Lighthouses

An aid for navigation and pilotage at sea, a lighthouse is a tower building or framework sending out light from a system of lamps and lenses or, in older times, from a fire. Lighthouses also provide coordinate location for small aircrafts travelling at night. more...

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More primitive navigational aids were once used such as a fire on top of a hill or cliff (see beacon).

Because of modern navigational aids, the number of operational lighthouses has declined to less than 1,500 worldwide. Lighthouses are used to mark dangerous coastlines, hazardous shoals away from the coast, and safe entries to harbors.

Perhaps the most famous lighthouse in history is the Lighthouse of Alexandria, built on the island of Pharos in ancient Egypt. The name of the island is still used as the noun for "lighthouse" in some languages, for example: French (phare), Italian and Spanish (faro), Portuguese (farol), Swedish, Danish and Norwegian (fyr), Romanian (far) and Greek (φάρος). The word "pharology" (study of the lighthouses), is also derived from the island's name.

History

Lighthouses originally contained an open fire. Later, they were gas-powered and electric.

Design

In order to conserve power, the light is concentrated.

In old lighthouses:

vertically the light is bundled into horizontal directions; horizontally the light is bundled into one or a few directions at a time, with the light beam sweeping around; as a result, in addition to seeing the light beam from the side, from every horizontal direction there are instants that one sees the light directly, hence from a larger distance.;

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 clockwork assembly wound by lighthouse keepers, sometimes as often as every two hours. 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 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 lens is 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, automated lighthouses, this system of rotating lenses is often replaced by a very bright light that emits brief omnidirectional flashes (concentrating the light in time rather than space). These lights are similar to the aerodrome beacons used to warn aircraft away from tall structures.

Read more at Wikipedia.org