The Astute Recorder



What gives the Fresnel lens its reputation for safety?

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Photo: Stock images of a Fresnel lens and Augustin-Jean Fresnel.

About the Fresnel Lens

The Fresnel (pronounced "fray'nell") lens resides in the lantern room of a lighthouse and encases a lamp that produces the light.

The light's maginfication brought on by the multiprism design, compounded with consistent rotation or non-rotating flashing, results in intermittent, far-reaching light that once made lighthouses prime for maritime safety.

Invented by French physicist Augustin-Jean Fresnel in 1822, the Fresnel lens replaced mirrors, which were used to reflect light but were only effective for short distances. On foggy and stormy days, the light would hardly be seen at all, thus defeating the purpose of lighthouses altogether—to ensure the safety of fishermen and ship masters along rocky shores and in turbulent weather.

Augustin-Jean Fresnel (1788-1927) invented the multi-prismed lens in 1822. He was considered a slow learner as a boy but eventually excelled in mathematics and engineering. The political climate during Napoleon's rise, fall, then return to France had an affect on Fresnel. Prior to Napoleon's exile, the Broglie, France native had started to work on engineering projects for the French government. At the same time, he had some scientific projects up his sleeve. When Napoleon returned from Elba in 1815, Fresnel lost his official post. This was a blessing in disguise as the physicist went on to master his investigations in the field of optics.

The beam resulting from the Fresnel lens can be seen for up to 20 miles. The U.S. started using this innovative device in its lighthouses in 1850, with each lens costing about $12,000 plus shipping from France.

The lens is categorized according to order depending on its focal length. The first order is the largest. The sixth and the last is the smallest.

As for the light source, the lamp was originally fueled by open fires then later candles. Part of the role of the lighthouse keepers in the 1800s was to ensure the flame was kept alive from dusk until dawn. In the U.S., the lamp's lighting was fueled by whale oil using solid wicks. The Argand hollow wick lamp was widely used in Europe.

Decades later, after the use and experimentation of multiple oil sources, electricty began to replace the use of manually produced flames. By the early 1900s, the need for staff to operate lighthouses started to dwindle.

In addition, other forms of technology brought about more sophisticated navigational aids, a development that began to render lighthouses irrelevant in terms of maritime safety.

Today, lighthouses that are still in operation are automated remotely.


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