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.

Lighthouses are used to mark dangerous coastlines, hazardous shoals and 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 aids.

Lighthouse technology

Lens technology

Prior to modern strobe lights, lenses were used to concentrate the light from a continuous source. Two tasks were involved:

• vertical light rays of the lamp are redirected into a horizontal plane
• horizontally the light is focused into one or a few directions at a time, with the light beam sweeping around; as a result, in addition to seeing the side of light beam, there are instants that one can see the light directly from a further distance away.

Fresnel lens: This concentration of light is accomplished with a rotating lens assembly. In classical period lighthouses, the light source was a kerosene lamp, and the lenses were rotated by a weight driven 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 motor drives were used, generally powered by diesel electric generators. These also supplied electricity for the lighthouse keepers.Crompton & Rhein (2002)

Efficiently concentrating the light from an omnidirectional light source requires a lens of very large diameter. This would require a very thick and heavy lens if naively implemented. The development of the Fresnel lens (pronounced 1/FREH nel or 2/frez null) in 1822 revolutionized lighthouses in the 1800s, focusing 85% of a lamps light versus the 20% focused with the parabolic reflectors of the time. Its design enabled the construction of lenses of large size and short focal length without the weight and volume of material in a lens of conventional design. Although the Fresnel lens was invented in 1822, it wasn't used in the US until the 1850s due to the parsimonious administrator of the United States Lighthouse Establishment, Stephen Pleasonton. With the creation of the United States Lighthouse Board in 1852, all U. S. lighthouses received Fresnel lenses by 1860.

Fresnel lens order of magnitude: Fresnel lenses were ranked by Order, with a first order lens being the largest, most powerful and expensive; and a sixth order lens being the smallest. The order is based on the focal length of the lens. A first order lens has the longest focal length, with the sixth being the shortest. Coastal lighthouses generally use first, second or third order lenses, while harbor lights and beacons use fourth, fifth or sixth order lenses.