​Classification The visual aids have always been classical because they are directed directly at the sailor’s senses and it is not necessary to have special on board instruments to detect them.  The message they give may easily be identified and it is possible to determine the approximate area where we are simply by consulting the books of Sea Lanes or the books of Lighthouses and the sea chart of the area. We can classify the visual aids in blind signals and luminous signals. The blind signal transmitts a message during the day whereas the luminous signal deals with transmitting the same message, but at night. The daytime message is formed by the colour, the shape and the marking on top of the signal but it can generally only be recognised at short distances of a few hundred metres.  These distances basically depend on one’s sight, the contrast with the background, the clearness of the air, etc. Their information is useful for all kinds of navigation, but especially for coastal navigation, for small fishing boats and sports vessels.  The night time visual signals transmit their message through the colour of their light and its rhythm, in other words the phases of light and darkness that succeed in it rhythmically or repetitively.  Its range is limited by the geographic scope determined by the Earth’s curvature.  Among the luminous aids we might distinguish many types such as lighthouses, beacons, port lights, satellites, buoys, lighthouse ships, etc. Lighthouses are aids with a daily mark of no standard shape and are located at singular points on the coastline and have a luminous range of over 10 nautical miles (criterion used in Spain). Beacons may be placed on land on fixed posts or on buoys in the sea. Their range is under 10 nautical miles and they are usually placed in canals or at danger points close to navigable waters. The port lights are placed at the harbour mouth, at the end of keys or on the edges of dredged areas and they are intended to guide sailors both to the entrance and exit.  Sector lights mark safe routes to reach port.History and evolution Visual aids to navigation date back to man’s first attempts to explore new trade routes, when they moved a considerable distance away from the coast. During the day the geography itself served to guide them but during the night they had to use the light given off by fires lit in sufficiently high strategic points for them to be able to locate the coast and return to port.  Later, to ensure that these fires would not go out with the wind or rain, they were protected by some kind of structure and then reflective surfaces were placed around them to amplify the light and make them more visible over large distances.  The device was not complicated but required someone always to keep the fire alight so that the aid would always be available and the sailor would feel protected.  This was the beginning of the lighthouse. The first permanent construction of this kind that we know about was located on the Pharoe Island in Alexandria and this is where the name of PHARE comes from.  This city, which was founded by Alexander the Great, had a very busy natural port so they considered it was necessary to build a large illuminated tower at its entrance so that sailors would easily find refuge and shelter.  From pre-Roman times we have no more references of constructions of this kind although the Colossus of Rhodes at the entrance to the port of the same name might also have been a lighthouse, but this has not been proven.  From Roman times we have news of many others for example in Italy, the Ostia lighthouse in the port of Rome, which was built by the Emperor Claudius, the Messina lighthouse in Sicily or the Capri lighthouse which was destroyed by an earthquake at the end of the reign of Tiberius.  In France, the best known from this time are those of Boulogne, Frejus and Marseille and in England the Dover lighthouse. With the fall of the Roman Empire, trade was paralysed and the countries focused more on war than on social and economic development, so not only were no new lighthouses built, with the exception of that on the isle of Cordouan, the first built in the open sea, but the existing ones disappeared.  After the 12th century, navigation was reactivated in the Mediterranean and Northern Europe and the safety of the routes meant that it was necessary to start building the signals again, which would turn Scandinavia and Germany (15 lighthouses in 1600) into the best lit coastline in Europe.  Trade in the Mediterranean also favoured the construction of numerous lighthouses and other fires were also built on existing watchtowers, as is the case of the Porto Pi lighthouse on Majorca. Coastline lighting started properly with the Middle Ages.  After the 18th century, commercial relations were increased between the states and sea traffic grew, so not only was it necessary to have guiding light in the most important ports, but it was also necessary to signal the coastline and the existing dangers such as wrecks and shallow waters. England was the country that had most lighthouses at this time, as it charged levies to the ships coming into its ports and used the money to maintain the existing lighthouses and to build others trying out new technologies.  Its level of lighting was followed by the French coasts and the American colonies.  Lighting in the rest of Europe and the world really didn’t exist. In the second half of the 19th century, the drive to building lighthouses was definitive due to the possibility of using new materials and new sources of energy.  In Spain in 1842, the first Permanent Lighthouse Commission was constituted and in 1847 the first Maritime Lighting Plan was approved for the Spanish coasts and gave rise to most of the lighthouses existing in our country today. Along with the traditional brickwork lighthouses there came much lighter metal lighthouses such as those of the Ebro estuary, Buda, El Fangar and la Baña. Another important lighthouse for its size is that of the Cape Palos which housed the School of Torreros or those of Chipiona and Trafalgar.Lighting systems Until the end of the 18th century, all of the lighthouses had wood or coal fires or oil or fat wicks that did not produce much light and caused a lot of smoke, and also involved having to carry heavy loads of fuel to such a great height. The flames were protected with closed lamps with glass, though conveniently ventilated. One of the first technological developments were the lamps in which a cylindrical cotton wick surrounded by a glass tube regulating the oxygen added to the combustion absorb the oil by capiliarity.  The wick could go up and down the glass chimney directing the air current to it end.  The amount of light given off was increased by placing several wicks in each lamp, but this was at the expense of consuming more oil.  Soon the possibility was studied of feeding them with different types of cheaper oils and much easier to obtain. Oil fell into disuse with the arrival of petrol, which was able to produce much more light.  The burners were perfected and as a result glow wicks were made in which the fuel rose through a pipe by means of pressurised air to a vaporiser where it was heated and became vapour that came out through an injector and burnt with the air. Then experiments were carried out with gaseous fuels despite the serious risks in their transport and handling until acetylene began to be used, which gave a very brilliant light and which when dissolved in acetone was less dangerous. The Swede Gustav Dalen made a gas device with automatic lighting, which was known as the solar valve and gave him the Nobel Prize for physics in 1812, and this immediately began to be used in distant lighthouses.  In Spain it was used for the first time in the Peña Horadada beacon on the entrance to Santander port. Electric power was definitive for supplying the lighthouse lights.  After several experiments in Beacons, the Villano lighthouse was lit, the first electric lighthouse on our coastline which was given an arc lamp. And so we come to the present incandescent lamps, sealed beams, halogen, etc. and the renewable energies such as photovoltaic and wind energy to supply them.Optical systems Another problem presented by lighthouse lamps was how to make the weak light produced visible over large distances and to enable sailors to distinguish some lines from others.  All of this gave rise to the development of optical systems. The first idea to amplify the light was based on reflection, placing a parabolic mirror behind the flame to concentrate the light while the limitation of the angle of vision created by the factor was resolved by turning the optical unit. Later lenses began to be used which concentrated the beams of light in a parallel beam.  But it was Agustín Fresnel who revolutionised the optical systems by placing total reflection prisms over and under the main lens to reinforce the emerging beam. To distinguish sunlight from others, the optics were built-in panels so as they turned they could achieve the required number of clashes in each case.  However as they were cut glass units, they were very heavy and a great effort was required to make them turn so between one and four minutes were needed to recognise the appearance and the ship along way from the lighthouse and subject to the movement of the waves could easily lose count of the number of flashes produced. This problem was resolved by placing the optical system on a mercury float which considerably reduced the friction and the turning speed could be increased sufficiently to make the whole characteristic visible in a much shorter time.  Today much lighter and smaller acrylic optics are used and achieve magnificent results.Lanterns These are the constructions that protect the light.  In the wood lighthouses they were a simple dome sustained by vertical supports with protection on the earth.  In oil lighthouses they were protected by glasses fitted into vertical columns. The shape of the lamp was polygonal and sometimes cylindrical and the number of sides coincided with the number of panels of the optic and was made to correspond to the number of columns. However, the vertical columns caused unwanted reflections that produced parasite flashing, forming areas of shade while turning that gave a false characteristic.  The problem was resolved by using cylindrical lamps with curved glass and helicoidal columns. The top part was enclosed with a metal, generate copper, dome open at the top for ventilation. When the roof was transparent, the portion of light getting through also served as a guide for a navigation, and special systems were used for this.