# Focusing Light - creating a hot zone

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Hi all, I'm looking for some input on an experimental setup. I need to create a hot zone of about 800 centigrade and move a 2x2 inch bronze base through the hot zone. Ideally, the bronze base needs to be in a ambient temperature zone, then as soon as it hits the hot zone it heats up to the 800 or so centigrade.

I initially thought of using a beam of light with a lens. I was thinking about using a 1000W halogen spotlight (8 inch diameter) and then use a lens to focus the light (the hot zone can be pretty small). Does anyone know any relationships of energy (temp) with respect to focal distance and diameter of lens? I was hoping to find a lens that would work for me based on that.

Or any other suggestions for creating a hot zone?

Thanks.

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You will lose quite a bit of that 1000W because it won't be within the numerical aperture of the lens. It's easier to do this experiment using a mode-locked high-power laser but that depends on your budget.

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big concave mirror+sun will do the trick. might take more than a thousand watts.

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Why mode lock the laser?

Anyway, if someone can be bothered to do the arithmetic on the stefan radiation law

http://en.wikipedia.org/wiki/Stefan-Boltzmann_law

then they can see if a 2 inch square at 800C radiates more than 1KW (I suspect it might). If it does then you have no chance with just 1KW.

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Anyway, if someone can be bothered to do the arithmetic on the stefan radiation law

http://en.wikipedia.org/wiki/Stefan-Boltzmann_law

then they can see if a 2 inch square at 800C radiates more than 1KW (I suspect it might). If it does then you have no chance with just 1KW.

It is - it's over 8 kW. But that means you can't heat the whole thing up to that temperature, not that you can't get a spot to that temperature.

edit: this is incorrect - see below

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Can you just run a current through it?.

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Thanks for all the input so far...I'm still getting through my heat transfer class...

It's for a heat treatment process, so I can't run a current through it. The heat treat has to be done in an inert environment, so the sample is enclosed with a He gas environment, which is why I was looking at the focused light. A laser is not in the budget.

What about an RF induction setup or something similar?

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I just re-checked the calculation (8kW seemed rather high) and I apparently had a slip of a digit somewhere. I now get ~375 Watts for the radiated power, which seems much more reasonable. (Assuming emissivity of 1). This also ignores and conduction and convection.

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would using three lamps and three lenses be above budget?

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RF heating seems a much better idea to me than light. Also, He is quite a good conductor, could you just heat the container and let the He carry the heat to whatever?

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I'm not exactly sure how this setup can use a relatively cheap and available lens and huge spotlight which loses alot of its output because all of it can't be focused (its bigger than the lens and divergence is higher than the NA of the lens).

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I'm not exactly sure how this setup can use a relatively cheap and available lens and huge spotlight which loses alot of its output because all of it can't be focused (its bigger than the lens and divergence is higher than the NA of the lens).

There are some larger lenses out there, but of course there is a payoff with focal distance.

Looks like this idea is out, probably go with a resistance or RF furnace set up of some sort.

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