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Old Aug 25th 2017, 10:32 AM   #1
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Deck prisms

I'm trying to develop a design for a "deck prism," a cast glass prism that refracts sunlight to illuminate the cabin in a boat. Commercially available deck prisms generally have a flat top surface and a prismatic lower surface, and are mounted in a small opening in the deck: the prism extends down a bit into the cabin to spread the sunlight and illuminate the space (without the prism, obviously, the sunlight would just create a small bright spot).

I am helping a friend refurbish an old boat that's missing its deck prisms, and the old openings are a little too small for any commercially available prisms. I have the ability to cast glass, so I thought I'd design my own prism and have it 3D printed so I can make a mold for casting.

My knowledge of optics is limited to what I can remember from high school physics, though, so I could use some help. The basic problem, I believe, is getting the parallel rays of sunlight to diverge. I'm thinking of developing the prism as a crude Fresnel lens, but I can't figure out if it should be a converging or a diverging lens. My understanding of converging lenses is that the light converges at the focal point of the lens -- but then diverges again. So maybe it doesn't matter which type I develop?

Does anyone have any insights or suggestions for the best way of approaching this problem? The deck is about 2" thick, and the existing prism openings are about 4" diameter at the top surface, and flare out slightly on the interior. The cabin measures (very) roughly 7' x 7' x 7', for whatever that's worth.
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Old Aug 27th 2017, 11:27 AM   #2
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The term prism confused me at first, but having done a quick Google I realise that this is more a generic name for a range of devices designed to disperse light from a small opening in the deck into the wider cabin, rather than necessarily an actual prism.

You would want to avoid a converging lens, since this would create a hot spot at the focus (before subsequently fanning out). You could end up burning the top of your head as you walked through the focus!

I would think that a diverging Fresnel lens should work ok,
but I am only going on gut feel rather than any firm knowledge.
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Old Aug 28th 2017, 09:57 AM   #3
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Oof, great point! Under the right circumstances, a converging lens could even be a fire risk! Thank you for bringing that up; it hadn't occurred to me.
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Old Dec 15th 2017, 08:32 AM   #4
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Be careful and don`t cause a fire. Always keep an eye on your prism.
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