The Light of the LighthouseThe luminous intensity of a light, or its candlepower (old definition), is now expressed in international units called candelas (cd). 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 the Earth, it can be readily calculated for any elevation by standard geometric methods. In observing a lighthouse, the observer is always assumed to be at a height of 4.5 meters, although on large vessels, 15 meters or more above the sea level is more likely. Assuming a light at a height of 30 meters, the range to an observer at 4.5 meters above the horizon will be about 29.6 km (16 nM). This is known as the geographic range of the light. (One nautical mile, the distance on the Earth’s surface traversed by one minute of arc longitude or latitude, is equivalent to 1.15 statute miles or 1.85 kilometers.) Powerful lights can usually be seen over the horizon because the light is scattered upward by particles of water vapor in the atmosphere; this phenomenon is known as the loom of the light.
The first lighthouses were built long before the time of Christ. The earliest known reference to a lighthouse dates back to 1200 BC. This reference appeared in the Iliad, Homer's Greek epic poem. The first onshore beacons that were used to guide vessels were bonfires. Eventually, these bonfires were replaced with iron baskets filled with burning wood or coal and suspended on long poles. It was not until the 18th century that these baskets were replaced by oil or gas lanterns. In the early to mid-20th century, electric beacons replaced oil and gas lanterns.
One of the most important developments in the technology of lighthouse beacons was the invention of the Fresnel lens. Developed in 1822 by French physicist Augustin Fresnel, the Fresnel lens is a collection of multiple glass prisms (transparent bodies consisting of two or more usually flat surfaces and used to bend a beam of light that bends nearly all the available light into a powerful central magnifying lens. The magnifying lens projects an intense beam of light that is visible more than 45 km (28 nM) away.
As mentioned earlier, atmospheric conditions have a marked effect on the luminous range of lights. The atmospheric conditions 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 33.3 km to 52 km (18 to 28 nM). 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 33.3 km (18 nM). In dense fog, with visibility down to 100 meters, a light of 10,000,000,000 candelas could scarcely be seen at 900 meters (0,5 nM).
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 18.5 km (10 nM), 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.
Modern lighthouse beacons vary in power from about 10,000 candelas to about 1 million candelas, depending on the prevailing weather conditions and the visibility requirements of vesselping traffic in the particular area. Every lighthouse emits a distinctive series of flashes known as its Light characteristic. These flash sequences allow seafarers to time intervals between flashes so that they can identify lighthouses with marks on sea maps or using a publication called the Light List. This publication, assigns a number to each lighted beacon and describes its identifying characteristics.
CandelaThe candela (symbol cd) is the SI base unit of light intensity. The light intensity indicates how much light is in each piece of a light beam. The candela is one of the seven basic units of the SI (International System of Units).
Origin of the CandelaPrior to 1948, various standards for luminous intensity were in use in a number of countries. These were typically based on the brightness of the flame from a "standard candle" of defined composition, or the brightness of an incandescent filament of specific design. One of the best-known of these was the English standard of candlepower. One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. (spermaceti is the name for a waxy substance from which the spermatozoa exist in the skull of the porpoise. A pot fish has about 3000 liters of it. It consists largely of acetyl palmitate.).
It became clear that a better-defined unit was needed. Jules Violle had proposed a standard based on the light emitted by 1 cm2 of platinum at its melting point (or freezing point), calling this the Violle. The light intensity was due to the Planck radiator (a black body) effect, and was thus independent of the construction of the device. This made it easy for anyone to measure the standard, as high-purity platinum was widely available and easily prepared.
Examples of the Candela• The light intensity of an ordinary flashlight or candle is about 1 candela
Definition of the CandelaDue to the big differences it became evident that a better defined unit was needed. The official definition in the SI is as follows: "The candela is the light intensity in a given direction of a source which emits monochromatic radiation at a frequency of 540 × 1012 Hz and whose radiation intensity in that direction is 1/683 watts per steroid."
Monochromatic means light of one colour, or more precise: light of one wavelength. The frequency of 540 × 1012 hertz corresponds to green light (wavelength of approximately 555 nm). Steradial is the unit of space angle. The space angle indicates how fast a light beam flares out. For a cone-shaped light beam, this is related to the opening angle. If a light source is equally strong in all directions, the space angle is 4π steradials.
How far a Lighthouse can be seen.