Lights may be either flashing, when the dark period exceeds the light, or occulting, when the dark period is equal to or less than the light; fixed lights are liable to cause confusion. The pattern of lighting is individually varied so that ships or aircraft can identify the lighthouse. In fog, sound signals may also be used, such as horns and sirens.

The light is magnified by one of two methods: catoptric, which uses reflection; dioptric, which uses refraction. Modern lighthouses are catadioptric, a combination of the two.

Lighthouses, UK

In the UK there are three lighthouse authorities: Trinity House, the Northern Lighthouse Board, and the Commissioners of Irish Lights.

Early lighthouses used fire as the source of the signal, and include the Pharos of Alexandria (c. 280 BC) and those built by the Romans at Ostia and Ravenna in Italy, Boulogne in France, and Dover, England. In England, beacons burning in church towers often served as lighthouses until the 17th century. In lighthouses such as Smeaton Eddystone (1698), as few as 24 candles were used. The arrival of electricity in the 1860s introduced the ‘carbon arc’, a dangerous system that used a high-voltage spark for the signal. The tungsten filament light bulb, invented by Thomas Edison in 1879, standardized the use of electricity in lighthouses. By the end of the 20th century, bulbs of 1000-3000 watts were being used.

Where it is impossible to install a fixed structure, unattended lightbuoys equipped for up to a year's service may be used. Fog bells and whistles are operated by the movement of the waves. Where reefs or sandbanks make erection of a lighthouse impossible, lightships may be moored.

The numbers of lighthouses and lightships are declining with the increased use of navigational aids such as GPS. In the United Kingdom all lighthouses are now uncrewed and in the United States most are. Most British lightships are now operated by solar power, but the brightest ones are powered by diesel-electric generators.

weblinks

Lighthouse Society of Great Britain

images

Faro Blanco, Florida

lighthouse, Maryland

Needles, The

Watch Hill, Rhode Island