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Light characteristic

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Light characteristic

Symbols and abbreviations for light characteristics

A light characteristic is a graphic and text description of a navigational light sequence or colour displayed on a nautical chart or in a Light List with the chart symbol for a lighthouse, lightvessel, buoy or sea mark with a light on it. The graphic indicates how the real light beacon may be identified when looking at its actual light output type or sequence. Different lights use different colours, frequencies and light patterns, so mariners can find out which light they are seeing.


  • Abbreviations 1
  • Light patterns 2
    • Fixed light 2.1
    • Flashing light 2.2
    • Occulting light 2.3
    • Isophase light 2.4
    • Quick light 2.5
    • Morse code 2.6
    • Fixed and flashing 2.7
    • Alternating 2.8
  • Class of Light 3
  • See also 4
  • Notes 5
  • References 6
  • External links 7


While light characteristics can be described in prose, e.g. "Flashing white every three seconds", lists of lights and navigation chart annotations use abbreviations. The abbreviation notation is slightly different from one light list to another, with dots added or removed, but it usually follows a pattern similar to the following (see the chart to the right for examples).

  • An abbreviation of the type of light, e.g. "Fl." for flashing, "F." for fixed.
  • The color of the light, e.g. "W" for white, "G" for green, "R" for red, "Y" for yellow. If no color is given, a white light is generally implied.
  • The cycle period, e.g. "10s" for ten seconds.
  • Additional parameters are sometimes added:
  • The height of the light above the chart datum for height (usually based on high water). e.g. 15m for 15 metres.
  • The range in which the light is visible, e.g. "10M" for 10 nautical miles.

An example of a complete light characteristic is "Gp Oc(3) W 10s 15m 10M". This indicates that the light is a group occulting light in which a group of three eclipses repeat every 10 seconds; the light is white; the light is 15 metres above the chart datum and is visible for 10 nautical miles (19 km).

Light patterns

Fixed light

A fixed light, abbreviated "F", is a continuous and steady light.

Flashing light

A flashing light is a rhythmic light in which the total duration of the light in each period is clearly shorter than the total duration of the darkness and in which the flashes of light are all of equal duration. It is most commonly used for a single-flashing light which exhibits only single flashes which are repeated at regular intervals, in which case it is abbreviated simply as "Fl". It can also be used with a group of flashes which are regularly repeated, in which case the abbreviation is "Fl.(2)" or "Gr Fl.(2)", for a group of two flashes. Another possibility is a composite group, in which successive groups in the period have different numbers of flashes, e.g. "Fl. (2+1)" indicates a group of two flashes, followed by one flash.

A specific case sometimes used[1] is when the flashes are longer than two seconds. Such a light is sometimes denoted "long flashing" with the abbreviation "L.Fl".

If the frequency of flashes is large (more than 30[2] or 50[1] per minute) the light is denoted as a "quick light", see below.

Occulting light

An occulting light is a rhythmic light in which the duration of light in each period is longer than the total duration of darkness. In other words, it is the opposite to a flashing light where the total duration of darkness is longer than the duration of light. It has the appearance of flashing off, rather than flashing on. Like a flashing light, it can be used for a single occulting light that exhibits only a single period of darkness or the periods of darkness can be grouped and repeated at regular intervals (abbreviated "Oc"), a group (Oc (3)) or a composite group (Oc (2+1)).

Isophase light

An isophase light, abbreviated "Iso", is a light which has dark and light periods of equal length. The prefix derives from the Greek iso- meaning "same".

Quick light

A quick light, abbreviated "Q", is a special case of a flashing light with a large frequency (more than 30[2] or 50[1] per minute). If the sequence of flashes is interrupted by regularly repeated eclipses of constant and long duration, the light is denoted "interrupted quick", abbreviated "I.Q".

Group notation similar to flashing and occulting lights is also sometimes[1] used (e.g. Q.(9)).

Another distinction sometimes[1] made is between quick (more than 50 and less than 80 flashes per minute), very quick (more than 80 and less than 160 flashes per minutes, abbreviated "V.Q") and ultra quick (no less than 160 flashes per minute, abbreviate "U.Q"). This can be combined with notations for interruptions, e.g. I.U.Q. for interrupted ultra quick, or grouping, e.g. V.Q.(9) for a very quick group of nine flashes. Quick characteristics can also be followed by other characteristics, e.g. V.Q.(6)+L.Fl. for a very quick group of six flashes, followed by a long flash.

Morse code

A Morse code light is light in which appearances of light of two clearly different durations (dots and dashes) are grouped to represent a character or characters in the Morse Code. For example, "Mo. (A)" is a light in which in each period light is shown for a short period (dot) followed by a long period (dash), the Morse Code for "A".

Fixed and flashing

A fixed and flashing light, abbreviated "F. Fl", is a light in which a fixed low intensity light is combined with a flashing high intensity light.


An alternating light, abbreviate "Al", is a light which shows alternating colors. For example "Al WGB" show white, green and blue lights alternatively.

Class of Light

Table I
Class of Light Feature Abbr. Definition Example Representation
1. Fixed
F Light that has a continuous and uniform and constant color F R
2. Occulting
The total duration of light emission in each period is greater than the total duration of darkness and the dark intervals (occultation) usually have the same duration.
2.1 Single-occulting Oc A blind is repeated regularly. Oc R 6s
2.2 Group occulting Oc(x) Group a number of concealments are repeated regularly. Oc(2) G 8s
2.3 Composite group-occulting Oc(x+y) Light similar to Withholds grouped, except that successive groups in the same period, contain different numbers of occultations. Oc(2+3) W 18s
3. Isophase
Iso The duration of the light emission and the duration of the dark are equal. Iso R 4s
4. Flashing
The total duration of light emission in each period is less than the total duration of the dark and light appearances (lightning) usually have the same duration.
4.1 Single-flashing
Fl Light lightning when lightning is repeated regularly (at a rate below 50 lightning per minute). Fl G 5s
4.2 Long-flashing LFl A light emission, duration of less than 2 seconds (long flash) is repeated regularly. LFl W 10s
4.3 Group-flashing
Fl(x) Group a number of lightning strikes, are repeated regularly. Fl(3) R 15s
4.4 Composite group-flashing Fl(x+y) Light grouped similar to lightning, with the exception that successive groups in the same period, contain different numbers of lightning. Fl(2+1) W 15s
5. Quick
flashes (scintillations) are repeated at a frequency of between 50 and 79 flashes per minute.
5.1 Continuous quick
Q Lightning are repeated regularly. Q W
5.2 Group quick
Q(x) Groups of a given number of flashes are repeated regularly. Q(3) G 9s
5.3 Interrupted quick
IQ The sequence of flashes is regularly interrupted by dark intervals of long and constant duration. IQ R 14s
6. Very quick
Lightning (scintillation) are repeated at a frequency between 80 and 159 lightning per minute.
6.1 Continuous very quick
VQ Lightning are repeated regularly. VQ W
6.2 Group very quick
VQ(x) Groups of a given number of flashes are repeated regularly. VQ(3) G 4s
6.3 Interrupted very quick
IVQ The sequence of flashes is regularly interrupted by dark intervals of long and constant duration. IVQ R 9s
7. Ultra quick
The flashes (scintillations) are repeated at a frequency equal to or greater than 160 flashes per minute.
7.1 Continuous ultra quick
UQ Lightning are repeated regularly. UQ W
7.2 Interrupted ultra quick
IUQ The sequence of flashes is regularly interrupted by dark intervals of long and constant duration. IUQ R 6s
8. Morse code
Mo(x) The luminous emissions have markedly different durations and are grouped together to form one or more characters MORSE code. Mo(K) G 6s
9. Fixed and flashing
FFl Light that combines a fixed light with a light lightning with a stronger light intensity. The flashes of light may have any of the features described in 4. FFl Y 5s
10. Alternating
Al Light that alternately displays different colors
Note - The alternating light can be used in conjunction with most of the lights earlier classes.
Al WR 3s

See also


  1. ^ a b c d e List of Lights
  2. ^ a b Light List states at one point that only "frequency not exceeding 30 flashes per minute" is considered "Flashing", yet at another that only "a rate of 60 flashes per minute" is considered "Quick", leaving the interval in-between undefined.


  • Light List, Volumes 1-7. - Glossary to Aids of Navigation Terms in any of the volumes  
  • Limits of List of Lights, Pubs. 110-116. p. XII - Characteristics of Lights, in any of the volumes  
  • Glossary of Lighthouse Terms

External links

  • U.S. ATON light characteristic terms illustrated
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