By the middle of the eighteenth century, the "state of the art" in
lighthouse optics consisted of single or multiple whale oil burning
lamps placed in the lantern at the top of a tower. This method was
extremely inefficient, as only three percent of the light ended up
being visible at any given point at sea. The only method available
through which the output of the lamp could be increased was to
increase the size of the flame, thus burning more fuel, and making
the light more labor intensive and costly to operate.
By
the beginning of the 19th century, US lighthouse optics
had progressed to the use of a silvered-metal parabolic reflector
placed behind the whale oil lamp, known as the Argand Lamp. With
this arrangement not only was the light source itself directly
visible at sea, but the parabolic reflector captured the light that
would have been lost behind the light, and concentrated it out to
sea in the desired direction.
While this system was workable, and a vast improvement over the
simple lamp, it too was very inefficient, with only thirty-nine
percent of the original light source transmitted in the desired
direction. Thus while the visible distance increased, it was still
limited to a maximum of a fifteen to twenty miles in clear
conditions. By the time a vessel saw such a light, they would have
precious little time available to turn away from the impending
danger of which the light was intended to warn.
With
shipping increasing throughout the world, an optical system was
desperately needed whereby the light could be cast many miles out to
sea, providing ample advanced warning of either danger or safe
harbor.
In 1819, the French Government commissioned 34
year old Augustin Jean Fresnel (pronounced Fruh-nell) to develop an
improved lighting system for French lighthouses. Fresnel was a
physicist who was well known for his experimentation with the
theories of light reflection and refraction.
Ignoring the
reflector paradigm, Fresnel instead began investigating ways that
glass lenses could be used to concentrate the light source. Since a
single lens of sufficient strength would be too large to be
practical, Fresnel began looking at ways that multiple lenses
surrounding the light source could capture the light rays emitted
from a single light source and direct them into a narrow horizontal
beam
In its simplest form, Fresnel's design was a
barrel-shaped array of lenses encircling the light source. In the
area immediately horizontal to the light source, dioptric
lenses magnified and concentrated the visible light as it passed
directly though them. At the same time, above and below the light
source, multiple catadioptric prisms mounted around the
periphery of the barrel each collected and intensified the light and
redirected it in the same plane as the dioptric lenses.
With Fresnel's optic array, output was increased
dramatically from the old reflector systems, with as much as eighty
percent of the light transmitted over twenty miles out to sea.
So
revolutionary was Fresnels' design that it was immediately adopted
world-wide as the standard lighthouse lens, a pre-eminence that it
maintained well into the twentieth century. While August Fresnel saw
the rapid adoption of his optic array, he never lived to see it
become universally adopted due to his death five years later in
1827.
The Fresnel lens design would eventually be
refined into eleven orders, with each order featuring a standard
focal length. Focal length being the distance from the center of
tight source (focal point) to the lens.
The huge Hyper-Radial, Meso-Radial and First Order
lenses were reserved for use in coastal salt water lighthouses, and
the smallest Sixth, Seventh and Eighth Order lenses for river and in
harbor navigation.
Francois Soleil Sr. was the first to build the
lenses for Fresnel. His son Francois Jr. took over the work, and
continued working in Paris, until he went to St. Petersburg Russia,
where he continued to build lenses. Several French companies, all
located in the vicinity of Paris were responsible for the
manufacture of almost all of the Fresnel lenses used in US
lighthouses during the nineteenth century.
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