Optical equipment
Paraboloidal mirrors
With the advent of the Argand burner, a reliable and steady illuminant, it became possible to develop effective optical apparatuses for increasing the intensity of the light. In the first equipment of this type, known as the catoptric system, paraboloidal reflectors concentrated the light into a beam. In 1777 William Hutchinson of Liverpool, England, produced the first practical mirrors for lighthouses, consisting of a large number of small facets of silvered glass set in a plaster cast molded to a paraboloid form. More generally, shaped metal reflectors were used, silvered or highly polished. These were prone, however, to rapid deterioration from heat and corrosion; the glass facet reflector, although not as efficient, lasted longer. The best metallic reflectors available in 1820 were constructed of heavily silvered copper in the proportion of 6 ounces (170 grams) of silver to 16 ounces (454 grams) of copper (compared with the 0.5 ounce [14 grams] of silver to 16 ounces of copper commonly used for plated tableware of the period). With such heavy plating, cleaning cloths were kept for subsequent recovery of the silver. These mirrors could increase the intensity of an Argand burner, nominally about five candlepower, almost 400 times.
Although the mirror could effectively concentrate the light into an intense beam, it was necessary to rotate it to make it visible from any direction. This produced the now familiar revolving lighthouse beam, with the light appearing as a series of flashes. Mariners were not favourably disposed to these early flashing lights, contending that a fixed steady light was essential for a satisfactory bearing. However, the greatly increased intensity and the advantage of using a pattern of flashes to identify the light gradually overcame their objections. The first revolving-beam lighthouse was at Carlsten, near Marstrand, Sweden, in 1781.
Rectangular and drum lenses
In 1821 Augustin Fresnel of France produced the first apparatus using the refracting properties of glass, now known as the dioptric system, or Fresnel lens. On a lens panel he surrounded a central bull’s-eye lens with a series of concentric glass prismatic rings. The panel collected light emitted by the lamp over a wide horizontal angle and also the light that would otherwise escape to the sky or to the sea, concentrating it into a narrow, horizontal pencil beam. With a number of lens panels rotating around the lamp, he was then able in 1824 to produce several revolving beams from a single light source, an improvement over the mirror that produces only a single beam. To collect more of the light wasted vertically, he added triangular prism sections above and below the main lens, which both refracted and reflected the light. By doing this he considerably steepened the angle of incidence at which rays shining up and down could be collected and made to emerge horizontally. Thus emerged the full Fresnel catadioptric system, the basis of all lighthouse lens systems today. To meet the requirement for a fixed all-around light, in 1836 English glassmaker William Cookson modified Fresnel’s principle by producing a cylindrical drum lens, which concentrated the light into an all-around fan beam. Although not as efficient as the rectangular panel, it provided a steady, all-around light. Small drum lenses, robust and compact, are widely used today for buoy and beacon work, eliminating the complication of a rotating mechanism; instead of revolving, their lights are flashed on and off by an electronic code unit.
Prior to Fresnel’s invention the best mirror systems could produce a light of about 20,000 candlepower with an Argand burner. The Fresnel lens system increased this to 80,000 candlepower, roughly equivalent to a modern automobile headlamp. With the pressure oil burner, intensities of up to 1,000,000 candlepower could be achieved. For a light of this order, the burner mantle would measure 4 inches (100 mm) in diameter. The rotating lens system would have four large Fresnel glass lens panels, 12 feet high, mounted about four feet from the burner on a revolving lens carriage. The lens carriage would probably weigh five tons, about half of it being the weight of the glass alone. The rotating turntable would float in a circular cast-iron trough containing mercury. With this virtually frictionless support bearing, the entire assembly could be smoothly rotated by weight-driven clockwork. If the illuminant was acetylene gas, the lens rotation could be driven by gas pressure.
Installations of this type entered common use, although many were converted to electric lamps with electric motor drives. Modern lens equipment of the same type is much smaller, perhaps 30 inches (75 cm) high, mounted on ball bearings and driven by an electric motor. Lens panels can be molded in transparent plastic, which is lighter and cheaper. Drum lenses are also molded in plastic. In addition, with modern techniques, high-quality mirrors can be produced easily and cheaply.
Intensity, visibility, and character of lights
Geographic range and luminous range
The luminous intensity of a light, or its candlepower, is expressed in international units called candelas. Intensities of lighthouse beams can vary from thousands to millions of candelas. The range at which a light can be seen depends upon atmospheric conditions and elevation. Since the geographic horizon is limited by the curvature of Earth, it can be readily calculated for any elevation by standard geometric methods. In lighthouse work the observer is always assumed to be at a height of 15 feet (4.6 metres), although on large ships the observer may be 40 feet (12 metres) above the sea. Assuming a light at a height of 100 feet (30.5 metres), the range to an observer at 15 feet above the horizon will be about 16 nautical miles (29.6 km). This is known as the geographic range of the light. (One nautical mile, the distance on Earth’s surface traversed by one minute of arc latitude, is equivalent to 1.15 statute miles or 1.85 km.)
The luminous range of a light is the limiting range at which the light is visible under prevailing atmospheric conditions and disregarding limitations caused by its height and Earth’s curvature. A very powerful light, low in position, can thus have a clear-weather luminous range greater than that when first seen by the mariner on the horizon. Powerful lights can usually be seen over the horizon because the light is scattered upward by particles of water vapour in the atmosphere; this phenomenon is known as the loom of the light.
Atmospheric conditions have a marked effect on the luminous range of lights. They are defined in terms of a transmission factor, which is expressed as a percentage up to a maximum of 100 percent (representing a perfectly clear atmosphere, never attained in practice). Clear weather in the British Isles corresponds to about 80 percent transmission, but in tropical regions it can rise to 90 percent, increasing the luminous range of a 10,000-candela light from 18 to 28 nautical miles (33 to 52 km). Conversely, in mist or haze at about 60 percent transmission, a light of 1,000,000 candelas would be necessary to maintain a luminous range of 18 nautical miles. In dense fog, with visibility down to 100 yards or metres, a light of 10,000,000,000 candelas could scarcely be seen at half a nautical mile (0.9 km). Because average clear-weather conditions vary considerably from one region of the world to another, luminous ranges of all lighthouses by international agreement are quoted in an arbitrary standard clear-weather condition corresponding to a daytime meteorological visibility of 10 nautical miles (19 km), or 74 percent transmission. This is known as the nominal range of a light. Mariners use conversion tables to determine the actual luminous range in the prevailing visibility.
Because lights of very great intensity yield diminishing returns in operational effectiveness, most very high-powered lights have been abandoned. A maximum of 100,000 candelas, with a clear-weather range of 20 nautical miles (37 km), is generally considered adequate. Nevertheless, there are still some very high-powered lights, which for special reasons may have to be visible at a distance in daylight.
Identification
Most lighthouses rhythmically flash or eclipse their lights to provide an identification signal. The particular pattern of flashes or eclipses is known as the character of the light, and the interval at which it repeats itself is called the period. The number of different characters that can be used is restricted by international agreement through the International Association of Marine Aids to Navigation and Lighthouse Authorities in Paris, to which the majority of maritime nations belong. The regulations are too lengthy to quote in full, but essentially a lighthouse may display a single flash, regularly repeated at perhaps 5-, 10-, or 15-second intervals. This is known as a flashing light. Alternatively, it may exhibit groups of two, three, or four flashes, with a short eclipse between individual flashes and a long eclipse of several seconds between successive groups. The whole pattern is repeated at regular intervals of 10 or 20 seconds. These are known as group-flashing lights. In another category, “occulting” lights are normally on and momentarily extinguished, with short eclipses interrupting longer periods of light. Analogous to the flashing mode are occulting and group-occulting characters. A special class of light is the isophase, which alternates eclipses and flashes of exactly equal duration.
Steadily burning lights are called fixed lights. For giving mariners accurate directional information in ports, harbours, and estuarial approaches, fixed directional lights display sharply defined red and green sectors. Another sensitive and very accurate method of giving directional instruction is by range lights, which are two fixed lights of different elevation located about half a nautical mile apart. The navigator steers the vessel to keep the two lights aligned one above the other.
Another use for fixed lights is the control of shipping at harbour entrances. A traffic signal consists of a vertical column of high-powered red, green, and yellow projector lights that are visible in daylight.
The daymark requirement of a lighthouse is also important, and lighthouse structures are painted to stand out against the prevailing background. Shore lighthouses are usually painted white for this purpose, but in the open sea or against a light background conspicuous bands of contrasting colours, usually red or black, are used.
