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
>