Light characteristic

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Symbols and abbreviations for light characteristics Light characteristics.svg
Symbols and abbreviations for light characteristics

A light characteristic is all of the properties that make a particular navigational light identifiable. Graphical and textual descriptions of navigational light sequences and colours are displayed on nautical charts and in Light Lists with the chart symbol for a lighthouse, lightvessel, buoy or sea mark with a light on it. Different lights use different colours, frequencies and light patterns, so mariners can identify which light they are seeing. [1]



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).

  • 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 the nominal range is 10 nautical miles.

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 [2] 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 [3] or 50 [2] per minute) the light is denoted as a "quick light", see below.

Occulting light

Clockwork occulting mechanism formerly in use at Coquet Island Lighthouse. Coquet Island Lighthouse, former occulting mechanism.jpg
Clockwork occulting mechanism formerly in use at Coquet Island Lighthouse.

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)).

The term occulting is used because originally the effect was obtained by a mechanism (e.g. a vertical or rotating shutter) periodically shading the light from view.

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 [3] or 50 [2] 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 [2] used, e.g. Q(9).

Another distinction sometimes [2] 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. VQ(6) LFl 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, abbreviated "Al", is a light which shows alternating colors. For example, "Al WG" shows white and green lights alternately.

Class of light

Table I
Class of LightFeatureAbbr.DefinitionExampleRepresentation
1. Fixed
FA continuous, steady, light.F R Light Code F R.gif
2. Occulting
The total duration of light in each period is greater than the total duration of darkness and the dark intervals (occultations) usually have the same duration.
2.1 Single-occultingOcA dark period is repeated regularly.Oc R 6s Light Code Oc R 6s.gif
2.2 Group-occultingOc(x)A group of dark periods are repeated regularly.Oc(2) G 8s Light Code Oc (2) G 8s.gif
2.3 Composite group-occultingOc(x+y)Light similar to group-occulting, except that successive groups in the same period contain different numbers of dark periods.Oc(2+3) W 18s Light Code Oc(2+3) W 5s.gif
3. Isophase
IsoThe duration of the light and dark periods are equal.Iso R 4s Light Code Iso R 4s.gif
4. Flashing
The total duration of light in each period is less than the total duration of the dark and light appearances (flashes) usually have the same duration.
4.1 Single-flashing
FlA flash is repeated regularly at a rate below 50 per minute.Fl G 5s Light Code Fl G 5s.gif
4.2 Long-flashingL.FlA light flash, duration of more than 2 seconds (long flash) is repeated regularly.L.Fl W 10s Light Code LFl 10s.gif
4.3 Group-flashing
Fl(x)A group of a specific number of flashes are repeated regularly.Fl(3) R 15s Light Code Fl(3) R 15s.gif
4.4 Composite group-flashingFl(x+y)Similar to group-flashing, but with several groups of flashes.Fl(2+1) W 15s Light Code Fl(2+1) W 5s.gif
5. Quick
flashes are at a frequency of between 50 and 79 flashes per minute.
5.1 Continuous quick
QQuick flashes are repeated regularly.Q W Light Code Q W.gif
5.2 Group quick
Q(x)Groups of a given number of quick flashes are repeated regularly.Q(3) G 9s Light Code Q (3) G 9s.gif
5.3 Interrupted quick
I.QThe sequence of flashes is regularly interrupted by dark intervals of constant duration.I.Q R 14s Light Code IQ R 14s.gif
6. Very quick
Flashes are repeated at a frequency between 80 and 159 flashes per minute.
6.1 Continuous very quick
VQVery quick flashes are repeated regularly.VQ W Light Code VQ.gif
6.2 Group very quick
VQ(x)Groups of a given number of very quick flashes are repeated regularly.VQ(3) G 4s Light Code VQ (3) G 4s.gif
6.3 Interrupted very quick
I.VQThe sequence of flashes is regularly interrupted by dark intervals of constant duration.I.VQ R 9s Light Code IVQ R 9s.gif
7. Ultra quick
The flashes are repeated at a frequency equal to or greater than 160 flashes per minute.
7.1 Continuous ultra quick
UQUltra quick flashes are repeated regularly.UQ W Light Code UQ.gif
7.2 Interrupted ultra quick
I.UQThe sequence of ultra quick flashes is regularly interrupted by dark intervals of constant duration.I.UQ R 6s Light Code IUQ R 6s.gif
8. Morse code
Mo(x)The flashes have markedly different durations and are grouped together to form one or more characters in Morse code.Mo(K) G 6s Light Code Mo(K) G 6s.gif
9. Fixed and flashing
F.FlA light that combines a fixed light with a light flashing with a stronger intensity. The flashes of light may have any of the features described in above.F.Fl Y 5s Light Code FFl Y 5s.gif
10. Alternating
AlLight that alternately displays different colors
Note - The alternating light can be used in conjunction with most of the lights earlier classes.
Al WR 3s Light Code Al WR.gif

See also


  1. Thompson, William. "On Lighthouse Characteristics" . Retrieved 21 August 2022.
  2. 1 2 3 4 5 List of Lights
  3. 1 2 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.

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