Al WR 3s
Fixed: a steady light that does not go off.
Alternating: it shows white and red in turn.
The whole pattern repeats every 3 seconds.
Watch it
Our drawing of one possible timing: the notation gives the period, not the length of each flash.
The abbreviations
| Written | Name | What you see |
|---|---|---|
| F | Fixed | a steady light that does not go off |
| Oc | Occulting | the light is on for longer than it is off: it seems to blink off |
| Iso | Isophase | on and off for equal lengths of time |
| Fl | Flashing | the light is off for longer than it is on: it seems to flash on |
| LFl or L.Fl | Long-flashing | a flash of 2 seconds or more, repeated |
| Q | Quick | quick flashes, 50 to 79 a minute |
| IQ | Interrupted quick | quick flashes broken by regular dark spells |
| VQ | Very quick | very quick flashes, 80 to 159 a minute |
| IVQ | Interrupted very quick | very quick flashes broken by regular dark spells |
| UQ | Ultra quick | ultra quick flashes, 160 or more a minute |
| IUQ | Interrupted ultra quick | ultra quick flashes broken by regular dark spells |
| Mo(x) | Morse code | long and short flashes that spell a Morse letter |
| F.Fl | Fixed and flashing | a steady light with brighter flashes on top |
| Al | Alternating | different colours in turn |
| W R G Y Bu | Colours | white, red, green, yellow, blue; no colour given usually means white |
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How to read the code
Take Fl(2) W 10s 15m 17M. It reads, in order:
- Fl(2): flashing, in groups of two flashes
- W: white
- 10s: the whole pattern repeats every 10 seconds
- 15m: the light is 15 metres above the chart's height datum
- 17M: a nominal range of 17 nautical miles
When no colour is given, a white light is usually meant. Lists of lights write the code a little differently, with dots added or left out, so Fl and Fl. are the same thing. The tool reads both.
Flashing or occulting?
A flashing light is dark for longer than it is lit, so you see flashes. An occulting light is lit for longer than it is dark, so it seems to blink off. Isophase means equal light and dark. Quick, very quick and ultra quick lights flash fast, and the rate tells them apart.
About the lamp
The code tells you the period, but not how long each flash lasts. Our lamp picks lengths that keep to the IALA rules quoted in the source, such as a single flash's dark being at least three times its light. Treat it as a picture of the pattern. For a real light's timing, use its entry in an official List of Lights.
Sources
- Wikipedia, Light characteristic (citing IALA Recommendations R0110 and R1001), read 30 October 2026
Craig Fearn runs Still & Wake. He writes the guides and journal.