Fresnel Lense for fire starting

Sesoku

Sesoku

Joined
Oct 19, 1998
Messages
498
I picked up a credit card size fresnel (sp?) lens from Meijers for a couple bucks a while back and it just got sunny a few days ago. I got it out and was focusing it on some wood with the light that was coming in from the window. I was able to get smoke it a few seconds, but I wasn't really trying to start the wood on fire. I think that these are a worthwhile addition to a mini kit, they take up hardly any space at all. They are in the Pharmacy section of Meijers next to they eyeglasses and related products.
 
I've been thinking about getting a couple of these myself. I'm glad to know they work well. I believe Spark has mentioned them in the past on here.
thanks,
Paul
 
Yeah,..they can be powerful.
Few ideas I had:
1) Sesoku, try it without filtering light through window!
2) With the same logic, they should work better in clear weather than in haze.
3) Light absorption is always higher when object is darker. Therefore, heating up faster. Try charcoal/charred cotton vs. dry light colored wood/white cotton.
4) Wind takes away heat (like from your body) accumulated locally at focus point. Try to windshade tinder when use lens.
5) Larger Fresnel or convex (?) lens focuses proportionally more light in the same point resulting in better efficiency. Size and weight with convex lenses is a factor, much less with Fresnels. At Borders and Barnes and Noble bookstores, art suppliers and Edmund Scientific (yum-yum..) you can even get letter paper size ones (or smaller...).
6) They are not good optical quality!!! but tremendous light gathering power (good for condensers) for the weight. For reading and longer use stick to old high quality glass convex lenses. Also, they are thin and therefore very little light is lost by absorption.
7) Grooves should face lightsource!! (Why??)

Hope, I help,

HM
 
One more thing last:
credit card size Fresnel fits nicely together with many credit card size commercial signal mirrors. I keep them together in a plastic bag (in the minikit) to avoid scratches.
Gone now,

HM
 
Were to make concentric circular cuts through a (one-face) convex lens, then slice horizontally through where that circular cut meets the curved surface, then remove the part of glass that is "inside" hte lens, and drop the curved part down, you have a fresnel lens. If that makes any sense, good. If not, just take my word for it that the grooved side is equvalent to the curved side of a one-sided convex lens, so it should face the sun.

--JB

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e_utopia@hotmail.com
 
E_utopia,

Thanks for the explanation. I still have not reached full understanding why the Fresnel (or the plano-convex lens) should face the light with the grooved (or convex) side. although that seems to be the case shown in the short descriptions and optical diagrams in the Edmund Scientific Optical Catalog.
When i scatched the lightpath going through such a plano-convex lens in the two directions i found a focusing effect with somewhat different focal lengths.
When light enters through the plano (plane surface), it does not difract due to the perpendicular angle of entry. When it exists the lens through the grooved or convex surface, it difracts toward the optical axis crossing the optical axis in the focal point.
If light comes from the other direction, it enters lens through the grooved or convex surface and difracts first toward the opt axis. When it exits lens through the plano surface, it difracts again -second time- toward the axis resulting in a double difraction by going through the lens. And it -as much as I could determine- gives a shorter focal length.
Well, probably I am offa bit but here is my two cents.
What did i miss?
Best,

HM
 
Go here to learn of the invention of the great Augustin Fresnel (pronounced FREH-nell): http://zuma.lib.utk.edu/lights/fresnel.html

He invented the Fresnel lens in 1822. It had two advantages. First it was made up of many independent sections, each of which could be replaced individually if broken. This had obvious advantages, and made shipping it (disassembled) easier as well.

Second, it was much more efficient; a flame with ordinary reflectors lost 83% of the light generated. A Fresnel CAPTURED 83% of the light.

Here is a short video showing exactly how a Fresnel lens works: http://www.pbs.org/legendarylighthouses/html/photoqt.html

The plastic cards are very very poor imitations of Fresnel lenses. I am amazed that anyone could start a fire with one.

Hope this helps, Walt

PS: as to which way to hold the card, consider what it does.

One side takes parallel light beams and bends them into a common point at the focal length of the lens.

The other side takes whatever light impinges at the focal length of the light and turns it into parallel beams. Light impinging from a source other than the focal length will be refracted according to the angle of impedence, so most of the light is scattered.

Obviously, you want the first side if your goal is to start a fire. Walt

[This message has been edited by Walt Welch (edited 03-04-2000).]

[This message has been edited by Walt Welch (edited 03-04-2000).]
 
Hi Walt,

You amaze me every time with your knowledge about.... well any subject I've encountered including my own Dutch history.

Walt did you receive my email some time ago?

Cheers,

Bagheera


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Hi Walt,

Thanks for the useful info and the cool little animation on the lighthouse Fresnels
smile.gif
.
First I would like to correct my typo: I meant to say refract instead of difract in my post. Heck, when am I going to learn proper English eventually?!!!!
More interestingly, as I looked it up it turned out that the relative higher number of grooves results in higher light gathering power, lower groove density allows higher quality image. Edmund Industrial Optics lists ones from 50 to 200 grooves per in. for the same price (they are NOT cheap). With a second magnifier and a ruler drawn on a white paper you can determine the groove density of your own lens (if you have the patience...).
To be honest, I have not started a fire yet with them. However, I generated small smoking and tiny hot ember in a small twig during a muggy hot summer trip and stopped at that point. Seemed to me still somewhat more promissing than the fire plough method
smile.gif
.
Thanks again and best regards,

HM


 
Bagheera; yep, got your e-mail. Am getting on plane to come see you and Eben-Emael in few months. Stock up on good Dutch beer, and if you happen to know any single women....
wink.gif


Walt

PS: let you in on my secret about knowing stuff. It is a parallel search engine. It runs all the common engines simultaneously, so you can skip the ones that come up dry, or don't have the info you want. This way, you don't have to run one search engine after the other. Took me about 10 min. to find the stuff on Fresnel. The search engine has a rather unfortunate name, but really works. Here it is:
http://www.dogpile.com/ See ya, Walt

PS; HM; Fresnel did a lot of work on diffraction as well, and those plastic lenses come awful close to being diffraction grids, so don't feel bad. Walt
 
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