Can I cut a sheet of fresnel, say 8x10, to smaller sizes and still get the magnifying capabilities in each piece? I'm thinking of getting some sheets of this and cutting them to psk size. They are sold as magnification sheet for a projector. They magnify 3x and a groove pitch of .3mm with a focal length of 30cm.
Regards
Tom

Based on doing something similar to that when I was young and stupid, I would say no. Those projector sheets you are talking about are designed and manufactured to function as a single unit. Based on memory, what I ended up with when I did that was a bunch of pieces of plastic that gave a neat prism/rainbow effect when held up to the sunlight, but neither magnified or concentrated the sun's rays.

Thanks, That's what I suspected.
Oh well Rainbows are nice tho <img src="images/graemlins/smile.gif" alt="" />
Tom

Tom,

Maggot hit the nail on the head. Fresnel lenses have circles etched in them that get smaller and smaller as they go toward the center of the lens. This concentrates the light in the direct center of the lens. You can cut an 8" X 10" lens down to 2" X 3" (not the cheapest method), as long as you get the center portion. I have to cut down the Fresnel lens in my smallest kit to make it fit, but since I maintain the center, it still works great.

Semper Fi, George

This is a little surprising to me...

When I first joined this forum, I didn't know where I could get a 2x3 Fresnel. So I did exactly what is being suggested here, I bought the bigger sheet from a vendor on eBay and then cut them to size.

I didn't seem to have a problem. The cards cut from the corners are off-axis in their focus (you have to hold them at about a 10 degree angle to your target), but they certainly seem to work.

I can't imagine my cutting technique was a lot different that the rest of you (I used scissors). I even radiused the corners so they wouldn't be sharp. I've got one in my wallet right now.

Tom,

If you live anywhere near a Walmart you can get a small (credit-card size) lens for $1. If you need to trim this one and screw up it is only a dollar. I bought several last fall. The local Wally-world here has them next to the office supply stuff.

Craig

interesting stuff about the frensel lens, i was going to say that as long as you got the centre part you'd be ok, but someone else got there first! i seem to recall that they were devoloped for lighting to make the intensity of the light even across the whole face of the light, important for photography and film making i suppose. hmm suppose i ought to get back to the lab and stop arsing about on the internet!
sc

I belive Fresnel lens have been used in lighthouses thoughout the 19th and early 20th cen. as well. Pete

Quite right, I'm thinking I even saw one in St. Augustine, Fl. I think the advantage of the Fresnel was a production/maintenance advantage.

A classic lens would have been a very thick piece of glass at the center. Since glass is a fluid, just one with a high viscousity, it would have quickly deformed around the edges under its own weight. A flatter lens solved that problem. The glass fluidity problem is why all big optical telescopes are reflectors... you can support a mirror lens with a strong backing material.

Optically, I don't know that a Fresnel is any different than a classic lens. I'd have to get out my physics book an re-read it to be sure, but you can break a classic lens and the big pieces will still bring light to a focus (even though it may be off center if its a piece from the edge). And if it can bring light to a focus, it can also form images (magnify).

For more info : see this web page. <img src="images/graemlins/laugh.gif" alt="" />

Regards.

Alain

Why not? each piece of the lens should focus the light individually on the same focal point - there's nothing magic about it.

You just need to remember that the focal point is generally in the centre of the lens. If you break a chunk off the corner, it will still focus the light to that point - so for example, if you cut the lens into 4 pieces and use one of the pieces, then it will focus the light at the corner which was in the centre of the original lens, not in the centre of the "new" lens

I did this accidentally when my Fresnel credit-card broke and a small piece, about an inch across separated from the main piece. It did focus the light, but not nearly powerful enough to start a fire or anything.

A large Fresnel lens may not have the same optical quality as a smaller one (i.e. the focal point may be larger) in which case the cut piece may or may not be as good as a lens the same size. But it should be easy to check - just mask the large lens with a sheet of paper and see what happens.

They don't work as well as a classic lens for things like photography. I think it was they prism the color just enough to see, but they work well for less demanding (color wise) applications. I wish I could remember exactly, it was a neat show on Discovery.

Frenchy's link above offered a little more info after he posted it and I read it.

Even a classic lens has chromatic abberation, but because the Fresnel has surface breaks its resolution isn't as good. I suspect that's why they aren't used in photography (or astronomy). I don't actually know for sure, just my guess.

I've never used this type of material for that purpose, and I dont know about its optical properties. Applying general priciples though, you should utilize the center of the fresnel sheet. The center should be the "sweet spot" as the fresnel rings converge there. If you cut a small piece from the edges, it may only direct light in one direction, or disperse the light in a scattered pattern. To create heat, the light should be focused from all directions to the lens' focal point.

Experiment yourself or maybe someone else has some good practical experience with this. Good luck.