As the scale along the top border is based on a meteorological visibility of 10 NM, the luminous ranges in the prevailing conditions obtained from the bold 10 miles curve will be identical to the nominal range started from the top border.
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The diagram can also be used to obtain an approximate meteorological visibility; when, for example, a light of an intensity of 100 000 candelas is sighted at 12 NM, the current meteorological visibility will be about 5 NM.
Caution When using this diagram keep in mind that:
- The ranges obtained are approximate.
- The transparency of the atmosphere is not necessarily consistent between the observer and the light.
- Glare from background lighting will reduce considerably the range at which lights are sighted. A light of 100 000 candelas has a nominal range of 20 NM; with minor background light as from a populated coastline this range will be reduced to about 14 NM, and with major background lighting as from a city or from harbour installations to about 9 NM.
Bottom border: candelas Approximate sighting ranges may be obtained by entering the diagram with the listed intensity divided by 10 for minor background lighting, and by 100 for major background lighting.
Geographic range
A light&#;s geographic range depends upon the heights of both the light and the observer.
The sum of the observer&#;s distance to the visible horizon (based on his height of eye) plus the light&#;s distance to the horizon (based on its elevation) is its geographic range, which is the dipping rangedipping range.
Geographic range = 2.08 × (&#;Elevation + &#;Eye height)
For this purpose the formula can be simplified, and solved without a calculator. Assume an opportune standard height of eye of 4 metres as well as rounding 2.08 down to 2.
2 × (&#;Elevation + 2)
Or if on a smaller yacht, with 3 metres Eye height, you can use &#;3 = 1.7.
Example with a light elevation of 25 metres:
2 × (5 + 2) = 14 NM
2 × (5 + 1.7) = 13.4 NM with 3 metres Eye height.
Download the geographic range table (PDF) or geographic range table (PNG) or use my online calculator.
See the geographic range table or use my online calculator.
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Visible range
When comparing the geographic range with the light&#;s luminous range, then the lesser of the two ranges is the range at which the light will first be sighted: the visible range.
Plot a visibility arc centered on the light and with a radius equal to the visible range. Extend the vessel&#;s dead reckoningdead reckoning
track until it intersects the visibility arc.
The bearing from the intersection point to the light is the light&#;s predicted bearing at first sighting.
Bobbing a light
When first sighting a light, an observer can determine if it is on the horizon by immediately reducing his height of eye. If the light disappears and then reappears when the observer returns to his original height, the light is on the horizon. This process is called &#;bobbing a light&#;.
Tide or no Tide&#;
There will be some judgement involved (luminous range is a rough estimate) resulting in a large margin of error in visible range. Therefore only when light elevation is rather low (<20m), while tidal range is high should it be necessary to include tide.
Loom
Because of the limiting factor of the geographic range, most major lights will never be seen from a sailing yacht 20 NM away.
Yet, due to atmospheric scattering, it is sometimes possible to take a compass bearing on the loom of the light: its reflection against the clouds. Additionally, it is sometimes even possible to observe a rotating beam of light.
Loom
Dipping distance / range
Figure 9.24 &#; A Looming light &#; A light at Dipping range.
Colours have different ranges
Different coloured lights with equal luminous intensity have different ranges.
White light is the most visible followed by yellow, green and then red.
Therefore, at extreme ranges an &#;AL WG&#; can resemble a &#;Fl W&#;.  
Distance of minor light
The range of a lighted buoy is never indicated, but on a clear night the maximum range is 3 NM, yet often considerably less.
There are 2 visual indicators to determine your distance from a buoy: at about 0.5 NM, the light will rise up from the horizon, and at about 200m, the light will reflect in the surface.
Buoy at less than 3 NM
Buoy at less than 0.5 NM
Buoy at less than 200m
Figure 9.25 &#; Visual clues to estimate the distance of a lighted buoy.
Glossary
- Loom: the diffused glow observed from a light below the horizon, due to atmospheric scattering of its light rays, usually the undersides of clouds.
- Looming: an apparent elevation of distant terrestial objects by abnormal atmospheric refraction. Because of looming, objects below the horizon are sometimes visible. The opposite is &#;sinking&#; during low visibility.
- ENC: the Electronic Navigation Chart.
- ECDIS: the Electronic Chart Display and Information System.
- AIS Aids to Navigation: Automatic Identification System AtoN
- Real AIS AtoN is attached to a physical mark such as a lighthouse or buoy.
- Synthetic AIS AtoN is also associated with a physical mark, but the actual AIS transmitter is in another location.
- Virtual AIS AtoN marks the hazard with the hazard&#;s coordinates, but there is no physical light, buoy.
- ODAS buoy: Ocean Data Acquisition Systems buoy &#; means a buoy intended for the collection of data on
properties of the ocean. It may be moored or free-floating.
- Aid to Navigation (AtoN) or Navigational aid (NAVAID): any mark, sign, device or system external to vessels that is designed
and operated to assist in determination of position, to define a safe course, or to warn of dangers, vessel traffic or obstructions.
- Nominal range is the luminous range when the meteorological visibility is 10 NM, equivalent to a transmission factor of T=0.74.
- Geographical range is the maximum distance at which an object or light from a light source can theoretically be seen by an observer, as limited only by the curvature of the earth, the refraction of the atmosphere, the elevation of the object or light and the height of the observer&#;s eye.
- Luminous range is the maximum distance at which a given signal light can be seen by the eye of the observer at a given time, as determined by the intensity of the meteorological visibility prevailing at that time. It takes no account of elevation, observer&#;s height of eye or the curvature of the earth.
Example &#; a light of an intensity of 500 candelas (nominal range of approx. 8 NM), can be seen up to 12 NM when the meteorological visibility is 20 NM, but will be seen only at 3 NM if the meteorological visibility is 2 NM.
- Visible range: the extreme distance at which an object of light can be seen, with current height of eye and current meteorological visibility: the lesser of the luminous and geographic ranges.
- Bobbing a light: quickly lowering the height of eye and raising it again when a navigational light is first sighted to determine if the observer is at the geographic range of the light.
- Luminous range diagram: a diagram used to convert the nominal range of a light to its luminous range under existing conditions.
- Meteorological Optical Codes: ranks from 0 (dense fog : less than 50 metres of visibility) to 9 (exceptionally clear : more than 27 kilometres of visibility).
- Mark, seamark, navigation mark: an artificial or natural object of easily recognizable shape or colour, or both, situated in such a position that it may be identified on a chart. A fixed artificial navigation mark is often called a beacon.
- Light characteristics: the sequence and length of light and dark periods and the colour or colours by which a navigational light is identified.
- Topmark: one of more objects of characteristic shape placed on top of a buoy or beacon to aid in its identification.
- Lateral mark: AtoN intended to mark the sides of a channel or waterway.
- Cardinal marks: black and yellow AtoN intended to show the location of a danger to navigation based on its position relative to the danger using the cardinal points of the compass: North, East, South and West, each with distinctive double-cone topmark variations. If lighted, the Q or VQ group flashes resemble the face of a clock.
- Isolated danger Mark: AtoN marking a danger with clear water all around it; it has a double ball topmark and is black with at least one red band. If lighted its characteristic is Fl(2).
- Sector light: a fixed AtoN that displays a light of different colours and / or rhythms over designated arcs. The
colour of the light provides directional information.
- Light sector: as defined by bearings from seaward, the sector in which a navigational light is visible or in which it has a distinctive colour difference from that of adjoining sectors, or in which it is obscured.
- Lighthouse: a tower, or substantial building or structure, erected at a designated geographical location to carry a signal light
and provides a significant daymark. It provides a long or medium range light for identification by night.
- Minor light: a light that has nominal range <10 NM. An automatic unmanned light on a fixed structure usually showing low to moderate intensity. Minor lights are established in harbours, along channels, along rivers, and in isolated dangers.
- Major light: a light that has nominal range >10 NM. A light of high intensity and reliability exhibited from a fixed structure (lighthouse) or on marine site (except leading lights). Major lights include primary sea-coast and secondary lights.
- Light List: a detailed list of navigational aids including lighthouses and other lighted navigation aids, unlighted buoys, radiobeacons, daybeacons and racons.
- Landfall: the first sighting, by eye or by radar, of land when approached from seaward.
- Range: two or more objects (leading marks) in line. Such objects are said to be in range. An observer having them in range is said to be &#;on range line&#;, or &#;on track&#;. Two beacons are frequently positioned together for the specific purpose of forming a range to indicate a safe route or the centerline of a channel.
- Leading line: a straight solid line, drawn through leading marks (a range) on the chart. A ship moving along such line will clear certain dangers or remain in the best channel.
- Leading lights: two or more lights at different elevations so situated to form a range and its leading line when brought into transit. The one nearest to the observer is the from light and the one farthest from the observer is the rear light. The front light is at a lower elevation than the rear light.
- Lights in line: two or more lights so situated that when observed in transit they define a position: the limit of an area, an alignment used for anchoring, etc. Not to be confused with leading lights, which mark a recommended direction to be followed.