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.
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, "Bu" for blue. 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:
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.
A fixed light, abbreviated "F", is a continuous and steady 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 is when the flashes are longer than two seconds. Such a light is sometimes denoted "long flashing" with the abbreviation "L.Fl".
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.
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".
A quick light, abbreviated "Q", is a special case of a flashing light with a large frequency (more than 30 or 50 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 used, e.g. Q(9).
Another distinction sometimes 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.
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 Light Feature Abbr. Definition Example Representation 1. Fixed F A continuous, steady, light. F R 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-occulting Oc A dark period is repeated regularly. Oc R 6s 2.2 Group-occulting Oc(x) A group of dark periods are repeated regularly. Oc(2) G 8s 2.3 Composite group-occulting Oc(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 3. Isophase Iso The duration of the light and dark periods are equal. Iso R 4s 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.
Fl A flash is repeated regularly at a rate below 50 per minute. Fl G 5s 4.2 Long-flashing L.Fl A light flash, duration of more than 2 seconds (long flash) is repeated regularly. L.Fl W 10s 4.3 Group-flashing
Fl(x) A group of a specific number of flashes are repeated regularly. Fl(3) R 15s 4.4 Composite group-flashing Fl(x+y) Similar to group-flashing, but with several groups of flashes. Fl(2+1) W 15s 5. Quick
flashes are at a frequency of between 50 and 79 flashes per minute.
5.1 Continuous quick
Q Quick flashes are repeated regularly. Q W 5.2 Group quick
Q(x) Groups of a given number of quick flashes are repeated regularly. Q(3) G 9s 5.3 Interrupted quick
I.Q The sequence of flashes is regularly interrupted by dark intervals of constant duration. I.Q R 14s 6. Very quick
Flashes are repeated at a frequency between 80 and 159 flashes per minute.
6.1 Continuous very quick
VQ Very quick flashes are repeated regularly. VQ W 6.2 Group very quick
VQ(x) Groups of a given number of very quick flashes are repeated regularly. VQ(3) G 4s 6.3 Interrupted very quick
I.VQ The sequence of flashes is regularly interrupted by dark intervals of constant duration. I.VQ R 9s 7. Ultra quick
The flashes are repeated at a frequency equal to or greater than 160 flashes per minute.
7.1 Continuous ultra quick
UQ Ultra quick flashes are repeated regularly. UQ W 7.2 Interrupted ultra quick
I.UQ The sequence of ultra quick flashes is regularly interrupted by dark intervals of constant duration. I.UQ R 6s 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 9. Fixed and flashing
F.Fl A 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 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
- Thompson, William. "On Lighthouse Characteristics". Retrieved 21 August 2022.
- List of Lights
- 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. United States Coast Guard. - Glossary to Aids of Navigation Terms in any of the volumes
- Limits of List of Lights, Pubs. 110–116. United States National Geospatial-Intelligence Agency. p. XII - Characteristics of Lights, in any of the volumes
- Glossary of Lighthouse Terms