When envisioning a lighthouse, regardless of the shape and size of the structure, it is always crowned by a mesmerizing beam of light that pierces through the darkness, guiding sailors to safety. At the heart of this captivating spectacle lies a remarkable invention: the Fresnel lens. In this blog post, we will explore the fascinating world of Fresnel lenses for lighthouses, delving into their construction, working principles, and the concept of orders of brilliance.
The Evolution of the Light:
Before the advent of modern lighthouses, early maritime beacons employed simple sources from open fires or torches on the beach to alert seafarers of their proximity to the shore or danger. These rudimentary systems were limited in their reach and were easily obscured by inclement weather.
By the beginning of the 19th century, oil lamps with a metal parabolic reflector called an Argand Lamp became the standard light source. While their reach was much better than their predecessors, they were limited to a single direction and still didn't provide enough warning time to avoid danger in inclement weather. It was not until later in the 19th century that the introduction of lenses began to transform how lighthouses functioned.
Enter Augustin-Jean Fresnel:
In the early 19th century, a French physicist named Augustin-Jean Fresnel further revolutionized lighthouse optics by designing a lens that drastically increased the intensity and range of the light emitted. Fresnel's lens design was a breakthrough in optics, utilizing a series of concentric prisms to capture and focus light in a barrel-shaped design. This enabled lighthouses to project light much farther than previous methods. The precise construction and design of the lenses allowed for the efficient capture and focus of light, extending the reach of lighthouses to several miles, significantly increasing their range and effectiveness.
The Fresnel Lens: A Marvel of Engineering:
Fresnel lenses are constructed from concentric glass prisms arranged in a stepped, circular pattern. The central area of the lens called the bull's eye, captures and redirects light rays through a system of concentric rings known as "Fresnel zones." Each zone acts as a separate prism, bending the light and focusing it into a powerful beam. The outer portions of the lens are designed to refract and expand the beam further. This unique design eliminates the need for a solid lens, making the construction lighter, thinner, and more efficient than traditional lenses. Another feature of the Fresnel lens was its modular construction, which allowed it to be manufactured in one location, disassembled, shipped in sections, and reassembled in the tight lantern rooms of lighthouses worldwide.
Types of Fresnel Lenses:
Fresnel lenses come in various sizes and configurations, depending on the specific needs of a lighthouse. The two primary types are "fixed" and "rotating" lenses. Fixed lenses project a steady beam of light. Rotating lenses employ a clockwork mechanism to rotate the lens, creating an adjustable and visually distinctive flashing pattern. The ability of a lighthouse to display a distinctive light pattern created an additional layer of navigational aid, helping to identify and confirm landmarks in adverse weather conditions. It allowed Mariners to count the number of flashes sent out by a lighthouse to calculate their ships’ location.
Orders of Brilliance:
To standardize the classification and comparison of lighthouse lenses, a system of orders of brilliance was introduced. The order of a lens determines its focal length and the distance at which mariners can see it. The higher the order, the greater the range and intensity of the light produced. Various orders were established, ranging from the lowest, sixth-order lenses to the largest and most powerful first-order lenses.
Browns Head Lighthouse in Vinalhaven, Maine. Fixed white with two red sections 4-th order Fresnel lens from 1902. Photos courtesy the current Keeper/owner, Nick Korstad, Nov. 2023.
1. First-Order Lenses: These are the largest and most impressive Fresnel lenses, typically measuring around 3.7 meters (12 feet) in diameter. They were used in major coastal lighthouses and possessed the ability to project their light over vast distances, often reaching up to 20 nautical miles.
2. Second-Order Lenses: Slightly smaller than first-order lenses, second-order lenses still commanded significant power and range. They were employed in prominent lighthouses along busy shipping routes, ensuring the safety of vessels within a range of around 15 nautical miles.
3. Third-Order Lenses: These lenses were utilized in lighthouses located in areas with moderate shipping activity. With a focal length smaller than their predecessors, third-order lenses illuminated the seas within a range of approximately 12 nautical miles.
4. Fourth-Order Lenses: Fourth-order lenses were employed in lighthouses along coastlines with less dense maritime traffic. They had a shorter focal length, limiting their range to 10 nautical miles.
5. Fifth- and Sixth-Order Lenses: These smaller lenses were used in lighthouses near harbors, estuaries, or other areas with low marine traffic. With a range of approximately 8 and 6 nautical miles, respectively, these lenses provided localized guidance to mariners.
In conclusion, Fresnel lenses revolutionized lighthouse technology, significantly enhancing navigational capabilities and saving countless lives at sea. The innovation of Augustin-Jean Fresnel's lens design represents a triumph of engineering and optics, leaving an indelible mark on maritime history.
While modern lighthouses and ships have transitioned to more advanced electronic navigational aid systems, including radios, GPS, and LED lights, the legacy of Fresnel lenses is still revered. Many historic lighthouses, particularly those that serve as architectural landmarks or museums, continue to operate with original or replica Fresnel lenses. As we admire the enduring beauty of lighthouses today, let us not forget the guiding light that Fresnel lenses have provided for generations of seafarers, forever etching their place in maritime lore.
As a Side Note:
The Fresnel lens of the Portage River Lighthouse was a fifth-order lens manufactured by The Henry-Lepaute Company of Paris. The lens was mounted in the lantern room along with two flash panels that revolved around the lens every four minutes. Ruby glass screens were affixed to the flash panels to produce a light characteristic of fixed white with a red flash every two minutes. On August 15, 1891, it was recorded that the period between the light’s red flashes was adjusted from two to one minute.
Sadly, the Portage River Lighthouse became obsolete when the neighboring Keweenaw Waterway Lighthouse went into service on August 1, 1920. At this time, we have not been able to locate records that document what happened with the fifth-order Fresnel lens that originally lit up the current Portage River Lighthouse and its predecessor that was first lit in 1856 and where it may have gone next.
I would love to find the lens and bring it home as part of our restoration. If anyone in the community knows anything or has information, we would be most grateful if you could contact us at:
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