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, 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.

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 the light beam, there are instants that one can see the light directly from a further distance away.

Fresnel lens

main: Fresnel lens

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.

Some lighthouses, such as those at Cape Race, Newfoundland, and Makapu'u Point, Hawaii, used a more powerful hyperradiant Fresnel lens manufactured by the firm of Chance Brothers.

In recent times, many Fresnel lenses have been replaced by rotating aerodrome beacons which require less maintenance. In modern automated lighthouses, this 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. Recent innovations are "Vega Lights", and initial experiments with LED panels.