lens display at Ponce Inlet, FL
The Fresnel Lens
In 1822 a French Physicist named Augustin Fresnel invented a lens that would make his name
commonplace along the seacoasts of Europe and North America. It looked like a
beehive, with a light at the center. The lens could be 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
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.
Note the enormous "bulls-eyes" in the original Cape Canaveral first-order lens,
which is on display at Ponce Inlet, Florida. The bulls-eyes bend the light
into a round beam to create a flashing effect.
Seven types, called "orders," of the lens were developed. The first three
largest orders 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.
- This is an example of a plain first-order (non-flashing) lens. This one
unusual green color. This lens still operates at the Southeast Light on Block
In Europe the new lenses were quickly adopted, but the head of the U.S.
Stephen Pleasonton, strongly resisted, feeling that it was a fad. It was not
until the 1850's that
most U.S. lights were converted.
- This is the original first-order fresnel lens from the Gay Head Light. It
modifications used to create a flashing effect. First, the lens is formed into
eyes" which bend the light into a beam rather than a flat sheet. Second, the
red panels cause
a red flash as the huge lens assembly rotates.
This is the first-order lens at Point Bonita, California. The plain curved
surface of the lens indicates a constant light. Click on this picture to get
the full impact of the 155k photo,
which clearly shows the Golden Gate Bridge in the background.