By Emily du Houx

October 17, 2012

The Fresnel lense in the Pemaquid Point Lighthouse, in Maine. Photo by Emily du Houx


The autumn colors in the cool, early afternoon were semi-precious and jeweled — emerald, ruby, gold, obsidian, sapphire — colors that consume light and radiate inner depth. It was one of those autumn days in Maine that makes it feel as though it’s possible to spot a leaf falling from a maple twenty miles away. In other words, it was the perfect day to go see a Fresnel Lens, a device that actually makes it possible to see the light from a lighthouse twenty-some miles out to sea with the naked eye. I had never seen a lighthouse up close, even though I grew up here; they always seemed like overrated tourist bait, impossible to experience in any meaningful way after having been barraged all my life by images of them on postcards, cups, calendars, bags, cookies, socks, and anything else that takes a decal or can be shaped into one of their famous silhouettes. But I was drawn by the lure of free admission, the possibility of an early fall drive, and the chance to visit the ocean, so I headed out to Pemaquid Point.

When I got to the lantern room at the top of the small lighthouse in Bristol, after ascending a short, winding staircase, passing through a hole in the floor, and pulling myself up with a series of rungs — all of which was made to sound like an especially complicated ordeal by the lighthouse guide at the bottom of the stairs, but was actually no worse than climbing up to my attic at home — I felt like I hadn’t seen a lighthouse before, even in pictures. The outside of the building was more or less what I had expected, but the lens itself is so rarely represented in lighthouse memorabilia that seeing it felt like a genuinely new experience. There are only six Fresnel lenses still in use in the state, but there was a time when this was the dominant technology used in lighthouses all up and down the Eastern Seaboard.

It’s a beautiful construction: a simple idea, elegantly executed. Lighthouses initially used whale-oil-burning lamps, which only transferred three percent of their light to any point out at sea. This was not a particularly great help to mariners at the time. Later, some lamps were outfitted with parabolic reflectors — large reflective discs placed behind the light — increasing the light visible at sea to about forty percent. But with the invention of the Fresnel lens, a whopping eighty percent of generated light could be transferred twenty miles over the ocean.

A lamp, or anything that emits light, does so in such a way that, the farther you get from the point of origin, the more diffuse the light is. Put simply, light gets dimmer as you get away from its source. Lenses can work to counter this by capturing the light before it scatters, re-concentrating it and organizing its path into a focused beam. Any lens has the potential to do this, but the beauty of a Fresnel lens is that it does so without the bulk of traditionally shaped, curved lenses, like those in glasses, telescopes, or cameras. In order for a traditional lens to magnify the light from a lamp so that it can be seen over twenty miles out to sea, it would have to be enormously thick and super heavy. Such a hefty lens probably wouldn’t fit up the staircase and through the hole in the floor at the Pemaquid Point tower.

The first Fresnel lenses were actually made from several rings of prisms which circled a central, curved lens in a bull’s-eye pattern. They could be taken apart and put back together for shipping, making installation in the cramped lantern rooms of lighthouses relatively simple. In later years, the individual prisms were integrated into one sheet of glass or plastic. This type of single-sheet Fresnel lens can get remarkably thin and is useful in many modern devices, from headlights in cars to movie production lighting, overhead projectors, and state-of-the-art solar panels. Fresnel lenses are not as clear or accurate as traditional lenses, which is why they are not used in cameras or eye-wear, but they are still used in places where it’s beneficial to magnify the brilliance of existing light.

There has been a Fresnel lens at Pemaquid Point since 1856; there is also a second lens in the lighthouse museum. When I went over to investigate it, the museum guide had me examine its outside closely. I could see the lamp on the inside, distorted and glowing through the thick glass surrounding it. After I stared at this for a while, I was allowed to peer around the back of the lens and see into the internal chamber with the lamp sitting inside. The magnifying power was amazing. What appeared to be a giant through the glass was simply a dwarf in reality, greatly enlarged by a Wizard-of-Oz-level optical trick.

I left the museum and drove back through the prismatic shimmer of the late afternoon, light refracting through the Fresnel lenses in my headlights, reflecting off the curved lenses of my eyes, as the giant lens I just left waited, dormant, for the daylight to give way to night, when its automated system would click in, and its concentrated beam would penetrate the darkness miles and miles out to sea, just as it had done for over 150 years.

Maine Open Lighthouse Day is the result of a collaboration between The U.S. Coast Guard, the State of Maine, and the American Lighthouse Foundation and has been held annually for four years to increase public knowledge of Maine’s maritime heritage. Hundreds of people have taken advantage of the free admission and increased lighthouse access. Check it out next year, even if you’re not a tourist.