Lighthouse Terminology Part 1

ATON

Acronym for aid to navigation. In addition to lighthouses, aids to navigation include buoys, fog signals, lightships and range lights.

Automatic Light

Unstaffed station projecting an automatic, electric-powered light. Some 400 U.S. lighthouses are still in active service, administered by the U.S. Coast Guard, including Sandy Hook, the oldest lighthouse in the United States, built in 1764. All but one of the active lighthouses is automated. The only one still staffed is Boston Light, built in 1783 on the exact site of America’s first lighthouse, built in 1716.

BIVALVE_NAVESINKBivalve Lens

Two-sided, rotating clam-shell Fresnel lens. On each side is a centered, round, convex bull’s-eye prism, surrounded by circular rows of three-sided prisms. Light from the interior light scatters in all directions. But the Fresnel prisms work together to capture more than 80% of the light and to refract or bend the light into a horizontal beam that is reflected for many miles.

The bivalve Fresnel lens shown, with an optical area measuring 8½ feet high, was activated in 1898 in the south tower of Navesink Twin Lights. The revolving lens, illuminated by an electric arc lamp, projected a beacon that could be seen 22 miles at sea. This bivalve lens is now on display at Twin Lights.

 

 

 

 

 

Bull’s-Eye PrismFresnel_Lens_bullseye

A convex lens in the center of a Fresnel lens. The inside surface of the bull’s-eye prism facing the light source, is flat. The exterior side curves outward. Depending on the size and style of the Fresnel lens, it could have one or several bull’s-eye prisms, each with associated surrounding prisms. The bull’s-eyes and the surrounding prisms capture more than 80% of the light, bend or refract the light into a horizontal beam that is reflected for many miles. The bull’s-eye prism orients, focuses and intensifies the beam at its center.

caisson_fourteenfoot_njlhsCaisson Lighthouse

Off-shore lighthouse whose foundation is a caisson, a cast-iron cylinder placed vertically into the water and extending down into the water’s floor and then filled with concrete and stone. The lighthouse – often a metal building but sometimes brick or stone – stands atop the concrete within the caisson.

The caisson lighthouse, developed by Englishman Lawrence Potts in 1845, was a superior and sturdier design to screw-pile lighthouses, which were built on platforms supported by metal piles that extended deep into the water’s floor of sand and rock. There are several surviving but deactivated caisson lighthouses in New Jersey waters: Brandywine Shoal, Fourteen Foot Bank and Ship John Shoal, all in Delaware Bay, and Great Beds in Raritan River, South Amboy.

Channel

The tower shaft through which drops weight(s) that drive gears causing a revolving style lens to rotate on its pedestal. While the light source is constant, the revolving lens creates the illusion of a pulsing light (weak, then bright, then weak and repeat) or a light blinking on and off, on and off. The light is dim as the leading edge of the revolving bull’s-eye prism captures and projects the light that then grows brightest as it hits the center of the convex bull’s-eye, only to dim as the light passes through the trailing edge of the moving bull’s-eye. In some lenses, the light appears to go out briefly. This occurs in lenses where the bull’s-eye prisms are separated by brass plates or doors that block the projection of light.

Character(istic)

The signature or unique appearance of a lighthouse’s beacon to enable mariners to distinguish it from the beacons of other lighthouses in the area. There are several variables or characteristics that determine a beacon’s signature: one or more lights, color of a beacon (white, red, green, etc.), one color or changing colors, a steady light or a blinking light, if a blinking light the length and number of flashes and dark periods per minute.

Chimney

A glass drum or globe, open at top and bottom, that is placed around the light source but inside the lens to give the projected beacon color. The chimney is either clear or colored glass. Sea Girt’s beacon was originally red, produced by a ruby chimney, that was replaced in 1912 by a clear chimney to project a white light. Also known as shade or filter.

Daymark

Lighthouses and other light stations have two principle functions – project a beacon at night and during daylight to serve as landmarks, also known as daymarks. The location of each lighthouse is carefully chosen to provide a beacon of light to guide ships to the next beacon of light. The lighthouse also stands as an identifier of local conditions, such as proximity to an inlet or harbor, or as a warning of hazardous conditions, including sandbars, shoals, etc. In addition to the distinctive characteristic of each beacon, each light station was built or painted to look different from other light stations in the area to help mariners figure out where they are.

Eclipse

The periods in the revolution of a lens when light appears to go dark.

(Time to) Exhibit Lights

Nighttime – sunset to sunrise – is when the light station lens is lighted, projecting its beacon. “Lights must be exhibited punctually at sunset and kept lighted at full intensity until sunrise, when the lights will be extinguished and the apparatus put in order without delay for relighting,” according to U.S. Lighthouse Service regulations. Lights were extinguished during the daytime to save on fuel, the biggest expense in operating a lighthouse. During daylight, mariners navigate by landmarks (see daymark) along the coastline.

Filter

See definition for chimney.

Fixed Light

A steady, unblinking light projected by a stationary lens.

Fixed Light Flashing

A stationary lens that projects a flashing light. There are different ways to project a flash with a stationary lens. If the light source is an electric bulb, a timer can be set to turn the electric current on and off. Another method, regardless of the light source, is to use one or more external flash panels that rotate back and forth or around the stationary lens. As light hits the flash panel, it projects a flash that appears to dim or go out as the flash panel moves away from the beam of light.

Flashing Light

A lens that projects a flashing beacon. When a fuel-burning lamp was the light source in the era before electrification, the number of flashes and the length of each flash and period of darkness per minute were determined by the speed of the revolving lens and the number of bull’s-eye prisms in the lens. The light is dim as the leading edge of the revolving bull’s-eye prism captures and projects the light that then grows brightest as it hits the center of the convex bull’s-eye, only to dim as the light passes through the trailing edge of the moving bull’s-eye. In some lenses, the light appears to go out briefly. This occurs in lenses where the bull’s-eye prisms are separated by brass plates or doors that block the projection of light. As lighthouses were electrified in the 20th century and powerful light bulbs became the light source, a stationary lens could also be made to project a flashing light by use of a timer to turn the electric current on and off. In the case of a revolving lens, electrification eliminated the need for a mechanical system of weights, pulleys and gears to power the revolving lens.

Flash Panel (Internal)

The vertical section of a lens assembly that includes a bull’s-eye lens and the curved rows of three-sided prisms above and below the bull’s-eye prism. As the lens revolves, the light source hits a flash panel, which refracts or bends the light into a horizontal bar that is reflected for many miles. The light appears to flash as it first hits the leading edge of the bull’s eye and grows more intense as it reaches the middle of the bull’s eye only to dim as it goes through the trailing edge of the bull’s eye. This pulsing is repeated as the next flash panel passes between the light source and the observer of the light. In some designs, the light appears to go out briefly, when it hits the brass framework between the flash panels. The number of flash panels in a lens depends on the design and size of the lens. There could be anywhere from 2 to 12 flash panels or sections.

Fog Signal

An audible warning device – bell, horn, siren or whistle – activated at the lighthouse and operated manually or powered to warn mariners of the coastline and hazards during times of fog when it was difficult to see the lighthouse beacon. The first fog signal was sounded at Boston Light in 1719, a cannon that was fired once every hour during fog. In 1921, Sea Girt Lighthouse became the first land-based station to be equipped with a radio fog system. Sea Girt Light, Ambrose Channel Lightship and Fire Island Lightship transmitted radio beams. Ships fixed their positions by tracking the radio signals. Originally Sea Girt’s distinct radio signal was 3 dashes for 60 seconds followed by silence for 6 minutes. The signal was changed April 18, 1923 to 3 dashes for 30 seconds followed by silence for 3 minutes. The radio transmitter remained in service at Sea Girt until 1928 when it was transferred to Barnegat Lightship.

Fresnel lens

The type lens, first developed in 1822 by French engineer Augstin Fresnel (pronounced “Fray-nel”), that produces a more concentrated, focused and intensified beam than other type lenses previously used at lighthouses.

FRESNEL_FIXED_REVOLVINGThe breakthrough Fresnel lens looked like a beehive, with concentric rows of three-sided prisms surrounding the convex bull’s-eye prisms. Light scatters in all directions. Earlier lenses, using reflectors, were able to capture only 17% of the light from a flame. But the Fresnel prisms surrounding the light source capture more than 80% of the light and refract or bend the light into a horizontal beam that is reflected for many miles. The bull’s-eye prisms orient, focus and intensify the beam at its center. The length of the reflection depends on the size of the lens.

The first Fresnel lens was installed in France’s Cardovan Tower Lighthouse on the Gironde River in 1822, projecting its beacon 20 miles. Fresnel developed refinements and variations on his basic design. By making the lens revolve, he was able to produce a flashing beacon as the light projected through each of the bull’s-eye prisms. When the light was between bull’s eyes, the beacon appeared to dim or go dark. A stationary Fresnel lens of 12 flat vertical panels arranged in a circle projected a constant circle of light. A drawback of the stationary lens was the constant light made it difficult for mariners to distinguish one lighthouse beacon from another. Since a revolving lens could project a distinctive beacon based on the speed of its revolution and number of bull’s-eye prisms, mariners could more easily distinguish one lighthouse from another and thereby fix their position. FRESNEL_REFRACTION_DIAGRAM

By the 1840s, Fresnel lenses came in two basic shapes (clamshell and beehive) and in seven sizes or orders (1st, 2nd, 3rd, 3rd½, 4th, 5th, and 6th). First order, the largest lens with an optical area measuring 8½ feet high, projects a beacon 22 miles. The first two Fresnel lenses in the U.S. were installed at Navesink Twin Lights in 1841 – a 1st order beehive lens in the South Tower and a 2nd order beehive lens in the North Tower. Sea Girt was equipped with a 4th order revolving Fresnel lens, 2½-feet-high, projecting a beacon 15 miles. A 6th order lens, the smallest lens measuring 1½ feet high, projects a beacon five miles.

Gallery

The railed exterior walkway or parapet around the tower’s lantern room (upper gallery) or the watch room (main gallery) immediately below the lantern room where the keeper prepares and repairs wicks and lanterns.

GPS

Acronym for global positioning system, the electronic navigational system that enables a shipboard, airborne or land-based receiver to fix its position by triangulating satellite signals. GPS and its terrestrial forerunner LORAN (long-range radio navigation) contributed to the declining reliance on lighthouses for navigation.

Illumination (Period of)

Nighttime – sunset to sunrise – when the lighthouse lens is lighted, projecting its beacon. “Lights must be exhibited punctually at sunset and kept lighted at full intensity until sunrise, when the lights will be extinguished and the apparatus put in order without delay for relighting,” according to U.S. Lighthouse Service regulations. Lights were extinguished during the daytime to save on fuel, the biggest expense in operating a lighthouse. During daylight, mariners navigated by landmarks (see daymark) along the coastline.