Structure designed to emit light to aid navigation
A lighthouse is a tower, building, or other type of physical structure designed to emit light from a system of lamps and lenses and to serve as a beacon for navigational aid for maritime pilots at sea or on inland waterways.
Lighthouses mark dangerous coastlines, hazardous shoals, reefs, rocks, and safe entries to harbors; they also assist in aerial navigation. Once widely used, the number of operational lighthouses has declined due to the expense of maintenance and the advent of much cheaper, more sophisticated, and more effective electronic navigational systems.
History.
Ancient lighthouses.
Before the development of clearly defined ports, mariners were guided by fires built on hilltops. Since elevating the fire would improve visibility, placing the fire on a platform became a practice that led to the development of the lighthouse. In antiquity, the lighthouse functioned more as an entrance marker to ports than as a warning signal for reefs and promontories, unlike many modern lighthouses. The most famous lighthouse structure from antiquity was the Pharos of Alexandria, Egypt, which collapsed following a series of earthquakes between 956 and 1323.
The intact Tower of Hercules at A Coruña, Spain gives insight into ancient lighthouse construction; other evidence about lighthouses exists in depictions on coins and mosaics, of which many represent the lighthouse at Ostia. Coins from Alexandria, Ostia, and Laodicea in Syria also exist.
Modern construction.
The modern era of lighthouses began at the turn of the 18th century, as the number of lighthouses being constructed increased significantly due to much higher levels of transatlantic commerce. Advances in structural engineering and new and efficient lighting equipment allowed for the creation of larger and more powerful lighthouses, including ones exposed to the sea. The function of lighthouses was gradually changed from indicating ports to the providing of a visible warning against shipping hazards, such as rocks or reefs.
The Eddystone Rocks were a major shipwreck hazard for mariners sailing through the English Channel. The first lighthouse built there was an octagonal wooden structure, anchored by 12 iron stanchions secured in the rock, and was built by Henry Winstanley from 1696 to 1698. His lighthouse was the first tower in the world to have been fully exposed to the open sea.
The civil engineer John Smeaton rebuilt the lighthouse from 1756 to 1759; his tower marked a major step forward in the design of lighthouses and remained in use until 1877. He modeled the shape of his lighthouse on that of an oak tree, using granite blocks. He rediscovered and used "hydraulic lime", a form of concrete that will set under water used by the Romans, and developed a technique of securing the granite blocks together using dovetail joints and marble dowels. The dovetailing feature served to improve the structural stability, although Smeaton also had to taper the thickness of the tower towards the top, for which he curved the tower inwards on a gentle gradient. This profile had the added advantage of allowing some of the energy of the waves to dissipate on impact with the walls. His lighthouse was the prototype for the modern lighthouse and influenced all subsequent engineers.
One such influence was Robert Stevenson, himself a seminal figure in the development of lighthouse design and construction. His greatest achievement was the construction of the Bell Rock Lighthouse in 1810, one of the most impressive feats of engineering of the age. This structure was based upon Smeaton's design, but with several improved features, such as the incorporation of rotating lights, alternating between red and white. Stevenson worked for the Northern Lighthouse Board for nearly fifty years during which time he designed and oversaw the construction and later improvement of numerous lighthouses. He innovated in the choice of light sources, mountings, reflector design, the use of Fresnel lenses, and in rotation and shuttering systems providing lighthouses with individual signatures allowing them to be identified by seafarers. He also invented the movable jib and the balance-crane as a necessary part for lighthouse construction.
Alexander Mitchell designed the first screw-pile lighthouse – his lighthouse was built on piles that were screwed into the sandy or muddy seabed. Construction of his design began in 1838 at the mouth of the Thames and was known as the Maplin Sands lighthouse, and first lit in 1841. Although its construction began later, the Wyre Light in Fleetwood, Lancashire, was the first to be lit (in 1840).
Lighting improvements.
Until 1782 the source of illumination had generally been wood pyres or burning coal. The Argand lamp, invented in 1782 by the Swiss scientist Aimé Argand revolutionized lighthouse illumination with its steady smokeless flame. Early models used ground glass which was sometimes tinted around the wick. Later models used a mantle of thorium dioxide suspended over the flame, creating a bright, steady light. The Argand lamp used whale oil, colza, olive oil or other vegetable oil as fuel, supplied by a gravity feed from a reservoir mounted above the burner. The lamp was first produced by Matthew Boulton, in partnership with Argand, in 1784, and became the standard for lighthouses for over a century.
South Foreland Lighthouse was the first tower to successfully use an electric light in 1875. The lighthouse's carbon arc lamps were powered by a steam-driven magneto. John Richardson Wigham was the first to develop a system for gas illumination of lighthouses. His improved gas 'crocus' burner at the Baily Lighthouse near Dublin was 13 times more powerful than the most brilliant light then known.
The vaporized oil burner was invented in 1901 by Arthur Kitson, and improved by David Hood at Trinity House. The fuel was vaporized at high pressure and burned to heat the mantle, giving an output of over six times the luminosity of traditional oil lights. The use of gas as illuminant became widely available with the invention of the Dalén light by Swedish engineer Gustaf Dalén. He used Agamassan (Aga), a substrate, to absorb the gas, allowing the gas to be stored, and hence used, safely. Dalén also invented the 'sun valve', which automatically regulated the light and turned it off during the daytime. The technology was the predominant light source in lighthouses from the 1900s to the 1960s, when electric lighting had become dominant.
Optical systems.
With the development of the steady illumination of the Argand lamp, the application of optical lenses to increase and focus the light intensity became a practical possibility. William Hutchinson developed the first practical optical system in 1777, known as a catoptric system. This rudimentary system effectively collimated the emitted light into a concentrated beam, thereby greatly increasing the light's visibility. The ability to focus the light led to the first revolving lighthouse beams, where the light would appear to the mariners as a series of intermittent flashes. It also became possible to transmit complex signals using the light flashes.
French physicist and engineer Augustin-Jean Fresnel developed the multi-part Fresnel lens for use in lighthouses. His design allowed for the construction of lenses of large aperture and short focal length, without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can also capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances.
The first Fresnel lens was used in 1823 in the Cordouan lighthouse at the mouth of the Gironde estuary; its light could be seen from more than out. Fresnel's invention increased the luminosity of the lighthouse lamp by a factor of four and his system is still in common use.
Modern lighthouses.
The introduction of electrification and automatic lamp changers began to make lighthouse keepers obsolete. For many years, lighthouses still had keepers, partly because lighthouse keepers could serve as a rescue service, if necessary. Improvements in maritime navigation and safety, such Global Positioning System (GPS), led to the phasing out of non-automated lighthouses across the world. Although several closed due to safety concerns, Canada still maintains 49 staffed lighthouses, split roughly evenly across east and west coasts.
The remaining modern lighthouses are usually illuminated by a single stationary flashing light powered by solar-charged batteries and mounted on a steel skeleton tower. Where the power requirement is too great for solar power alone, "cycle charging" of the battery by a Diesel generator is provided. The generator only comes into use when the battery needs charging, saving fuel and increasing periods between maintenance.
Famous lighthouse builders.
John Smeaton is noteworthy for having designed the third and most famous Eddystone Lighthouse, but some builders are well known for their work in building multiple lighthouses. The Stevenson family (Robert, Alan, David, Thomas, David Alan, and Charles) made lighthouse building a three-generation profession in Scotland. 
Richard Henry Brunton designed and built 26 in Meiji Era Japan, which became known as Brunton's "children". Blind Irishman Alexander Mitchell invented and built a number of screw-pile lighthouses. Englishman James Douglass was knighted for his work on the fourth Eddystone Lighthouse.
United States Army Corps of Engineers Lieutenant George Meade built numerous lighthouses along the Atlantic and Gulf coasts before gaining wider fame as the winning general at the Battle of Gettysburg. Colonel Orlando M. Poe, engineer to General William Tecumseh Sherman in the siege of Atlanta, designed and built some of the most exotic lighthouses in the most difficult locations on the U.S. Great Lakes.
French merchant navy officer Marius Michel Pasha built almost a hundred lighthouses along the coasts of the Ottoman Empire in a period of twenty years after the Crimean War (1853–1856).
Technology.
In a lighthouse, the source of light is called the "lamp" (whether electric or fuelled by oil) and the light is concentrated, if needed, by the "lens" or "optic". Power sources for lighthouses in the 20th–21st centuries vary.
Power.
Originally lit by open fires and later candles, the Argand hollow wick lamp and parabolic reflector were introduced in the late 18th century.
Whale oil was also used with wicks as the source of light. Kerosene became popular in the 1870s and electricity and acetylene gas derived on-site from calcium carbide began replacing kerosene around the turn of the 20th century. Carbide was promoted by the Dalén light, which automatically lit the lamp at nightfall and extinguished it at dawn.
In the second half of the 20th century, many remote lighthouses in Russia (then Soviet Union) were powered by radioisotope thermoelectric generators (RTGs). These had the advantage of providing power day or night and did not need refuelling or maintenance. However, after the collapse of the Soviet government in 1990s, most of the official records on the locations, and condition, of these lighthouses were reportedly lost. Over time, the condition of RTGs in Russia degraded; many of them fell victim to vandalism and scrap metal thieves, who may not have been aware of the dangerous radioactive contents.
Energy-efficient LED lights can be powered by solar panels, with batteries instead of a Diesel generator for backup.
Light source.
Many Fresnel lens installations have been replaced by rotating aerobeacons, which require less maintenance.
In modern automated lighthouses, the system of rotating lenses is often replaced by a high intensity light that emits brief omnidirectional flashes, concentrating the light in time rather than direction. These lights are similar to obstruction lights used to warn aircraft of tall structures. Later innovations were "Vega Lights", and experiments with light-emitting diode (LED) panels.
LED lights, which use less energy and are easier to maintain, had come into widespread use by 2020. In the United Kingdom and Ireland about a third of lighthouses had been converted from filament light sources to use LEDs, and conversion continued with about three per year. The light sources are designed to replicate the colour and character of the traditional light as closely as possible. The change is often not noticed by people in the region, but sometimes a proposed change leads to calls to preserve the traditional light, including in some cases a rotating beam. A typical LED system designed to fit into the traditional 19th century Fresnel lens enclosure was developed by Trinity House and two other lighthouse authorities and costs about €20,000, depending on configuration, according to a supplier; it has large fins to dissipate heat. Lifetime of the LED light source is 50,000 to 100,000 hours, compared to about 1,000 hours for