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I've got a letter-paper sized fresnel that was like $1 at American Science and Surplus. Making a big glass one might be expensive, but plastic ones (which, adm

I've got a letter-paper sized fresnel that was like $1 at American Science and Surplus. Making a big glass one might be expensive, but plastic ones (which, admittedly, are going to be a bit bad in terms of oil) shouldn't be that much worse than any other formed plastic sheet. Now, the glass ones for lighthouses, those are expensive.

Tade0 4y ago
That's my problem - the small ones are $5 at most, but you cross a certain radius and bam - $10k. I wanted to build a "fake sun" in my apartment, but lenses of proper size were either unavailable or had to be custom made, ballooning the price heavily. Plan B is to use a parabolic dish to collimate the light, but such a setup is pretty awkward.
- dzhiurgis 4y ago
  I still want to make giant parabolic mirror from mylar space blanket
- marcus_holmes 4y ago
  Is there any way of building an array of small lenses that can output the same power? Optics is not my thing, but I'm fascinated at the possibility of a solar-powered forge
  - Gravityloss 4y ago
    I think there is. Furthermore, the machining should be very simple, as the triangles can be all the same shape as the focus doesn't need to be so sharp. The lenses can be angled so that they focus the sun in the same spot.
    - marcus_holmes 4y ago
      thanks, that's interesting. I'm moving back to Australia next year, might give this a try there (where the sun is a little more reliable than here in Berlin)
- bradyd 4y ago
  You should look for old rear projection TVs. The screen in these have a fresnal lens the size of the viewable area.
- cwkoss 4y ago
  All the fresnel lenses i've seen have the focus in the center of the panel, because they only expect you to use one at a time. I would imagine it wouldn't be too hard to make an array of n^2 (9?) fresnel lenses where they each have the appropriate pattern for their position in the array. I think you might be able to get away with three molds for a 9-array (middle, side, corner). The resulting lenses would be much cheaper to manufacture and ship because shipping a stack of 9 6-ft squares is much easier logistically than shipping an 18ft square.
femto 4y ago
Looking at the video posted elsewhere in the comments, I'd say he made his own lens by buying a large rectangular sheet of acrylic and using a (homemade?) lathe to machine correctly shaped groves on it.

Tade0 4y ago

That's my problem - the small ones are $5 at most, but you cross a certain radius and bam - $10k. I wanted to build a "fake sun" in my apartment, but lenses of proper size were either unavailable or had to be custom made, ballooning the price heavily. Plan B is to use a parabolic dish to collimate the light, but such a setup is pretty awkward.

dzhiurgis 4y ago
I still want to make giant parabolic mirror from mylar space blanket
marcus_holmes 4y ago
Is there any way of building an array of small lenses that can output the same power? Optics is not my thing, but I'm fascinated at the possibility of a solar-powered forge
- Gravityloss 4y ago
  I think there is. Furthermore, the machining should be very simple, as the triangles can be all the same shape as the focus doesn't need to be so sharp. The lenses can be angled so that they focus the sun in the same spot.
  - marcus_holmes 4y ago
    thanks, that's interesting. I'm moving back to Australia next year, might give this a try there (where the sun is a little more reliable than here in Berlin)
bradyd 4y ago
You should look for old rear projection TVs. The screen in these have a fresnal lens the size of the viewable area.
cwkoss 4y ago
All the fresnel lenses i've seen have the focus in the center of the panel, because they only expect you to use one at a time. I would imagine it wouldn't be too hard to make an array of n^2 (9?) fresnel lenses where they each have the appropriate pattern for their position in the array. I think you might be able to get away with three molds for a 9-array (middle, side, corner). The resulting lenses would be much cheaper to manufacture and ship because shipping a stack of 9 6-ft squares is much easier logistically than shipping an 18ft square.

femto 4y ago

Looking at the video posted elsewhere in the comments, I'd say he made his own lens by buying a large rectangular sheet of acrylic and using a (homemade?) lathe to machine correctly shaped groves on it.