Lighthouses:

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.
In any of these designs an observer, rather than seeing a continuous weak light, sees a brighter light during short time intervals. These instants of bright light are arranged to create a light characteristic or, pattern specific to the particular lighthouse.[5] For example, for the lighthouse of Scheveningen the time intervals between flashes are alternately 2.5 and 7.5 seconds.
To assist in distinguishing between lighthouses, the time interval of the light or the color pattern of the lens is varied. Sector lights may additionally have a red or green filter on parts of the lantern house to distinguish safe water areas from dangerous shoals.