Isophase, occulting, Morse and alternating
1The lesson in plain words
Not every light flashes. Isophase, occulting, Morse and alternating lights have their own patterns, and safe water marks use three of them.
Isophase
In an isophase light (Iso) the light and the dark last the same time. Iso 4s shows 2 seconds of light and 2 of dark. IALA sets a minimum period of 2 seconds and prefers at least 4, so that it is not confused with flashing or occulting lights. A white isophase light marks a safe water mark.
Occulting
An occulting light is mostly lit over its complete period. For single occulting (Oc), IALA specifies light lasting at least three times the eclipse. Oc 4s might show 3 seconds of light and 1 of dark. A group-occulting light, such as Oc(2) 10s, shows two eclipses close together, then a long spell of light. White occulting can mark safe water; yellow group occulting is used on special marks.
Morse
A Morse code light (Mo) shows shorter and longer flashes that spell a letter. Mo(A) is a dot then a dash (· –); in white it marks safe water. A dot lasts about half a second and a dash at least three times as long. Racons also use Morse letters, but on radar; they are in the electronic aids unit.
Alternating
An alternating light (Al) shows different colours in turn, for example Al.WR: white, then red. It is used with care, because both colours must look equally bright. The emergency wreck mark has its own alternating rhythm: 1 second blue, half a second dark, 1 second yellow, half a second dark, in a period of 3 seconds.
Fixed and combined
A fixed light (F) never changes. It is not used on buoys, but appears on some leading lights and structures. A fixed and flashing light (FFl) shows a steady weak light with brighter flashes. Vessel lights follow the COLREGs, the International Regulations for Preventing Collisions at Sea (/colreg/rule-20).
2Key data
- Iso
- Light = dark; period at least 2 s, preferably 4 s
- Single occulting (Oc)
- Light at least 3 × the eclipse
- Mo(A)
- Dot then dash; white = safe water
- Safe water lights
- White Iso, Oc, LFl 10s or Mo(A)
- Emergency wreck
- Bu 1 s, dark 0.5 s, Y 1 s, dark 0.5 s; period 3 s
- Al
- Different colours in turn, e.g. Al.WR
3Key points
- 1
Isophase alternates equal light and dark phases. Over a complete period, occulting has more total light than dark.
- 2
Safe water marks show white Iso, Oc, LFl 10s or Mo(A).
- 3
Mo(A) is dot–dash: a shorter flash, then a longer one.
- 4
Alternating lights change colour; the wreck mark alternates blue and yellow.
4Frequent mistakes
Wrong: Iso and Oc are the same, since both are mostly on.
Right: Iso has equal light and dark. Oc has clearly more light than dark.
Wrong: Mo(A) is a dash then a dot.
Right: A is dot–dash: a shorter flash, then a longer one. Dash–dot is the letter N.
Wrong: A light that shows white then red must be two separate lights.
Right: It may be one alternating light, such as Al.WR. Check the List of Lights.
5Practice case & exam keys
You see a white short–long pattern. A crew member calls it an alternating light because one flash is longer. What should you check before accepting that identification?
Show the answer
Alternating means changing colour, not flash length. Watch several complete cycles. A repeating white dot–dash is Mo(A), a safe-water rhythm; an alternating light changes colour. Match the complete rhythm and period to the chart.
Exam keys
- Iso: equal light and dark phases. Oc: more total light than dark over one complete period.
- Safe water: white Iso, Oc, LFl 10s or Mo(A).
- Mo(A) = dot–dash; a dash is at least 3 dots long.
- Wreck mark: alternating blue and yellow, period 3 s.
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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