If anyone is good with a lathe and has some decent wood round stock, you might be able to tool out a Fresnel master, and then make a mould from that. It would certainly be easier to work with than foam (you could just sand the wood once it was carved, seal it with a varnish or something and with a little Pam Cooking Spray as a mould release be on your way, rather than having to first cover the foam with a Bondo-like substance, smooth the surface, and then seal it before getting to the Pam Spray stage).
And if anyone is that clever, then this may be of help:
It's the profile of a standard Fresnel lens that I pulled from an old Lighthouse Maintenance Manual I have sitting on the bookshelf. (And I cleaned it up a little as well.) There are dozens of different lens profiles in there, but this one came the closest to what we've seen on the "Anchor Lamp" style: 3 prisms on top and bottom with "mounting flanges" on each end, and a "bull's eye" section in the middle. (Yes, the wall of the lens
is that thick; I've got a 6-inch Fresnel on top of that bookcase; remember, these things were built to take serious heat!)
If someone does cast a mould from a carved wooden lens form like this, keep in mind that you'll need to also insert a "plug" in the center of the mould. So once the "outer" mould is made from RTV rubber or some other moulding rubber, if you don't make a simple cylindrical plug to fill that center then you'll end up with a solid paperweight of a lens. (And don't forget to really lube up that center plug when you cast the lens or you'll never get the plug out without serious drilling.) As to how clear the casting will come out? I have no idea, but I'm sure someone here has used clear resin to cast things and I'll defer that to them (as well as how to polish something like this to get a decently smooth as well as clear finished lens).
Anyway, I hope this is helpful to someone who's planning on making their own lens.
Dino.
