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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.
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]
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Annotation:
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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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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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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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try making one out of plywood and Al foil. |
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I'm thinking about this, and I think there may be an issue.
A front silvered fresnel would work, but I think a back
silvered one probably won't. |
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In a fresnel lens, the light is bent to pass straight out the
back of the lens. In a mirror, it will be coming back out of
the front. So either the light will be bent perpendicular to
the silvering, in which case it will return along the same
path it entered, or it won't be, in which case it has a
significant risk of crossing one of the discontinuities that
make up the reliefs in the fresnel, in which case it will be
scattered in undesired directions. |
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I could be wrong, it's difficult to visualize ray paths for the
the entire mirror, but as [csea] said, back silvering is a lens
in front of a mirror, not the same thing at all. |
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So why does this extremely baked idea have nine croissants? (Mine seems to be the sole [-].) < And no, that is not a new emoticon... |
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//back silvering is a lens in front of a mirror, not the same
thing at all.// |
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Perhaps and perhaps not. I think it could work that way, if
the lens were right. If you put a lens in front of a flat
mirror, the net result is the same as putting two lenses in
series, separated by twice the distance between the lens
and the mirror. (Plus, of course, the light path is folded
back on itself.) |
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So, a regular-lens-in- front-of-a-mirror arrangement can
work, and a Fresnel lens is formally equivalent, so a back-
silvered Fresnel lens should work. |
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//[-].) <// cyclops fishbone? |
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But in a back silvered mirror, the distance between the lens
and the mirror is zero. What happens if you put light
through to identical but opposite lenses with no space in
between? |
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//What happens if you put light through to identical but
opposite lenses with no space in between?// |
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It's exactly the same as putting it through a single lens of
twice the stength. |
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(To visualize: you can get convex lenses which are flat on
one side, or which bulge on both sides; two flat lenses back
to back work the same as the equivalent one-piece bulgey
lens) |
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But the convex on both sides lenses (or concave), if
identically so, only have an optical effect related to the
thickness of the lens, since the two curves cancel each other
out, and any changes occur during path through the glass. |
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Since Fresnel lenses have a minimal and irregular glass
thickness, this will produce a minimal concentration effect
and a distorted path. |
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The two curves don't cancel - they add. |
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Draw it out and trace a half a dozen rays and you'll see that it
works fine. |
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You're right about that, I admit optics is not my strong suit. |
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I still have my doubts about the effectiveness of this. This is
still a lens and reflector in place of a simple reflector. |
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I think the point is that a Fresnel lens can be made for not
too much more than a reflector sheet. Then, all you need is
a flat surface on which to fix it. |
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With a reflector, the material is cheaper, but then you have
a huge problem to make an accurate parabolic structure to
support it. |
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