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Many of you know what a fresnel sheet lens is... a piece of clear plastic with grooves cut in it's surface, making it a large focusing lens.
I propose to do something similar, but add a mirrored coating to the non-grooved side of the plastic, making it, in effect, a parabolic mirror.
Once the right
shape of grooves was calculated, and a stamping mold manufactured, these could be mass produced very cheaply -- they wouldn't cost much more than a regular fresnel lens, and large sized ones would certainly much cheaper than any comparably sized regular parabolic reflectors.
The big advantage, of course, of using a fresnel reflector instead of a fresnel lens, is that it can be supported by any simple flat surface, such as a sheet of plywood.
A "solar trough" style fresnel reflector could also be manufactured, possibly onto a continous piece of plastic, and shipped (rolled up!) from the factory to an under construction solar power station. It might be a little less efficient than a traditional glass reflector, but it would very much decrease the capital cost of the power station.
Ridged mirror
http://en.wikipedia.../wiki/Ridged_mirror
[21 Quest, Mar 23 2009]
reflective arrays
http://books.google...&resnum=7&ct=result
[4whom, Mar 24 2009]
The actual idea
http://www.ragareco...rrorPeelFresnel.JPG
[loonquawl, Mar 24 2009]
[link]
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All of the Fresnel reflectors that I found were about arrays of flat mirrors, with the set acting as a parabolic mirror. |
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I found no mention of anything resembling a single sheet of material, with grooves in it. |
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The only Fresnel sheets I could find were lenses. |
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The flat overhead projectors use a reflector beneath a fresnel lens, but a reflectivly coated fresnel lens was not to be googled by me, either. [+] |
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Ridged mirrors are baked, and have been for a long time. Try a Wikipedia search. |
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The linked article has no connection to the idea [goldbb] proposed, the atomic reflection is another thing completely. I found that article too, and owing to the extreme simpleness of the fresnel-mirror idea thought 'got it', too, but then realized this was something else. The link to 'Fresnel reflector' on the wiki page is also no good, as it redirects to fresnel lenses with no metioning of the fresnel reflector... |
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I don't see any real advantage to it. The big thing with fresnel lenses is that they take up a lot less glass than normal. A reflector, however, can be as thin as you like. |
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// add a mirrored coating to the non-grooved side of the plastic, making it, in effect, a parabolic mirror.// |
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Non-grooved side? Does this really make a parabolic mirror? I'd have thought it would make a lens in front of a flat mirror, not the same at all. |
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The Polaroid SX-70 camera had a Fresnel mirror, IIRC. |
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Ah. Well, ok then. That's not my fishbone, after all. |
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If your mirror can be shipped rolled up then surely you could just have a plastic mirror that you can bend to make a trough reflector. |
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The reason no links appear for this splendiforous idea, is that is baked, overbaked and centuries old. You are just calling it the wrong name. Planar reflective arrays, or just plain planar arrays are what they are called. |
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The main advantage of these *arrays* is that the focal point can be adjusted. Making for ideal ship burning weapons, fresnel antennas, etc. |
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I linked an example of what this idea probably is about - [4whom]'s link is to theoretical work on antenna arrays, which, given a wavelenght in the visible spectrum and an implementation as a // a piece of clear plastic with grooves cut in it's surface // would coincide with the idea, but... |
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While it is true that reflectors can be made thin, and in many applications the "flattening" of the space required for the mirror be only a slight advantage compared with the additional light loss such a mirror would cause, I can nonetheless imagine that there would be situations where an inexpensive "flat" focusing mirror could be useful. |
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One significant caveat, though: from an optical perspective, the "ideal" lens would have essentially flat sides and a nearly-infinite coefficient of refraction. The fact that lenses are actually curved means that they cannot focus perfectly at every distance; Fresnel lenses don't have that problem to such a degree. With mirrors, however, the reverse situation would apply: a full curved mirror would be able to focus perfectly at different distances, but a flattened one would not. |
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So many new words, so little time. |
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I suggest trying this with an ordinary Fresnel lens. I'm not sure the ridge pattern would be any different. You can use aluminium foil or reflective cellophane for the mirrored backing, though both tend to get creased, so they're not ideal |
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