What a light’s character tells you
1The lesson in plain words
A rhythmic light’s character is its rhythm, colour and period. Match all three to the chart to help identify the mark.
The character
At night, a buoy’s shape and colour may be hard to see. Its light’s character helps you identify it. The character has three parts: the rhythm, the pattern of light and dark; the colour; and the period, the time in seconds for one complete sequence before it repeats. The International Organization for Marine Aids to Navigation (IALA) standardises these rhythms in Recommendation R0110.
Fixed and rhythmic
A fixed light (F) shines steadily. A rhythmic light shows a repeating pattern. Each piece of the pattern, a spell of light or of dark, is a phase, and a spell of dark is an eclipse. IALA does not allow a single fixed light on a mark of the buoyage system, because it is too easily confused with lights ashore.
Light shorter or longer than dark
Compare the total time lit with the total time dark over one complete period. A flashing light (Fl) is mostly dark. An occulting light (Oc) is mostly lit. An isophase light (Iso) alternates equal spells of light and dark. For grouped flashing or occulting lights, add all the light spells and all the dark spells before comparing.
Colours
Charts and the List of Lights, a reference listing navigation lights and their details, use letters: W white, R red, G green, Y yellow, Bu blue. When no colour is written, the light is white. Each mark type has its own colours: red and green for lateral marks, white for cardinal, isolated danger and safe water marks, yellow for special marks, and blue and yellow for the emergency wreck mark.
Timing the period
Count the period from the start of one sequence to the start of the next, with a watch or by counting seconds. Time two or three periods: a sequence is easily missed, and from far away a short flash looks even shorter. Check the colour too, since a white light can look reddish in haze.
2Key data
- Character
- Rhythm + colour + period
- Period
- Time for one full sequence, in seconds
- Flashing (Fl)
- Total light time shorter than total dark time per period
- Occulting (Oc)
- Total light time longer than total dark time per period
- Isophase (Iso)
- Equal spells of light and dark, alternating
- No colour letter
- White light
3Key points
- 1
A light’s character is its rhythm, colour and period.
- 2
Over a complete period, compare total light and dark times: Fl has less light; Oc has more. Iso: equal light and dark phases.
- 3
No colour letter means white.
- 4
Time several periods with a watch before you name a light.
4Frequent mistakes
Wrong: Occulting and flashing are the same: both blink.
Right: Compare light and dark. Flashing is mostly dark with short flashes; occulting is mostly light with short eclipses.
Wrong: A light with no colour letter on the chart is of unknown colour.
Right: No colour letter means white.
Wrong: One sequence is enough to time the period.
Right: Time two or three periods. A flash can be missed, and distance makes flashes look shorter.
5Practice case & exam keys
After one cycle you call a light Fl 4s. Your crew says it was on for 3 seconds and dark for 1 second. Should you keep that identification, and what should you check?
Show the answer
No. Three seconds lit and one dark suggests an occulting light with a 4-second period. Watch several cycles, confirm the colour, and match rhythm, colour and period to the chart before accepting the identification.
Exam keys
- Character = rhythm, colour and period.
- Over one complete period: Fl has less total light than dark; Oc has more. Iso: equal light and dark phases.
- No colour letter = white; W, R, G, Y, Bu.
- Period: from the start of one sequence to the start of the next.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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