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Date:         Wed, 8 May 2002 09:17:36 -0600
Reply-To:     Ben McCafferty <ben@KBMC.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Ben McCafferty <ben@KBMC.NET>
Subject:      Re: fresnel lens
Comments: To: Dan Snow <dieselvanagon@HOTMAIL.COM>
In-Reply-To:  <F5PJA1rJvdeD8i2Gqfp00011543@hotmail.com>
Content-type: text/plain; charset="US-ASCII"

While we're on the topic of Fresnel lenses, those of you in or near CA, take a trip in your Westy (RVC) to the Pigeon Point Lighthouse, about 15 miles south of Half Moon Bay. You can take a tour, and they are one of the only lighthouses I know of where you actually get to go up into the top where the Fresnel lens is. It is an order 1 lens (I think--it's the biggest one ever made), and is nothing short of spectacular. Once a year, in November, they light the bulbs inside the lens and it is a working Coast Guard beacon for 2 hours. Other times, the have a modern spotlight that rotates. It is magnificent--24 beams of light in all directions, slowly rotating.....it's like being under a giant umbrella. And there's a hostel there, so you can camp or get a bunk. bmc :) "Faith will move mountains, but you'd better bring a shovel...."

> From: Dan Snow <dieselvanagon@HOTMAIL.COM> > Reply-To: Dan Snow <dieselvanagon@HOTMAIL.COM> > Date: Wed, 8 May 2002 00:19:44 -0700 > To: vanagon@GERRY.VANAGON.COM > Subject: Re: fresnel lens > > My dad brought one home when I was a kid, I think it was from an old > overhead projector. On sunny days in Colorado Springs (~6300 ft elevation) I > could spot weld coins together with it. And I don't mean sit around for an > hour and wait for things to heat up. I mean put on welding goggles, bring it > into focus on a stack of coins, and almost instant molten pennies. If you > ever see an overhead projector at a thrift store or junkyard, I HIGHLY > recommend getting the lens. It beats frying ants and beetles (VW Content?) > with a little reading glass. > > >> From: John Carpenter <Trvlr2001@AOL.COM> >> Reply-To: Trvlr2001@AOL.COM >> To: vanagon@GERRY.VANAGON.COM >> Subject: fresnel lens >> Date: Tue, 7 May 2002 18:37:53 EDT >> >> Question >> >> I have a thin piece of plastic mounted on the back window of my RV. It >> magnifies things so I can see better when I am backing up. Why can such a >> thin piece of plastic magnify things? A normal glass magnifying lens would >> have to be curved on both sides and would be much thicker. Answer >> >> >> >> >> If you have ever looked at the lens of a magnifying glass, you know it is >> thick in the middle and tapers to nothing at the edges. In other words it >> is >> shaped like a lentil, which is where the word lens comes from. It would not >> be very easy to make a big magnifying glass lens for your RV because it >> would >> be thick, heavy and hard to mount. The thin piece of plastic you are using >> is >> called a Fresnel lens. It is flat on one side and ridged on the other. >> Fresnel lenses we first used in the 1800's as the lens that focuses the >> beam >> in lighthouse lamps. Plastic Fresnel lenses are used as magnifiers when a >> thin, light lens is needed. The quality of the image is not nearly as good >> as >> that from a continuous glass lens, but in lots of applications (like your >> RV) >> perfect image quality is not necessary. The basic idea behind a Fresnel >> lens >> is simple. Imagine taking a plastic magnifying glass lens and slicing it >> into >> a hundred concentric rings (like the rings of a tree). Each ring is >> slightly >> thinner than the next and focuses the light toward the center. Now take >> each >> ring, modify it so it flat on one side, and make it the same thickness as >> the >> others. To retain the rings' ability to focus the light toward the center, >> the angle of each ring's angled face will be different. Now if you stack >> all >> the rings back together, you have a Fresnel lens. You can make the lens >> extremely large if you like. Large Fresnel lenses are often used as solar >> concentrators. >> >> > > > > > Daniel Snow > > '82 Vanagon Diesel > '78 Puch Maxi Luxe Moped > '01 Xootr Scooter > http://faculty.haas.berkeley.edu/snow/vanagon/vanagon.html > > > _________________________________________________________________ > Chat with friends online, try MSN Messenger: http://messenger.msn.com >


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