You said in a pm a while back "what we really need is a lenicur/fresnel screen to inhanse the brightness (of focus it)."
What type of fresnel is this? What is its function?
Full Version: Joechevvy
The lenticular RPTV screen is a diffusion screen with linear parallel grooves in it and the peaks painted black for contrast. There are newer designs of lenticulars with higher viewing angles, but I have little experience with them. Check some of the links from google here for more information.
You'd probably think this is a ridiculous idea and you'd be right but I was thinking of how to make a fresnel.
Obviously the groves would be much fatter, but it still might work well.
The idea is to get a large sheet of plexiglass and set it up so its perfectly flat, then pour a thin layer of melted wax on it, the wax will most likely cool in somespots before it is all poured and leveled so you might need to shine a heat lamp on it to remelt the wax. Like a halide or a common reptile heat lamp with a dome reflector.
Then perhaps is if knew the pattern of the groves in the fresnel make a knife edge out of sheet metal, the exact shape of the groves, then using a rotating mechansim, slowly rotate this knife across the surface of the cooled wax, slowly etching out the grooves. This would obviously require multiple knives each to carve out the particular angle if this type of fresnel is not regular.
The drawbacks of this idea are obvious. Getting the wax to flatten out all the way is a tricky buisiness. The edges of the sheet metel would be hard to cut accuratly and the tips might bend when cutting. While wax carving can be done, it might not be the cleanest cut.
Obviously the groves would be much fatter, but it still might work well.
The idea is to get a large sheet of plexiglass and set it up so its perfectly flat, then pour a thin layer of melted wax on it, the wax will most likely cool in somespots before it is all poured and leveled so you might need to shine a heat lamp on it to remelt the wax. Like a halide or a common reptile heat lamp with a dome reflector.
Then perhaps is if knew the pattern of the groves in the fresnel make a knife edge out of sheet metal, the exact shape of the groves, then using a rotating mechansim, slowly rotate this knife across the surface of the cooled wax, slowly etching out the grooves. This would obviously require multiple knives each to carve out the particular angle if this type of fresnel is not regular.
The drawbacks of this idea are obvious. Getting the wax to flatten out all the way is a tricky buisiness. The edges of the sheet metel would be hard to cut accuratly and the tips might bend when cutting. While wax carving can be done, it might not be the cleanest cut.
Most are cast or extruded. I don't see any advantage in making your own low quality fresnels when they are already cheap.
Do they come cheap in 5 foot diagonal pieces?
I get old lenticulars for free locally. 
QUOTE (Hyper Smiley @ May 1 2005, 04:18 PM)
I get old lenticulars for free locally.
From old TV's?
Mikau's idea reminded me of something I read a few months ago. It talks about various ways of how to manufacture a spiral fresnel, and one of them is very similar to the diagram above.
http://brainwagon.org/archives/2004/05/22/246/
http://brainwagon.org/cgi-bin/axs/ax.pl?/f...snel_spiral.pdf
http://www.google.com/search?q=spiral+fresnel+reflector
Yeah, it's easy to get old rear projection tv's for nothing. Anyway, that's pretty interesting of using fibonacci instead of concentrics.
Every angle is the sum of the two angles before it? Is that the formula for the fresnel?
How is the quality of an old rptv screen with the lumenlab projector?
I wonder if you could use 2 rear projection tv screens. I'm not sure what the edges are like so I don't know if you could join them.
The old lenticular RPTV screens have horrible viewing angles. With a little work you could join them. It's hard to find a good cheap rear projection screen that can match front projection.
Well my guess would be that those screens have a collination fresnel designed for a projector at a specified distance. Which is probably not the distance required for the lumenlab projector.
All rear projection screens work at nearly any distance. Although small lenticulars are low resolution. A collimating fresnel doesn't factor in to it's functionality. It's more a matter of finding a RPTV screen material that's cheap, high contrast, highly transmissable, and has a wide viewing angle. It's pretty bad when a professional RPTV screen manufacturer gets outperformed by a cheap shower curtain. 
I think the manufacturers charge so much because they can being a luxury item. :angry:
Our best bet is to get samples from every film manufacturer to test. I'm thinking microperforated films would work quite well. Right now I haven't had the time to mess with it.
I think the manufacturers charge so much because they can being a luxury item. :angry: Our best bet is to get samples from every film manufacturer to test. I'm thinking microperforated films would work quite well. Right now I haven't had the time to mess with it.
Those things just seem to be a fuzy material that are meant to deflect light. I think it needs to be a fatter stiffer material designed to deflect light to a specific range, not just wildly all over.
Yep, that's exactly what we need. A non fuzzy material of fat (like white shortening) smeared all over a highly aerated concrete wall to give us the narrowest viewing angle possible. You're a genius! We're still discussing rear projection lenticulars aren't we? 
We're still discussing rear projection lenticulars aren't we?
lol. I think you missinterpreted my meaning.
Actually I have no idea what you meant by any of it. "those things" Are we still discussing rear projection screens?
professional rear projection materials as upposed to rear projection screens.
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