The Fresnel Lens: How a French Design Made Lighthouses Shine Farther

Lighthouse lenses were first used in England in the 18th century and reached the United States by 1810. Those early lenses were thick, heavy and made of poor glass, so much of the light was lost in them. In 1811 the French Commission on Lighthouses set up a committee to improve lighthouse illumination. One of its members, Augustin Fresnel, completed the design of a flashing lens in 1822, using thin, bull's-eye-shaped panels that bent light both horizontally and vertically into a much stronger beam.

A first-order Fresnel lens at the San Francisco Maritime Museum. NPS

How it works

The Fresnel lens (pronounced "Frey-nel") was a monumental step forward for lighthouse technology and maritime safety. Hundreds of specially cut pieces of glass surround the lamp, gathering light that would otherwise scatter and focusing it into a single intense beam aimed in a particular direction. The lens could create any number of flashing patterns, and its light could be seen much farther away, making navigation near shore far safer.

Most Fresnel lenses look like a beehive or a barrel and contain from two to twenty-four panels.

Flashes that identify a light

A Fresnel lens can be fixed, showing a steady light all around the horizon, or revolving, producing flashes. The number of flashes per minute depends on how many flash panels the lens has and how fast it turns, and varying the periods of light and dark gives each lighthouse its own pattern. One might flash every five seconds; another might show ten seconds of darkness and three of light. This pattern is the light's characteristic. Mariners looked up a light's characteristic and color on a light list or chart to work out their position relative to land.

Revolving lenses were turned around the lamp by a clockwork mechanism that, before automation, had to be wound by hand every few hours, and timed so that a panel passed the lamp exactly when a flash was due.

Sizes, or orders

Fresnel lenses came in sizes called orders, from the largest, the hyper-radial, to the smallest, the eighth order; not all were used in the United States.

• First-order lenses, such as those still in place at Fowey Rocks lighthouse in Biscayne National Park and Bodie Island lighthouse in Cape Hatteras National Seashore, were used on major seacoasts, with beams reaching up to twenty-one miles out to sea.
• Fifth- and sixth-order lenses, the smallest used in the United States, served bays and rivers. The Jones Point lighthouse on the Potomac River in Alexandria, Virginia, used a fifth-order lens for its shorter range, yet it was essential to the hundreds of merchant, passenger, fishing and naval vessels on the waters around Washington, D.C., every day.

Preserving the lenses

Nearly every lighthouse in the national parks originally had a Fresnel lens. Many have since been removed or replaced with modern lights, but the lenses' beauty and importance have kept many of them preserved, some in museums at their own lighthouses and others elsewhere. Many more Fresnel lenses survive in American lighthouses outside the National Park System.

Sources

• National Park Service, "Fresnel Lens" (Biscayne National Park and Cape Hatteras National Seashore): https://www.nps.gov/articles/fresnel-lens.htm
• The source spells the name "Fresnal" in its photo caption; this page uses Fresnel.