A light characteristic is all of the properties that make a particular navigational light identifiable. It is defined as the distinctive colour or periodic rhythm (or both) of a navigation light, enabling it to be identified. 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 (IALA) publishes recommendations that standardise light characteristics worldwide. Key documents include:
- Recommendation R1001 – "The IALA Maritime Buoyage System (MBS)", which sets out recommended light characteristics for different types of marine aids to navigation, including lateral marks, cardinal marks, isolated danger marks, safe water marks, special marks, and emergency wreck marks.
- Recommendation R0201 – "Marine Signal Lights – Colours", which specifies for each named colour a polygon within the CIE 1931 color space for acceptable shades.
- Recommendation R0110 – "Rhythmic Characters of Lights on Aids to Navigation", which defines light patterns and their abbreviations.
Abbreviations
Light characteristics are described using a standardised notation. The abbreviation typically includes:
- An abbreviation of the type of light, e.g. "Fl." for flashing, "F." for fixed.
- The colour of the light, e.g. "W" for white, "G" for green, "R" for red, "Y" for yellow, "Bu" for blue.
- The cycle period in seconds, e.g. "10s".
- Additional parameters such as height above chart datum (e.g. "15m") and nominal range in nautical miles (e.g. "10M").
A complete example is "Gp Oc(3) W 10s 15m 10M", indicating a group occulting light with three eclipses, white, repeating every 10 seconds, 15 metres above datum, with a nominal range of 10 nautical miles.
Classes of Light Characteristics
The IALA classification system defines the following main classes:
Fixed Light (F)
A continuous and steady light. The IALA Maritime Buoyage System does not include use of a fixed light pattern due to lack of differentiation.
Occulting Light (Oc)
A rhythmic light in which the total duration of light in each period is clearly longer than the total duration of darkness. The intervals of darkness (eclipses) are usually of equal duration. Variants include single-occulting (Oc), group-occulting (Oc(x)), and composite group-occulting (Oc(x+y)).
Isophase Light (Iso)
A light in which all durations of light and darkness are clearly equal.
Flashing Light (Fl)
A rhythmic light in which the total duration of light in each period is shorter than the total duration of darkness. Variants include single-flashing (Fl), long-flashing (LFl, flash duration ≥ 2 seconds), group-flashing (Fl(x)), and composite group-flashing (Fl(x+y)).
Quick Light (Q)
A light in which flashes are repeated at a rate of not less than 50 flashes per minute but less than 80 flashes per minute. Variants include continuous quick (Q), group quick (Q(x)), and interrupted quick (IQ).
Very Quick Light (VQ)
A light in which flashes are repeated at a rate of not less than 80 flashes per minute but less than 160 flashes per minute. Variants include continuous very quick (VQ), group very quick (VQ(x)), and interrupted very quick (IVQ).
Ultra Quick Light (UQ)
A light in which flashes are repeated at a rate of not less than 160 flashes per minute. Variants include continuous ultra quick (UQ) and interrupted ultra quick (IUQ).
Morse Code Light (Mo)
A light in which appearances of light of two clearly different durations (dots and dashes) are grouped to represent a character or characters in Morse Code, e.g. Mo(A) for the letter "A".
Fixed and Flashing Light (FFl)
A light in which a fixed low intensity light is combined with a flashing high intensity light.
Alternating Light (Al)
A light which shows alternating colours, e.g. Al WR for alternating white and red.
Use in the IALA Maritime Buoyage System
Different light characteristics are assigned to specific types of marks within the IALA system:
- Lateral marks: Use red or green lights with any rhythmic character except composite group flashing (2+1), which is reserved for preferred channel marks.
- Cardinal marks: Use white lights with specific patterns — North (continuous VQ or Q), East (VQ(3) or Q(3)), South (VQ(6)+LFl or Q(6)+LFl), West (VQ(9) or Q(9)).
- Isolated danger marks: White group-flashing light with two flashes (Fl(2)).
- Safe water marks: White light with isophase, occulting, long-flashing (10s period), or Morse "A" characteristics.
- Special marks: Yellow light with any rhythmic character not reserved for other marks.
- Emergency wreck marks: Blue and yellow alternating light with a period of 3 seconds.
Period and Phase
The period of a rhythmic light is the time interval between the beginnings of two successive cycles. Each successive element of a rhythmic light's cycle (e.g. a flash, an eclipse) is called a phase. Maximum periods are specified by IALA: 12 seconds for isophase lights, 15 seconds for single-occulting and single-flashing lights, 20 seconds for group-occulting of two eclipses, long-flashing, and group-flashing of two flashes, and 30 seconds for group-occulting of three or more eclipses, group-flashing of three or more flashes, composite group-flashing, and Morse code lights.
