Our Fourth-Order Fresnel Lens

The bright beacon that reached out from Admiralty Head Lighthouse for the first time on January 21, 1861 to ships 16 miles away could hardly have been more modest. It was just a whale-oil lamp.

What made that lamp so brilliant was an ingenious invention engineered in France in the 1800s - the Fresnel lens. These lenses are so well designed they remain in use today all over the world, typically in combination with a 1,000-watt electric bulb.

For 61 years, a fourth-order Fresnel lens at Admiralty Head Lighthouse guided mariners safely into Puget Sound. There are two lenses currently on display at Admiralty Head Lighthouse. One, like the one pictured above, is on loan from the United States Coast Guard Station, Port Angeles, Washington. The history of it is unknown, but it is possible that it was originally at Admiralty Head Lighthouse.

This lens # 435, It has script letters along the base and says L. Sautter, _________, Paris. According to Thomas Tag, Fresnel Lens expert, this lens was definitely made in Paris between 1852 and 1869. He suggested it may have come from Patos Island Ligthouse, Washington.

The second Lens on display is a Bull's-Eye Lens or flashing lens . This lens was given to Washington State Park years ago and has been on display at the lighthouse for many years. The prisms concentrate the light to the bull's eyes, and when rotating, it appears to flash.

This lens, #457, according to Thomas Tag, definitely came from Alki Point Liighthouse in Seattle and was made between 1870 and 1889

How the Lens Works

Augustine Fresnel developed his lens in the early 1800s and it became the standard for most U.S. Lighthouse Service lighthouses across the country. Fresnel lenses are comprised of many smaller prisms, individually crafted and suspended in a complex arrangement. They concentrate the light into parallel, horizontal beams that may be directed into a narrow, cylindrical beam or a wide, flat beam visible far out to sea. The concentration is accomplished by passing the light through glass prisms in which the beam is bent by refraction or reflection, and also may add reflection from polished surfaces or mirrors.

First-order lenses, the largest made, are 12 feet in diameter and typically are placed at major coastal stations such as Cape Flattery on the Olympic Peninsula. Sixth-order lenses, the smallest, are just eight inches in diameter. Fourth order lenses, such as the one at Admiralty Head, are customary for lighthouses on inland waters.

Special Effects

A lens system enables not only the concentration of light onto a plane or beam, but also any desired special effects such as color and flashing. The light at Admiralty Head was white until 1921, at which time red was used. It was visible over an area of 270 compass degrees. A fixed lens such as the one at Admiralty Head does not move. Lighthouses that employ flashing or occulting do it by rotating the lens. The cycle or period is controlled by rotation speed and number of features around the circumference, such as the round prism sections called bulls eyes, which appear to a distant observer as a flash. Colored sectors may be stationary or rotating, and some sectors may be blocked out, usually with mirrored sections.

The Cleaning Chore

When the Admiralty Head light came on line in 1861, it was powered by a whale- or sperm-oil burner. Over the years, light sources were improved and upgraded. New systems usually were introduced on the East Coast and later spread to the West Coast. In 1867, whale oil gave way to lard oil as standard. In 1877, kerosene (also called mineral oil) was introduced. Admiralty Head Lighthouse made the conversion from oil to kerosene in 1880. By 1885, kerosene had become the principal illuminant throughout the lighthouse system. Various kinds, numbers and shapes of wicks drew the liquid up to the flame, but all the oil-powered burners shared the same big drawback - they produced soot or smoke.

For the early lighthouse keepers, cleaning the lens and lantern house was a constant and important chore.

A new-technology kerosene lamp eventually increased the burn temperature and light intensity, while decreasing the cleaning problem. It was the incandescent oil vapor lamp, in which kerosene under pressure was forced into a heated chamber where the liquid vaporized and was passed into a mantel burning very brightly.

By 1886, electricity was introduced to the lighthouse at the Statue of Liberty. But the unavailability of electrical power at many stations meant that electric light was slow to be introduced elsewhere. When burners were removed and electric bulbs installed, a fourth order lens such as the one on display at Admiralty Head used about a 500-watt bulb, probably of a cylindrical design, rather than spherical, with the bulb placed with its axis in the vertical.

No Two Alike

Mariners today, as in the past, determine which light they are viewing by consulting marine navigation charts and light lists that identify, among other things, the light's unique features. These characteristics include color, fixed/flashing/occulting, and period or cycle time. The characteristics of any light will be different than its visible neighbors.