Author Topic: Best shape for fresnel lens...square or rectangular?  (Read 10337 times)

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riversedge

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Best shape for fresnel lens...square or rectangular?
« on: April 04, 2011, 12:27:56 PM »
Hi Everybody,

I yanked a fresnel lens out of a large rear-projection TV today, something I have been looking around for a while. When I was prying the lens from the TV casing, it cracked...the top left quarter of the lens basically broke off, with a further crack extending down the middle of the lens somewhat. So I sort of have an L-shaped solid part of the lens remaining. I figure I can cut this l-shaped piece into two smaller lenses. But I wanted to know what you thought about what shapes I should cut out from this l-shaped piece...should I cut out 2 pieces that are roughly the same square-shaped? Or should I cut one piece so that it is a long, wide rectangle, and then have a smaller square piece from the remainder. I was wondering if a long, rectangular piece would give me a better concentration of the beam then a square piece. Does anyone here have any experiences in what shapes of fresnel lenses work best, square or rectangular?

Thank you

Jake

wooferhound

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Re: Best shape for fresnel lens...square or rectangular?
« Reply #1 on: April 04, 2011, 12:39:56 PM »
I'm not completly sure that there is a fresnel lens in a projection TV or what it's function would be.
I work doing theatrical lighting and have played a lot with fresnel lenses. A broken lens works the same as a full lens, What matters is the area, so bigger is better and shape doesn't matter. It will also work as it did originally by piecing it all back together into one big lens.

zap

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Re: Best shape for fresnel lens...square or rectangular?
« Reply #2 on: April 04, 2011, 01:57:21 PM »
It will also work as it did originally by piecing it all back together into one big lens.

You wouldn't loose much at all by sticking them back together.  Mostly likely some type of acrylic solvent/weld would work best if you want the thing back to the full size.

Theoretically... a round one would probably give better results but a TV won't have a round one... a square shape would probably give better results... but that's guessing on my part.  I've busted a few and they still seem to work (more or less) no matter what shape.

Most of the ones I've removed off of newer TVs are a little more opaque than my oldest lens.  The latter will start asphalt paving smoking instantly and catch it on fire in about 2-5 seconds with clear skies.

MagicValleyHPV

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Re: Best shape for fresnel lens...square or rectangular?
« Reply #3 on: April 04, 2011, 10:53:47 PM »
I posted this elsewhere.

These were purchased from a large experimental project in the early '80s, near the Solar One project just NE of Barstow, Calif. Five individual cells anchored to a stick of 1/2" copper tubing. The front of this aluminum 'wedge' shaped container is capped with plastic Fresnel lens - one lens for each cell. I don't remember what the cell specs are but each cell lens is about 5 maybe 6" square. I have about 75 or so of these units littering my back yard. The problem of course, is that they need to track the sun to be effective.



jaysicle

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Re: Best shape for fresnel lens...square or rectangular?
« Reply #4 on: April 07, 2011, 10:35:14 PM »
I'm not completly sure that there is a fresnel lens in a projection TV or what it's function would be.
I work doing theatrical lighting and have played a lot with fresnel lenses. A broken lens works the same as a full lens, What matters is the area, so bigger is better and shape doesn't matter. It will also work as it did originally by piecing it all back together into one big lens.

Fresnel ('fray-nell') lense columates light evenly from a single point source (lighthouse). Before CFL's and LEDs were used, this is how an even backlight would have been acheived in the TV to its extents. May be used in reverse to concentrate light at a given focal point distance - but not design intent, and potentially on the very hot side of the street. I'd imagine it is the dot in the centre of the circular lense (they all are whether or not the OD's are square or rectangular or pear shaped etc.... ) that you'd like to centre on your target, covering all of target and not spilling wider than it, - while at correct distance/focal point so as not to burn a hole in same target. Any broken pieces will be angled toward this centre as designed, and trying to refract and columate/concentrate (orientation) light in that direction.
 
« Last Edit: April 07, 2011, 10:37:10 PM by jaysicle »
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madlabs

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Re: Best shape for fresnel lens...square or rectangular?
« Reply #5 on: April 08, 2011, 10:07:28 AM »
As others have said, it will work ok if just glued back together. I have a 3'x4' lens, also from a TV, and it can melt pennies. Fun stuff, had hours of fun with that one.

Jonathan

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Re: Best shape for fresnel lens...square or rectangular?
« Reply #6 on: April 08, 2011, 12:10:55 PM »
Be careful where you leave it, not near any windows or where it can start a fire. I was playing with one and left it leaning against a post  while I went in to eat lunch when I came back there was a small fire at the base of the post. I was at a flee market and one of those magnifying makeup mirrors about 5" was in a box and set it on fire. We had a 2' stainless steel mirror from a spotlight it focused to 1/8 in. it would boil a 1/8 spot on rocks, we needed welding helmets to use it. We were able to make small rubies with it. We gave these away to promote solar energy. (Rubies made by the power of the sun)
I had an idea about making diamonds using pure graphite and explosives but never pursued it.
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