Fresnel Lens…Light Across the Water  

The Lighthouse Preservation Society defines a lighthouse as “a structure from which light is projected at night.” Other significant characteristics of a lighthouse include its geographic location and specific source of illumination. Located throughout the world, the existence of any number of lighthouse structures is well documented. The world’s first man-made lighthouse, located on the island of Pharos off the coast of Alexandria, Egypt, was a spectacular example of superior construction and design. Soaring 450 feet above Alexandria’s harbor, The Pharos of Alexandria as it is m in histories, guided trading ships through the difficult passage. A limestone tower provided the necessary navigational light by means of a beacon fire.

Literary and historical documents reference any number of lighthouses. Homer mentions a lighthouse in his epic Greek poem The Iliad and the Odyssey written between 760 – 710 BCE.  The lighthouse at Ostia (50 CE), located in the port of Rome, is depicted on ancient coins. Fragments of the Roman lighthouse at Dover, England remain on-site.  By 1500, lighthouses were routinely mentioned in travel books and on navigational charts. One hundred years later, at least 30 major beacons aided mariners through dangerous waters.

The earliest light aids were simple: bonfires on a high point above the water. Eventually open fires were replaced with wire baskets filled with burning wood suspended from a pole. By the first century CE, candles or oil lamps contained in lanterns of glass or horn, were in use. Methods of illumination continued to evolve as man evolved. Technical and mechanical advancements continued to improve the accuracy of the beam and ease of operation. By the mid-1700s, oil and gas lanterns were the standard of the day.

lighthouse top with fresnel lens

Cape Meares First Order Fresnel Lens

The most significant advancement in lighthouse technology, however, was Augustine Fresnel’s remarkable lens, developed in 1822. A collection of multi-faceted glass prisms mounted in a framework resembling a large beehive, the lens’ genius was based on the principles of physics: refraction (bending) and reflection (magnification). This unique lens provided a light that was visible for great distances. Secondly, the Fresnel lens could provide specific light patterns. The “characteristic” of a lighthouse became its’ calling card, and included any number of combinations, including flashing lights.

The Fresnel lens was located in the lantern room at the top of a light tower. Within the lens a wicked oil lamp provided the light source which was then magnified and directed outward by means of the prisms, creating a beam of light. Initially, the lamp burned lard oil. By the 1880s, however, kerosene was the universal fuel. Acetylene gas and eventually batteries, automated the lights.

Fresnel designed the lens in seven sizes called “orders,” ranging from the largest First Order to the smallest lens called a Sixth Order. The location of each lighthouse determined the size of lens that would be installed. A First Order Fresnel lens, 12 feet high and 6 feet wide, is visible more than 20 miles out to sea and were implemented along the coastlines of large bodies of water.

Unfortunately, American lighthouses were slow in implementing the Fresnel lens, due in large part to Treasury Auditor Stephen Pleasonton’s resistance to change. Beleaguered by complaints from sailors and shippers, the federal government created a “blue ribbon panel” to investigate and found the lighthouses to be in fairly poor condition. Pleasonton was discharged from his position, and the Light-House Board was created in 1852, resulting in a sweeping overhaul of the lighthouses. Twelve districts were drawn up, and an inspector was assigned to each. On the eve of the Civil War, all American lighthouses were utilizing Fresnel’s technology.

fresnel lens closeup

Eagle Bluff Lighthouse Fifth Order Fresnel Lens

When Eagle Bluff’s lighthouse was constructed, a Third-and-a-Half Order lens was installed. The light utilized a wicked oil lamp fueled by lard oil. In the 1880s, the lamp was converted to a kerosene-burning light. In 1918, a new and improved incandescent oil vapor lamp (IOV) was installed at Eagle Bluff. Unfortunately, the light was now too bright and intense for accurate navigational purposes, and the larger Fresnel was replaced with a smaller Fifth-Order lens which proved more suitable.  Eagle Bluff Lighthouse was officially unmanned when an acetylene gas mechanism was installed in 1926. The lamp was eventually converted to solar energy in the mid-1980s.

Today, the solar light is comprised of six bulbs (wattage: .55 amp lamps).   Illumination is provided by a single bulb which when exhausted is automatically replaced by one of the remaining five bulbs. (For purposes of definition: the small light bulb inside the lantern is called a “lamp” and the complete structure is defined as a” light.”)

According to the U.S. Coast Guard every lighthouse is identified by a specific flashing code. Each evening at dusk, Eagle Bluff’s solar light is sensor-activated by a Daylight Control (DLC) that measures the amount of daylight and completes the circuit when the sun goes down.  A flashing coded white beam of light with a sequence of one second on, six seconds off, is visible for seven miles. The U.S. Coast Guard reports that signal lights can be green, red, yellow, or white. The choice of color is selected based on the waterways location and pattern.

Although a majority of the historic lighthouses have been abandoned, sold, or destroyed, approximately 400 active lighthouses are currently administrated by the U.S. Coast Guard, the majority of which are automated. In honor of its heritage as the “first,” the Boston Light on Little Brewster Island remains the only manned station in the United States.

For fun facts about lighthouses visit www.us-lighthouses.com.

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