A lighthouse is a tower, building, or framework 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 and to pilots at sea or on inland waterways.
Lighthouses are used to mark dangerous coastlines, hazardous shoals and reefs, and 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 aids.
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 – as along the west coast of the United States. In these cases, lighthouses are placed below clifftop to ensure that they can still be seen at the surface during periods of fog, as at Point Reyes Lighthouse.
Another victim of fog was Point Loma Light (old) which was replaced with a lower lighthouse, Point Loma Light (new).
As technology advanced, prefabricated skeletal iron or steel structures tended to be used for lighthouses constructed in the twentieth 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 the St. George Reef Light off California.
In shallower bays, screw pile 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, in northern climates steel caisson lighthouses such as Orient Point Light are used.
Orient Long Beach Bar Light (Bug Light) is a blend of a screw pile light that was later 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.
Most of these have now been replaced by fixed light platforms (such as Ambrose Light) similar to those used for offshore oil exploration.