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Lighthouse Lenses
The Beauty of Genius

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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.

 

Fresnel Lens Listings
1st Order
This lens is the most powerful and largest of the six.  Primarily installed at primary landfall or coastal lighthouses to give warning to the approach of land.
Lens height: 101.97"
Focal length:  36.22"
Weight of assembly:  3,530 lbs
Relative brightness (over sixth order): 17.69x
Useful range:  Up to 20 nautical miles
Oil consumption per hour 26.25 oz.
2nd Order
The Second Order lens was used in seacoast and warning lights to mark headlands, dangerous oceanic shoals, rocks or reefs. It was used quite frequently in the Great Lakes Lights, on islands, and in marking sounds.
 
Lens height: 81.46"
Focal length:  27.6"
Weight of assembly:  3,530 lbs
Relative brightness (over sixth order): 11.54x
Useful range:  Up to 20 nautical miles
Oil consumption per hour 17.5 oz.

 

3rd Order
The Third Order lens was used in coastal lights which lead the mariner from one point to another along the coast. It also marked sounds, the entrances to rivers, bays, and channels.
 
Lens height: 62.05".
Focal length:  19.7"
Weight of assembly:  1,985 lbs.
Relative brightness (over sixth order): X3.85
Useful range:  Up to 18 nautical miles.
Oil consumption per hour 7 0z.
4th Order
The Fourth Order lens was used in major harbor lights which lead the mariner into the channel at the entrance of a harbor mouth. It was also frequently used on rivers and in harbors to mark shoals and islands.
 
Lens height: 28.43"
Focal length:  9.8"
Weight of assembly:  440 to 660 lbs.
Relative brightness (over sixth order): 2.31x
Useful range:  Up to 15 nautical miles.
Oil consumption per hour 5.25 oz.
5th Order
The Fifth Order lens was used in leading lights, showing the line of a channel, or the entrance to a harbor. It was also used in river navigational lights to mark small shoals and islands in sounds, and to mark breakwaters.
 
Lens height: 21.3".
Focal length:  7.4"
Weight of assembly:  265 to 440 lbs.
Relative brightness (over sixth order): 1.23
Useful range:  Up to 10 nautical miles.
Oil consumption per hour: 3.15 oz.

 

 

6th Order
The Sixth Order lens, the smallest of the regular Fresnel Orders, was used in minor lights, showing the line of branch or secondary channels or the ends of piers, breakwaters and jetties.
 
Lens height: 17.05"
Focal length:  5.9"
Weight of assembly:  220 lbs
Relative brightness (over sixth order): 1x
Useful range:  Up to 5 nautical miles
Oil consumption per hour: 3.14 oz.
 

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