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B Fresnel lens concentrator DIY

  1. Jan 10, 2018 at 11:34 PM #1
    Just running an idea for a diy fresnel lens past ..

    [ In the context that parabolic mirrors have been created by rotating a liquid ] The volume left above
    the parabola is also parabolic, so ...if that volume is used as a mold for casting it should form a reasonable solid lens, at least for concentrating a parallel beam to a point. (?)

    It then follows that a set of concentric cylinders should form a set of shallow concentric ring-shaped parabolic curves, which would be inefficient as a reflection mirror as reflections near the edge are blocked by the edges - but the shape should then act as a good template for casting a transmission fresnel lens (?) In fact a simple rectangular grid should be ok. Eg an ice-cube mold should work at least for some resins.

    So - if anyone wants to try it - the idea would be to partially fill an ice-cube tray with resin, rotate it slowly while it sets, then use as a mold to cast the inverse shape.The inverse shape should then be a diy fresnel (I think ) Does that sound realistic ? (The intent being, the lens use as a fairly crude concentrator, not a high-grade optic ). Could get kind of messy though.
     
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  3. Jan 10, 2018 at 11:46 PM #2

    mfb

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    You'll probably need different parabolas for the different rings in the lens - with well-defined relations. Not impossible, but it doesn't sound easy. And then you have to combine them to a single lens.
     
  4. Jan 11, 2018 at 6:11 AM #3

    sophiecentaur

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    I can't think of a reason why the slope on any rotating ring would be different from the slope of the full paraboloid at that radius. Once equilibrium is reached, wouldn't the hydrostatic pressure on the walls of the ring be the same as the pressure at that radius in the full paraboloid, making the curve the same? To make the best fresnel lens, it would be necessary to get the initial levels in the rings right. You have to minimise the masking of the walls in the direction of the focus so the walls would need to slope 'inwards' so that they are parallel with the rays going to the focus. The sloping sections are 'wasted' parts of the fresnel lens area, though. But that aspect of the design is common to all fresnel reflectors so there would be loads of info about it.
    I just realised; you could probably fill the mould from the inside and the resin would move outwards, filling each ring to the outside rim. You'd need to have a retardant to get the levels right before it sets.
     
  5. Jan 11, 2018 at 6:01 PM #4

    mfb

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    That's not what I said. I was talking about the slopes you need in the lens, and in particular the slope as function of radius of the lens.
     
  6. Jan 11, 2018 at 6:08 PM #5

    sophiecentaur

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    You are right that the focus for each ring would be different by the amount the section is displaced back from its natural position on the full parabola. But if the reflector is chosen to have a long focus this would not matter for an energy concentrator (Low optical quality) and also, a shallow fresnel reflector would be more efficient.
    If the optical quality had to be better then it's true that the different rings should be spun at different rpm. The focussing could be adjusted during manufacture - one ring at a time with a sensor at the required focus point. Later, the reflection would be improved by silvering.
     
  7. Jan 11, 2018 at 7:07 PM #6
    Will your resin be adequately UV stable ?

    Aside from that, you can get inexpensive 'legal-sized' Fresnel magnifier panels for reading aids. If you cannot use them due UV degradation, can you take casts with a UV-stable resin ?? You may have to take a ''negative' cast with silicone,. then cast from that...
     
  8. Jan 12, 2018 at 4:16 AM #7

    sophiecentaur

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    I understood the OP to be referring to a reflector. That would be coated with a reflecting surface so UV stability need not be an issue. Also, an ideal lens would not be a paraboloid so this particular method of manufacture wouldn't apply.
     
  9. Jan 12, 2018 at 4:30 AM #8
    OP wrote, "...but the shape should then act as a good template for casting a transmission fresnel lens ..."
     
  10. Jan 12, 2018 at 7:47 AM #9
    Good points, yes I was thinking of a lens but the mirror would be good as well. The actual polymer for a lens would be a careful choice , wrt UV and heat loss.
    The idea would be to align the light from multiple zones without a precise focus effect within each element. I hadn't thought of using different rotation rates to bring the zone focii into alignment, that could be quite nifty.
     
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