View Full Version : Fire starting---Fresnel lens
December 11, 2005, 01:48
How many here use a credit size (or larger) Fresnel lens to start a camp fire?
If you do, give us some input on size, kind, material etc.
How many here have no clue what a Fresnel lens is?
fire for effect
December 11, 2005, 04:31
don't use one. Yes I know what it is.
December 11, 2005, 08:52
i have a big magnifying lens i got out of a boat radar, i play with once or twice a year,, its really easy to start fires with , but i dont carry it around regularly.
December 11, 2005, 10:07
Yes you can use a fresnel lens to start a fire.
Basically a fresnell is a series of circular prisms cut into a thin sheet.It is based on the idea that a convex (magnifying) lens can be thought of as two triangular prisms back-to-back. The bulk & weight of the thick glass needed to make the lens can be replicated by using much thinner & lighter material.
The focussing ability of a fresnel is inferior to a real "condenser" lens though. It will certainly focus enough to make a good "hot spot".
The bigger the better, as it will compress down the area of the lens into a spot, so the more area focussed the more intense the spot. Fresnel lenses are readily available at book stores as page magnifiers, or wallet magnifyers, for abou$5~$10.
Most of them are on a flexible clear plastic, so they are fairly durable also.
They come in a bunch of sizes, varying from 3"X2" up to 8 1/2" X11", but the thinness & lightness of them means that an 8 1/2 X11" will slide into a pack just about anywhere, while only weighing an ounce or two. The bigger sizes can also be cut down with a good pair of sissors. just remember if you do this to keep the center, well centered!
Darn it! Now you've given me a good idea for something else for the BOB.:uhoh: :? :sad: And I thought it was complete! Great idea. I'm just wondering if I'm so flippin' smart why I didn't think of it. Oh well....off to Borders.
December 11, 2005, 23:32
more grooves or lines per inch.... not size makes them hotter at focal length .....get the 12"x12" ones from EDMUND SCIENTIFIC 10 buck ....makes a good solar furnace ....ive got one and it will melt and fuse basalt rock /gravel..i think they claim 2000 farenhieght..shorter focal length heats a 5x5 area ...enough to heat up an MRE....lens comes with a big warning ...not like the kind in the back of a motor home window.....they have a good selection of optics and lenses at E.S.
December 12, 2005, 19:24
Edmunds is a good source! good catch.
On the subject of grooves per inch, That's not the whole story. more grooves are definately better because they give better resolution, so a hotter, more focussed spot.
BUT you need length between the lens & the thing to be set on fire, otherwise toasted fingers & a melted fresnel!
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