LIGHTHOUSES & THE FRESNEL LENS

 

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The earliest lighthouses were simply bonfires built on hilltops to guide ships.  The light from a bonfire could not have been very bright or traveled vary far.  No doubt it was better than no light at all, but things were to improve, once the Admiralty became convinced of the safety value.

 

 

Augustine Fresnel

 

Then in 1822 a French Physicist named Augustine Fresnel invented a lens that would make his name commonplace along the coasts of Europe and North America. A fresnel lens looks like a giant glass beehive, with a light at the center. The lens could be (and usually is) as tall as twelve feet, with concentric rings of glass prisms above and below to bend the light into a narrow beam. At the center the lens was shaped like a magnifying glass, so the concentrated beam was even more powerful. Tests showed that while an open flame lost nearly 97% of its light, and a flame with reflectors behind it still lost 83% of its light, the fresnel lens was able to capture all but 17% of its light. Because of its amazing efficiency, a fresnel lens could easily throw its light 20 or more miles to the horizon.  Because of the improved range, it became necessary to build taller light houses to enable the full range to be achieved - hence overcome the curvature of the earth.

 

 

 

Rhode island - non flashing fresnel lens

 

The above is an example of a plain first-order (non-flashing) lens. This one has an unusual green color. This lens still operates at the Southeast Light on Block Island, Rhode Island, USA.   Orders of lenses were developed. There were seven types, the first three orders and largest were for seacoast lights, while orders four through six were smaller, for harbor or bay lights. There was also a 3.5 order lens which was used mostly in the Great Lakes, USA..  In Europe the new lenses were adopted more quickly, but the head of the U.S. Lighthouse Board, Stephen Pleasonton, strongly resisted what he believed was a fad.  Nevertheless, by the 1850's most U.S. lights had been converted.

 

 

 

Fresnel Lens - Paris Maritime Museum

 

 

If you have ever looked at the lens of a magnifying glass, you know that it is thick in the middle and tapers down 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 1800s 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 that it's 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.

 

The Inventor

 

The Fresnel lens is named for its inventor, French physicist Augustin Jean Fresnel. Fresnel studied light and optics in the 19th century. Click here to find out more.

 

 

 

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