The Fresnel Lens
This page last updated 07/18/06
During discussions of lighthouses, you'll usually see mention of a this-order or that-order Fresnel lens. Too infrequently, however will you see any mention of what the heck a Fresnel lens is, how it works, what the different "orders" mean, or who this Fresnel person was. To some extent, I'll try to correct that oversight here.
Augustin Jean Fresnel (pronounced "Fruh-nel") (1788-1827), was a French physicist and an early adherent of the wave theory of light. He conducted many experiments with light. Fresnel was the first person to demonstrate that two beams of light polarized in different planes do not exhibit interference effects and, from this experiment, theorized that light waves are transverse, rather than longitudinal (like that of sound). Fresnel was the first to produce circularly polarized light. He is also credited with discovering several basic optical formulas, including those for reflection, refraction, double refraction, and the polarization of light reflected from a transparent substance. Fresnel's work on optical effects caused by the motion of objects was important in Einstein's theories of relativity. In the field of applied optics, Fresnel, in 1822, designed the lens that has been used for many years to produce parallel beams of light from lighthouses. Fresnel's scientific work was known only to a small group of scientists during his lifetime, and some of his papers were not published until after his death.
The Fresnel lens, also known as the dioptric lens, consists of a central glass disk and concentric glass rings (on flashing lenses) or a central glass band surrounded by parallel glass rings (on steady lights). These rings or bands decrease in thickness as one moves further from the center. A flashing Fresnel lens produces a flash each time a rotating lens aligns with an observer's eye. The single, nonrevolving lens, which is the same type of lens one sees on many 19th century hand-held lanterns, sends a steady beam in all directions unless the lamp inside employs some sort of rotating shutters, either solid or colored glass, which cause a flashing effect to the observer.
In American lighthouses, Fresnel lenses are categorized by their strength, with a first-order lens being the most powerful and a sixth-order lens being the least powerful. A first-order lens is almost fifteen feet tall, with a diameter of over six feet. Sixth-order lenses, the smallest, are about the size of a table lamp. The powerful first-, second-, and third-order lenses were used along coastlines, with the smaller lenses used for smaller channels, inlets, lakes, and harbors. A third-and-one-half order lens was the seventh size in the series.
When the Fresnel lens was introduced into marine navigation, it caused a revolution of sorts. The powerful beam of light made possible by the lens sparked the construction of taller lighthouses on the coastlines. Beams of light could now be sent far out onto the horizon. These lenses were employed rapidly in France and England. In the United States, the implementation of Fresnel lenses was slowed by bureaucratic mismanagement in the government. This changed when the Light-House Board was formed in 1852. When the Board came into being, only 3 U.S. lighthouses had Fresnel lenses (Navesink, NJ, Sankaty Head, MA, and Brandywine in the Delaware Bay). When Congress created the Lighthouse Board, they stipulated that Fresnel lenses be installed in all new lighthouses and those which needed new lights. By the time the Civil War started, nearly all U.S. lighthouses were equipped with Fresnel lenses.
One of the major advantages to the Fresnel lens was its ability to reduce the amount of energy required to produce a good light. Instead of several lamps with reflectors burning a great deal of oil, a single lamp could be used. This increased efficiency reduced fuel consumption, even though the single lamps would use a multi-concentric-wick design. The multi-wick design would, in essence, house several lamps within one, using up to five wicks to produce a single flame.
At first, the light beams that passed through Fresnel lenses were created by whale oil. Other fuels (lard oil, colza oil, kerosene, etc.) were tried up until around 1900, when electric lamps began to show up in U.S. lights. By the 20's and 30's most lights had been converted to electric lamps.
Today, many lighthouses use rotating beacons similar to those found at airports. These beacons generally consist of a high-powered light source, with a reflecting mirror on one side of the light source and a condensing lens on the other. The newest standard optic in US lighthouses, the Vega VRB-25 (manufactured in New Zealand), is a small drum-like plastic version of the Fresnel design. These new lights and lenses are smaller, lighter and cheaper and easier to maintain than the Fresnel lenses. As with the lighthouses themselves, beauty has been replaced with pure function and ease of maintenance.
Below are some images of Fresnel lenses which I hope will give you a good idea of what these lenses look like. You may see more on the US Lighthouse Society's Long Island Chapter web site. If you ever have the chance to visit a museum or lighthouse which has one on display, please do so. They are quite impressive in person.
This First Order lens, on display at Ponce Inlet, Florida, once shed its light from the lantern of the Cape Canaveral lighthouse. On Long Island, the Montauk Point, Shinnecock Bay, and Fire Island lights used this type of lens at one time.
This Second Order lens, on display at the East End Seaport Museum in Greenport, previously resided in the Little Gull Island tower.
This Third Order lens once resided atop the Ponce Inlet tower. It now sits in the lens exhibit near the base of the tower.
This Third and One Half Order bivalve lens welcomed and warned mariners near Montauk Point from 1903 to 1987.
Visitors at the lovely Horton Point lighthouse will notice this 1850s Fourth Order lens, manufactured by L. Sautter, on display in the keeper's quarters.
Fifth Order lenses, such as this one on display at Montauk, were used to mark smaller waterways.
Here's another Fifth Order lens.
The smallest order lens was the Sixth Order, of which I do not currently have a digital image.
Lighthouses weren't the only beneficiaries of Fresnel technology. The oil lantern shown below, from my personal collection, is of the type used by the New York Transit Authority (I hear that some of these may still be in use). On the left is the lantern without its Fresnel lens in place. The photo on the right shows the same lantern with the lens (this lens happens to be orange, though they were made in several colors). Without the lens, the light is unfocused and diffuse. The shape of the Fresnel lens bends the light beams into a more focused, useful arrangement. The type of lens pictured below is the type used used in lighthouses for fixed lights and lights which flashed due to the actions of shutters inside the lens. (Personal note: After taking these photographs, I washed the lantern and got a reminder of what a dirty job lighthouse keeping must have been. The soot from this lantern clings to everything it touches. Despite repeated washings, I still have soot on my hands as I type this.)
Fresnel technology is still very much in use. The navigation light pictures below serves aboard the Orient Point ferry North Star.
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