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I am working on a project to bring light into a bunker. The bunker is 3-4 meters below the ground.

I am looking into bringing natural light into the space, instead of using electricity. The space is difficult to access and light must therefore be transported by flexible cable for about 20 meters from the surface to the areas I want to light up. I am thinking of pipe around 1 cm diameter maximum.

I am looking into a cheap solution to create an optical pipe that is effective at transporting light.

  1. I am wondering what techniques are typically used.
  2. What kind of pipes would you use ?
  3. Is it possible to fill the pipe with other materials to increase its effectiveness ?
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With those restrictions you will probably be stuck with using fibre optics, which are expensive. Something like the Parans SP3 system for example. Making your own by purchasing fibers is a possibility but still not cheap and there are a variety of problems with actually implementing it (such as focusing enough light into the fiber and controlling heat issues). This is discussed in this Youtube video.

There was a relatively inexpensive option called 'The Light Bandit' which had a Kickstarter campaign which unfortunately has failed.

If you could somehow get a relatively straight run the much more economical sun tunnel type systems, such as Solatube may be a possibility but they usually need a much larger diameter than 1cm to collect any reasonable amount of light.

Some additional information which my be interesting/useful: Before Nightfall: Advanced Daylighting Technologies

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  • $\begingroup$ Second fiber optics. Only thing I can think of that will transfer the light that far below ground and be reasonable "hardened." $\endgroup$ – grfrazee Apr 1 '16 at 14:48
  • $\begingroup$ While looking for the cost of Parans SP3, I came across this paper academia.edu/7278813/… which has a good summary of the options including costs. Parans is about €5000 per unit. $\endgroup$ – pjc50 Apr 1 '16 at 14:52
  • $\begingroup$ Actually i had found Parans too. But the price was the main motivation for looking at a DIY solution. I watched the video but I don't understand what the guy is using ? Glass fiber ? $\endgroup$ – Sylvain Apr 4 '16 at 15:54
  • $\begingroup$ The problem of the Solatube solution is it's a direct tube from roof to ceiling, I thought about this solution but because the place is a former bunker this implementation is not possible. $\endgroup$ – Sylvain Apr 4 '16 at 15:55
  • $\begingroup$ @Sylvain He is using a plastic tube with a water-clear resin in it (perhaps something like: easycomposites.co.uk/#!/resin-gel-silicone-adhesive/…). Probably only realistic to make such a thing in short sections, but could be cheaper than glass fiber cables. $\endgroup$ – atom44 Apr 13 '16 at 10:02
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Considering your difficult geometry, you might look into solar panels above ground, with wires to the underground lights. This has the added advantage that you can store some of the energy in batteries for use when there is no sunlight at the surface.

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You could try adapting the litre of light principle, where a hole is made in the roof of a shed which is then plugged with a re-purposed 1.25 L soft drink bottle filled with water. It provides light in the interior of the shed while the sun is shining.

If a bottle was fitted to both ends of a internally reflective plastic tube and the whole system was filled with water, one bottle could act as the light collector, the tube would transport the light and the bottle at the other end of the tube would disperse the light.

The bottles don't have to be 1.25 L soft drinks bottle, but a proof of concept rig could be constructed cheaply this way. If it proves success then different "bottles" can be used.

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  • $\begingroup$ I cannot do this, the place is a former bunker. $\endgroup$ – Sylvain Apr 4 '16 at 15:58
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You can send your light via internal total reflection. You need to research a flexible material that's hollow inside (to fill with liquid) and the material's index of refraction match the formula for total internal reflection.

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