A lighthouse is a type of navigational aid for ships sailing in coastal waters. It shines by night flashing a distinct signal which aids the ship in determining its location and in warning the ship away from rocky dangers. By day it becomes a landmark.
The 1st recognised lighthouse was built in 280BC called Pharos of Alexandria and was considered one of the 7 wonders of the world. The structure was 137 metres high made of thick stone with elaborate carvings and other ornamentation. Sitting at the top of the lighthouse was a cylinder shaped container where a wood fire lit the harbour by night and a mirror reflecting the sun by day. This system was said to be visible for up to 55 km. The lighthouse survived several invasions and many earthquakes until it was completely demolished in the 14th century by an earthquake.
Early lamps used in lighthouses were sooty, hard to keep burning in cold and drafty conditions, not very effective as beacons, and often had toxic fumes,and were a great fire risk. The most widely used lamp until 1812 was the spider lamp, which had a shallow brass pan as a reservoir and four to eight adjustable solid round wicks (without chimneys) that surrounded the pan.
During the height of the Shipping Age in the 18th century, France was looking for a way to make new lighthouses along the coast of Normandy and Brittany. The lenses that were used in the lighthouses were huge pieces of glass that were both bulky and expensive. In 1748, Georges de Buffon realized that only one side of a lens is needed to bend light. In fact, only the outer surface of the lens is needed. de Buffon cut away the inside of the lens and left rings with edges on the outside. Later, Augustin Fresnel ( see below ) modified this idea and the modern Fresnel lens was created. His lenses were first used on the French coast as a lightweight and less-expensive alternative to the old, bulky lighthouse lenses.
In 1781 a Swiss chemist named AMEE ARGAND invented a LAMP that was as bright as seven candles. It had two cylinders of brass: one inside to feed oil to the lamp's wick, and one outside to bring oxygen around the wick. This lamp used half the oil of the spider lamp. A parabolic reflector helped to magnify the light. By 1800 the lamp was in wide-spread use in europe, and it is believed that the first Argands lamp was used in an American lighthouse in 1809.
In 1822 Augustin Jean Fresnel (pronounced fra nell) introduced the lens that would change the world of lighthouses .He in conjunction with the Scotish enginering family firm( Stevenson's) are credited with the early development of light and lense units that are or have been use world wide ever since. They were able to magnify the flame of a typical lamp by nearly three times. It resembled a huge cut glass beehive surrounding a single light source. The light was bent through the prisms at the top and bottom to create a narrow beam of light. A magnifying glass at the centre intensified the light even further making it visible for many miles.Or for you technical minded people it went like this .. The French physicist had made a lens shaped like a bullet. Directly over and under the central glass drum are curved dioptic prisms and then above those are curved catadioptic prisms. Around the fixed lens' waist is a central drum of convex glass; the revolving optic employs a series of convex bullseye lenses. The lens has a delicate balance, is easily rotated and was extremely well-made. The principles of physics applied here are of refraction (bending) and reflection. The two types of prisms bend and reflect light rays: by positioning the prisms around the outside of the lens so that all emerging light rays are parallel to each other, the lens is capable of collecting up to 90% of the lamp's light and focusing it into an intense horizontal beam.Did you get all get all that ??
The problem with these early lights was that they weighed several tons and needed a strong mechanism to keep the system gears, pinions, shafts,and bearings moving. This restricted the size of lanterns that could be used and reduced the power and rotating speed of the light. In 1880 a new system of floating the lights on a bath of mercury was introduced making the revolution of the light easier and faster. This system was continued in New Zealand until about the 1950's when the dangers of mercury on ones health was discovered.
The lamps were eventually fuelled by kerosene, this sytem used a pressurized vapourised kerosene that was burnt injunction with a mantle,much the same as a lamp that is sometimes used when camping. This was introduced as it was more economical and had a brighter light. These were by far the most successful lights and lasted until 1950 when lighthouses began their conversion to electricity. Electricity made it possible to have a electrical light source most lighthouses remained with the fixed light but still retained the revolving lense system. As the introduction of electricity decreased the need for lighthouse keepers the lights were progressively demanned, today there are now no longer resident keepers.The old revolving lense's were removed ( does anyone know where ??) to be replaced by high tech electronic light units that are fully automatic fitted with self changing high powered lamps illuminating when sensors detect a change in daylight intensity.
Any altering of a beam produces each light station's light characteristic, by which they are classified. Moving screens (also called eclipsers) could be placed in front of the optic. Rotating the Fresnel's vertical segments of prisms resulted in funneling the light into vertical or pencil beams. Sometimes a colored light sector is used. It is an arc of light seen as a ship approaches the lighthouse.
The Fresnel orders - the lens was designed with different strengths, or orders:
1st - the most powerful, was installed in primary coastal (landfall) lighthouses. Inside diameter = 72 7/16"
2nd - lights marking coastal headlands and approaches to bays and sounds. Inside diameter = 55 1/8"
3rd - assigned to lights covering wide bays and large lakes and coastal inlets. Inside diameter =39 3/8"
4th - fourth thru sixth order lenses mark prominent headlands, points and shoals large bays and sounds, or to warn of obstructions in rivers, to mark piers and wharves. Inside diameter = 19 11/16" 5th - Inside diameter =14 3/4" 6th - Inside diameter =11 3/16"
All lighthouses are identified by their designated light patterns and these are noted on charts with a series of abbreviations giving mariners the most important features of the light.
Some of the abbreviations are.
F......... Fixed light with a steady beam
Alt....... Alternating light showing 2 or more colours successively
Fl........ Flashing light showing series of brief flashes, darkness longer than light
Occ..... Occulting light opposite to F1 fixed light is broken by short periods of darkness.
FFl.......Fixed and flashing light has steady beam varied by flashes of greater brilliance.