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Minnesota Historical Society Split Rock Collection
Split Rock Lighthouse
As created by Augustin Jean Fresnel, lighthouse lenses were intricate and beautiful in their own right and intensified the beam from lighthouses by focusing the beam through prismatic elements.
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National Park Service
Split Rock Lighthouse
The lens required intensive daily cleaning and maintenance of both the lamp and rotating mechanism.
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Lee Radzak
Split Rock Lighthouse
The Split Rock lens is a 3rd Order Fresnel and has an official range of 22 miles.
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Minnesota Historical Society Split Rock Collection
Split Rock Lighthouse
Augustin Jean Fresnel introduced his eponymous lens in 1822.
Lighthouses reach back more than 2,300 years to 280 B.C. when the ancient Egyptians erected a 450-foot wood-fired tower at Alexandria to guide mariners to their port. It’s likely that shore-based fires had guided ships well before the Alexandria Light, but that structure was so impressive that it’s listed as one of the seven wonders of the ancient world.
While the structure was wondrous, its beacon would have been disappointingly meager because there was no means of focusing the light produced by the flames. Furthermore, the fire was exposed to wind and rain, making it less effective in foul weather – when it was most needed.
Through the next two millennia, small steps in lighthouse technology led to better sources of light and the use of mirrors to focus the light. It would not be until the late 17th century, however, that an enclosed glass lantern room was first installed at Eddystone, England. Candles fired the light and mirrors in large wooden bowls directed the light outward, making it much more efficient than previous lights.
Another 130 years passed before the next significant improvement in lighthouse technology occurred, and that event proved to be revolutionary rather than the earlier evolution in lights. It came from an unlikely French inventor and physicist named Augustin Jean Fresnel (fray NELL).
A slow learner with little interest in language arts or tests of memory, Augustin could barely read by the time he was 8 years old. Nonetheless, his boyhood friends called him “The Genius” for his ability to make their toys perform more efficiently.
His boyhood nickname would prove prescient when he turned his attention to the physics and characteristics of light in the early 1800s. Rather than looking at the results of lighting, he studied the dynamics of light itself. From that research and the success of earlier efforts to direct light using parabolic mirrors, he developed an intricate system of reflective lenses that intensified the beam of light and could be controlled as to frequency of the flash.
From its 1822 introduction, the Fresnel lens became the standard for lighthouses throughout the world. Measured in seven orders, with the 1st Order being the most powerful, the importance of Fresnel’s invention is analyzed by Split Rock site manager Lee Radzak: “If the light of a candle is rated at 1, a parabolic reflector would bring it up to about 17 times better and a Fresnel lens would be 85 times better.”
The 3rd Order Fresnel lens installed at Split Rock was one of the most widely used fixtures and is one of the key features of the site. Very few lighthouses still have an original lens in such good shape. Floating on seven quarts of mercury that are strictly monitored by the Environmental Protection Agency, the 2.5-ton, double bull’s-eye lens mechanism was rotated every 20 seconds by a wind-up clockwork system of weights and pulleys, emitting a white flash every 10 seconds. From its 130-foot cliff, the tower’s light had an elevation of 168 feet and was rated at a 25-mile range – although it has reportedly been seen more than 45 miles away in certain weather conditions.
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