Lighthouse Designs

An aid for navigation and pilotage at sea, a lighthouse is a towerbuilding or framework sending out light from a system of lamps and lensesor, in older times, from a fire. More primitive navigational aids were once used such as a fire on top of a hill or cliff. Because of modern navigational aids, the number of active lighthouses has declined to fewer than 1,500 worldwide. Lighthouses are used to mark dangerous coastlines, hazardous shoals away from the coast and safe entries to harbors.

In order to conserve power, the light is concentrated. In old lighthouses: vertically the light is bundled into horizontal directions and horizontally the light is bundled into one or a few directions at a time, but sweeping around, so that it can be observed from every horizontal direction.

This concentration of light is accomplished with a rotating lens assembly. In very old lighthouses, the light source was a kerosene lamp and the lenses were rotated by a clock work assembly wound daily by lighthouse keepers. The lens assembly usually floated in mercury to reduce friction. In more modern lighthouses, electric lights and clock drives were used, generally powered by diesel burning electric generators. These also supplied electricity for the lighthouse keepers.

Efficiently concentrating the light from an omnidirectional source of the type used in lighthouses requires a lens of very large diameter. This would lead to a very thick and heavy lens if naively implemented. A Fresnel lensis a type of lens developed for lighthouses. Its design enables the construction of lenses of large size and short focal length without the weight and volume of material which would be required in a lens of conventional design. Some lighthouses, such as those at Cape Race, Newfoundland, and Makapu'u Point, Hawaii, used a special hyperradiant lens manufactured by the firm of Chance Bros.

In modern, unmanned lighthouses, this system of rotating lenses is often replaced by a very bright light that emits brief omnidirectional flashes. These lights are similar to the aerobeacons used to warn aircraft away from tall structures.

In any of these designs an observer, rather than seeing a continuous weak light, sees a brighter light during short time intervals. These instants of bright light are arranged to create a characteristic lightpattern specific to the particular lighthouse. For example, for the lighthouse of Scheveningen the time intervals between these instants are alternately 2.5 and 7.5 seconds.