A lighthouse is a tower, building, or other type of structure designed to emit light from a system of lamps and lenses or, in older times, from a fire, and used as an aid to navigation for maritime pilots at sea or on inland waterways.

Lighthouses mark dangerous coastlines, hazardous shoals, reefs, safe entries to harbors, and can also assist in aerial navigation. Once widely used, the number of operational lighthouses has declined due to the expense of maintenance and replacement by modern electronic navigational devices.

Design

Architect's design of Split Rock Lighthouse The lighthouses in Finland as of 1909 showing differing architecture

To be effective the lamp must be high enough to be seen before the danger is reached by a mariner. The minimum height is calculated according to trigonometry by taking the square root of the height of the light above the water in feet and multiplying it by 1.17 to yield the distance to the horizon in nautical miles.

Where dangerous shoals are located far off a flat sandy beach, the prototypical tall masonry coastal lighthouse is constructed to assist the navigator making a landfall after an ocean crossing. Often these are cylindrical to reduce the effect of wind on a tall structure, such as Cape May Light. Smaller versions of this design are often used as harbor lights to mark the entrance into a harbor, such as New London Harbor Light.

Where a tall cliff exists, a smaller structure may be placed on top such as at Horton Point Light. Sometimes, such a location can be too high, for example along the west coast of the United States, where frequent low clouds can obscure the light. In these cases, lighthouses are placed below clifftop to ensure that they can still be seen at the surface during periods of fog or low clouds, as at Point Reyes Lighthouse. Another victim of fog was the Old Point Loma lighthouse, which was replaced in 1891 with a lower lighthouse, New Point Loma lighthouse.

As technology advanced, prefabricated skeletal iron or steel structures tended to be used for lighthouses constructed in the 20th century. These often have a narrow cylindrical core surrounded by an open lattice work bracing, such as Finns Point Range Light.

Sometimes a lighthouse needs to be constructed in the water itself. Wave-washed lighthouses are masonry structures constructed to withstand water impact, such as Eddystone Lighthouse in Britain and the St. George Reef Light off California. In shallower bays, Screw-pile lighthouse ironwork structures are screwed into the seabed and a low wooden structure is placed above the open framework, such as Thomas Point Shoal Lighthouse. As screw piles can be disrupted by ice, steel caisson lighthouses such as Orient Point Light are used in cold climates. Orient Long Beach Bar Light (Bug Light) is a blend of a screw pile light that was converted to a caisson light because of the threat of ice damage.

In waters too deep for a conventional structure, a lightship might be used instead of a lighthouse, such as the former lightship Columbia. Most of these have now been replaced by fixed light platforms (such as Ambrose Light) similar to those used for offshore oil exploration.