The Lighthouse of Alexandria, the world’s first known lighthouse, was built in 280 B.C. Since then thousand of lighthouses have been built resorting to the most advanced technologies for their time. In order to obtain a longer range light it was fundamental to “concentrate” the emitted light. Swiss physician Aimé Argand (1755-1803) had already invented in 1781 a new type of lamp with a wick that concentrated the light as it performed an almost full circle. However, the lens systems that were used at the time to concentrate the light and to carry the light signal in concentrated beams were very expensive. In 1822 Augustin Jean Fresnel (1788-1827) solved the problem when he designed a lighter and more economical lens system.
There are no identical lighthouses both in architectural terms and in what concerns the “language” used … The light signal they emit is unique and there is a simple reason for this: the ship has to identify the place it is passing by … This identification consists of three elements. The first element is the way how the light flashes: if it exhibits only short flashes of light it is called a flashing light (code “Fl”); if it emits an almost continuous light with short interruptions it is called occulting light (code “Oc”); and finally if the flashing light has dark and light periods of equal length it is called isophase light (code “Iso”). The second element is the colour of the light – white, red or green – which is usually identified by the initial of the English word. The third element is the duration in seconds of a full light cycle.
