Still & Wake

Lighthouse light characteristics, explained

Every lighthouse flashes its own pattern, and charts write it in a short code beside the light. Type the code and see it in plain words, then watch it flash.

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What does that light do?

Try: F R, Oc R 6s, Iso G 4s, Fl G 5s, LFl W 10s, Fl(3) R 15s, Fl(2+1) W 15s, Q(3) G 9s, VQ(6)+LFl W 10s, Mo(K) G 6s, Al WR 3s

LFl W 10s

Long-flashing: a flash of 2 seconds or more, repeated.

Colour: white.

The whole pattern repeats every 10 seconds.

A framed painting of a lighthouse on a rock in a storm, waves breaking and a small sailing boat nearby, above a sideboard with a lamp.
A lighthouse painting in a room setting.

The abbreviations

WrittenNameWhat you see
FFixeda steady light that does not go off
OcOccultingthe light is on for longer than it is off: it seems to blink off
IsoIsophaseon and off for equal lengths of time
FlFlashingthe light is off for longer than it is on: it seems to flash on
LFl or L.FlLong-flashinga flash of 2 seconds or more, repeated
QQuickquick flashes, 50 to 79 a minute
IQInterrupted quickquick flashes broken by regular dark spells
VQVery quickvery quick flashes, 80 to 159 a minute
IVQInterrupted very quickvery quick flashes broken by regular dark spells
UQUltra quickultra quick flashes, 160 or more a minute
IUQInterrupted ultra quickultra quick flashes broken by regular dark spells
Mo(x)Morse codelong and short flashes that spell a Morse letter
F.FlFixed and flashinga steady light with brighter flashes on top
AlAlternatingdifferent colours in turn
W R G Y BuColourswhite, 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.

Three framed lighthouse paintings in a row above a green sofa: one at sunset, one by day and one at night.
Three lighthouses in a room setting.

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

Written by Craig Fearn, Still & Wake. Last updated .

Craig Fearn runs Still & Wake. He writes the guides and journal.

Questions, answered

What does Oc mean on a chart?
Occulting: the light is on for longer than it is off, so it seems to blink off rather than flash on.
What does the M at the end mean?
The nominal range in nautical miles. A small m before it is the height of the light in metres.
Does the tool hold data for real lighthouses?
No. Type the characteristic from your chart or List of Lights and the tool explains it. For lighthouses you can visit, see our map of UK lighthouses.