Light characteristic

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.

Standardisation

The International Association of Marine Aids to Navigation and Lighthouse Authorities publishes a recommendation R1001, "The IALA Maritime Buoyage System ", which is stated within to be adopted by almost all marine aids to navigation authorities. This recommendation sets out recommended light characteristics for different types of marine aids to navigation within the categories of lateral marks, cardinal marks, isolated danger marks, safe water marks, special marks, emergency wreck marks and other marks such as lighthouses. Depending on the marine aid to navigation, this recommendation includes specification of light colours and light patterns.
Light colours are defined in recommendation R0201, "Marine Signal Lights-Colours ". This recommendation specifies for each named colour a polygon within the CIE 1931 color space for acceptable shades of the named colour a marine aid to navigation may exhibit.
Light patterns, including abbreviations, are defined in recommendation R0110, "Rhythmic Characters of Lights on Aids to Navigation".

Abbreviations

While light characteristics can be described in prose, e.g. "Flashing white every two 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.

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 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

Fixed lights, abbreviated "F", are a continuous and steady light. The IALA Marine Buoyage System does not include use of a fixed light pattern due to lack of differentiation as a marine aid to navigation light.

Flashing light

Single flashing light

Single flashing lights continuously repeat a flash of light of duration followed by an obviously longer duration of darkness. Every repetition is identical. Single flashing lights are abbreviated as "Fl".
The IALA Marine Buoyage System allows single flashing lights to be used for the following purposes:

Port hand lateral marks, if the single flashing light has a colour of red and the mark is in region "A".
Starboard hand lateral marks, if the single flashing light has a colour of green and the mark is in region "A".
Port hand lateral marks, if the single flashing light has a colour of green and the mark is in region "B".
Starboard hand lateral marks, if the single flashing light has a colour of red and the mark is in region "B".
Special marks, if the single flashing light has a colour of yellow.

IALA recommendation R0110 specifies that a single flashing light should:

Have a period of repetition within the range of seconds for countries which do not use "quick lights". This corresponds to frequency of 4 to 30 flashes per minute.
Have a period of repetition within the range of seconds for countries which do use "quick lights". This corresponds to frequency of 4 to 24 flashes per minute.
Have a duration of light less than 2 seconds within each period.
Have a duration of darkness between two successive flashes being three times or more the duration of a flash. For example, if the frequency is 20 flashes per minute, the duration of a flash may be 0.75 seconds and duration of darkness following may be 2.25 seconds to satisfy constraint.

If the duration of light is greater than or equal to 2 seconds, the flashing light may alternatively be classified as a "long flashing light".
If the frequency is greater than or equal to 50 flashes per minute, the flashing light may alternatively be classified as a "quick light".

Long flashing light

Long flashing lights are a variant of single flashing lights where the duration of light is within the inclusive range of 2 to 5 seconds. Long flashing lights are abbreviated as "LFl", and in certain publications abbreviated alternatively as "L.Fl".
The IALA Marine Buoyage System allows long flashing lights to be used wherever a single flashing light is allowed to be used. The long flashing light must be used for the same purpose and have the same light colour as an equivalent single flashing light.
A white coloured long flashing light with a period of 10 seconds is additionally specified as an option for safe water marks.
To satisfy the constraint specified by IALA recommendation R0110, a long flashing light has a minimum period of 8 seconds. The recommendation also specifies a maximum period of 20 seconds for long flashing lights, restricting duration to be no greater than 5 seconds. A long flashing light is therefore specified by the recommendation to have a frequency within the inclusive range of 3 to 7.5 flashes per minute.

Group flashing light

Group flashing lights continuously repeat a pattern consisting of a group of flashes followed by a duration of darkness between successive groups. The duration of darkness between successive groups must be obviously longer than the duration of darkness between flashes within a group. Every repetition is identical. Group flashing lights are abbreviated as "Fl" for a group of two flashes, and in certain publications abbreviated additionally as "Gr Fl".
The IALA Marine Buoyage System specifies group flashing lights be used for isolated danger marks where each group consists of two flashes of white light.
The IALA Marine Buoyage System allows group flashing lights to additionally be used wherever a single flashing light is allowed to be used. The group flashing light must be used for the same purpose and have the same light colour as an equivalent single flashing light.
IALA recommendation R0110 specifies that a group flashing light should:

Have a period of repetition of no more than 20 seconds if each group consists of two flashes.
Have a period of repetition of no more than 30 seconds if each group consists of three or more flashes.
Have between 2 and 5 flashes in a group. By exception, the recommendation also allows groups to have 6 flashes.
Have the same number of flashes in every group.
Have the same duration of each flash of light.
Have the same duration of darkness between every flash in a group.
Have the same duration of darkness between every group of flashes.
Have a duration of darkness between groups which is at least three times the duration of darkness between every flash in a group. For example, if duration is 1 second of darkness between each flash in a group, duration must be 3 seconds or more of darkness between groups of flashes.
Have a duration of at least 1 second where, if each group consists of two flashes.
Have a duration of at least 2 seconds where, if each group consists of three or more flashes and the country deploying the group flashing light does not use "quick lights".
Have a duration of at least 2.5 seconds where, if each group consists of three or more flashes and the country deploying the group flashing light uses "quick lights".
Composite group flashing light

Composite group flashing lights are a variant of group flashing lights with two groups of flashes in each period. Group flashing lights are abbreviated as "Fl" for a group of two flashes followed by a single flash. There are three durations of darkness for composite group flashing lights:

as the duration of darkness between flashes in the same group.
as the duration of darkness between the first group of flashes and second group of flashes in a period.
as the duration of darkness between the second group of flashes of one period, and the first group of flashes of the next period.

The IALA Marine Buoyage System specifies composite group flashing lights be used for the following purposes:

Port hand preferred channel marks, if the composite group flashing light has a colour of red, there are 2 flashes followed by 1 flash and the mark is in region "A".
Starboard hand preferred channel marks, if the composite group flashing light has a colour of green, there are 2 flashes followed by 1 flash and the mark is in region "A".
Port hand preferred channel marks, if the composite group flashing light has a colour of green, there are 2 flashes followed by 1 flash and the mark is in region "B".
Starboard hand preferred channel marks, if the composite group flashing light has a colour of red, there are 2 flashes followed by 1 flash and the mark is in region "B".

The IALA Marine Buoyage System allows composite group flashing lights to be used for special marks if the light colour if yellow.
IALA recommendation R0110 specifies the following differences with group flashing lights:

Composite group flashing lights have a period of repetition of no more than 30 seconds.
The first group of flashes in a composite group flashing light period should only consist of two flashes. By exception, the recommendation also allows the first group to consist of three flashes.
The second group of flashes in a composite group flashing light period should only consist of one flash.
Composite group flashing lights should adhere to constraint. For a typical "Fl" light, the period of darkness between a single flash and group of two flashes must be at least the same duration of darkness as exists between a group of two flashes and a single flash.