I talked about? Here's my post from 2004 in sci.optics:

Hi,

Thanks I searched but wasn't able to find it before, I read it now but I prefer to use though experiments rather than math anyway hehe (but thanks for posting it)

Can you focus a thermal source down to a smaller size (extreme case sun to 1cm) if you accept massive losses? I think the answer is YES. ie. simple example focusing with a magnifying glass on earth to a 1cm spot or a more complex example, surrounding the sun with the same small magnifying glasses, with their focal points coupled to individual fiberoptic cables and routed/spliced together to aim at a suitable sized target. Of course the output energy of the fiberoptics will approx zero energy, as the losses will approach 100% in the very long cables, but still this is focusing the thermal source down to a given size, just with huge losses.

If somehow these losses could be reduced, then the energy of the sun could be routed to a given location in space rather than have it expand into space, but still there would be losses proportional to the technology used.

Imagine a high tech optical solution to focus the sun's energy that had only 99.99% losses to focus all the sun's energy to a disc that was 1km in diameter.

So 0.01% of that is 3.846 x 10^22 watts

spread over 1km disc, (785398 square meters), that is

48968803078184563 watts per square meter on the surface of the 1km disc. (that's about 48 million trillion watts per square meter)

So with that much energy per square meter, can the surface of the disc stay below the temperature of the surface of the sun? I am not sure but I really doubt it is possible! :D

The only other question is can there be some futuristic passive optical routing that can route 0.01% of the suns energy to a 1km disc.. I would think it is theoretically possible, given that it is accepted that photovoltaics in ie a Dyson sphere could do this, and electromagnetics, plasmonics, electrons, etc are all just forms of energy, so I see nothing special to say why electromagnetic energy from a thermal equilibrium source can't be focused to create higher temperatures, even if it requires an intermediate non-optical stage or something to help route the energy, that could still be considered a passive system possibly, which is kind of what my argument was originally, using second harmonic generation ideas to increase the frequency of the source etc.

cheers, Jamie

I'll stay with you if you're actually willing to make an effort.

Reply to
Jamie M

actual argument? I'll stay with you if you're actually willing to make an effort.

Hi,

The conservation of entendue says you can't use optics concentrate the entire emitted radiation to a smaller area than the original emitting area. That makes sense, but still you can take all the light emitted and concentrate it (with losses) to an area smaller than the source.

This smaller area that the light is concentrated onto can have a higher watts/m^2 than the surface of the source too.

cheers, Jamie

Reply to
Jamie M

how much of the sun's light does that focus?

ignoring the possibility of a gravitational collapse, the cross section of all those fibres (added together) would be the surface area of the sun (or more)

such a thing cannot be done.

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Reply to
Jasen Betts

That's not enough. The outgoing radiation is not fully intercepted, unless you grab a full 2pi steradians (watts/m^2/steradian is the efflux). The entire emitted radiation is more than the part that was shining in the direction that reached your lens. You can focus to a smaller image, but that only keeps it balanced, after correcting for illumination angular coverage.

Reply to
whit3rd

In other words, No, you aren't willing to make the effort. Suit yourself.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

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