Light Characteristics on Charts — Fl, Oc, Iso, Q Explained

A light's characteristic tells you its rhythm, colour, period, height and range. For example Fl(3)WR.15s25m18/14M means: group of 3 flashes, white and red sectors, repeating every 15 seconds, 25 m above mean high water springs, visible 18 miles (white) and 14 miles (red). At night you identify a light by timing its period with a stopwatch.

The red and white Needles lighthouse at the tip of the chalk stacks off the Isle of Wight — The Needles lighthouse, Isle of Wight. At night it is identified by its characteristic, not its shape. Photo: David P Howard, CC BY-SA 2.0.

Timelines over 10 seconds for fixed, flashing, group flashing, quick, very quick, isophase, occulting and long-flash lights — How each common light rhythm looks over 10 seconds.

Common rhythms.
Abbreviation	Name	What you see
F	Fixed	A steady light
Fl	Flashing	Light shorter than dark
Fl(3)	Group flashing	Groups of 3 flashes
LFl	Long flashing	Flash of at least 2 seconds
Oc	Occulting	Light longer than dark; eclipses
Iso	Isophase	Equal light and dark
Q	Quick	50 to 60 flashes a minute
VQ	Very quick	100 to 120 flashes a minute
Al.WR	Alternating	Changes colour, e.g. white then red
Mo(A)	Morse code	Dot-dash: the letter A

The rest of the description

Colour: W white (often omitted), R red, G green, Y yellow, Bu blue.
Period: the time for one full cycle, e.g. 10s.
Height: in metres above mean high water springs.
Range: nominal range in nautical miles (M). A light may be seen further on a clear night, but also may dip below the horizon before its range if you are low down.
Sectors: some lights show different colours over different arcs, drawn on the chart. A white sector often marks the safe channel between red and green.
Dir: a directional light shows a narrow beam along a channel.

Point Reyes lighthouse, a white tower with a red roof on a rocky headland above the Pacific — Point Reyes, California. By day you see the tower; by night only its flashes. Photo: Frank Schulenburg, CC BY-SA 4.0.

Identifying a light at night

Count the flashes and time the full period, from the start of one group to the start of the next. Compare with the chart. Cardinal buoys follow a clock face: continuous for north, 3 for east, 6 plus a long flash for south and 9 for west. Practise with the lights at night drill.

Related guides

Frequently asked questions

What does 'M' mean after a light?

Nautical miles of nominal range: how far the light can be seen in clear weather, ignoring the curvature of the earth.

What is the difference between flashing and occulting?

Flashing lights are on for less time than they are off. Occulting lights are on for longer than they are off, so they look like a steady light with brief eclipses.

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