Light Characteristic Decoder
Type a charted light like Fl(3)WR 15s 21m 18M and get it back in plain English, with the flash pattern drawn out.
How to read the line
A charted light is always written in the same order: rhythm, group, colour, period,
elevation, range. So Fl(3)WR 15s 21m 18M is a light that flashes three
times, shows white and red sectors, repeats every fifteen seconds, stands twenty-one metres
above chart datum and is nominally visible for eighteen miles.
The trap is the two ms. Lower case is metres of elevation; upper case is miles of range. They sit next to each other, they mean entirely different things, and mixing them up turns a distant light into a very tall one.
Rhythms
| Code | Name | What you see |
|---|---|---|
| VQ | Very quick flashing | Roughly 100 to 120 flashes a minute — fast enough that it reads as a stutter. Used where a quick light would be confused with another nearby. |
| UQ | Ultra quick flashing | Over 160 flashes a minute. Rare, and almost always marking something you badly want to avoid. |
| IQ | Interrupted quick flashing | Quick flashing with a regular dark pause. The pause is the identifying feature. |
| LFl | Long flashing | A single flash of two seconds or more each period — deliberately unmistakable at a distance. |
| FFl | Fixed and flashing | A steady light with a brighter flash superimposed. The steady part holds your eye between flashes. |
| Fl | Flashing | Light shorter than dark. The most common characteristic by far. |
| Oc | Occulting | Light longer than dark — a steady light briefly eclipsed. Easy to mistake for flashing until you time it. |
| Iso | Isophase | Equal light and dark. Distinctive precisely because it is so regular. |
| Q | Quick flashing | Roughly 50 to 60 flashes a minute. Common on cardinal marks, where the number of flashes tells you which side to pass. |
| Al | Alternating | Alternates between colours rather than between light and dark. |
| Mo | Morse | Flashes a morse letter. Mo(A) — short then long — marks safe water or a landfall. |
| F | Fixed | A steady light that does not change at all. Increasingly rare, and easily lost against shore lighting. |
Colour codes: W white · R red · G green · Y yellow · Bu blue · Vi violet · Or orange. No letter means white.
Cardinal marks — count the flashes like a clock face
| Mark | Light | Topmark & colours | What to do |
|---|---|---|---|
| North | Q or VQ, continuous | Black over yellow, two cones pointing up | Pass to the north of it. The flashing never stops, which is the identifier. |
| East | Q(3) or VQ(3) every 5 or 10 s | Black, yellow band, black; cones base to base | Pass to the east. Three flashes for three o’clock. |
| South | Q(6) + LFl or VQ(6) + LFl every 10 or 15 s | Yellow over black, two cones pointing down | Pass to the south. Six flashes for six o’clock, and the long flash confirms you counted six rather than losing track. |
| West | Q(9) or VQ(9) every 10 or 15 s | Yellow, black band, yellow; cones point to point | Pass to the west. Nine flashes for nine o’clock. |
Three, six and nine sit at three, six and nine o'clock on a clock face pointing east, south and west — which is the whole mnemonic. North gets the continuous flashing that is left over, and south gets a long flash on the end so you know you counted six rather than losing your place partway through nine.
Red right returning — but only in half the world
Lateral marks reverse between regions. In IALA Region A — Europe, Africa, most of Asia and Australasia — red is on your left as you come in from seaward. In Region B — the Americas, Japan, Korea and the Philippines — red is on your right, which is the "red right returning" most Americans learn first. The shapes are the same in both; only the colour convention flips, and it flips mid-ocean.
How to use
- Type or paste the characteristic exactly as the chart prints it.
- Read the plain-English sentence and the flash pattern for one period.
- Check the breakdown table for elevation and range — the two m’s mean different things.
Frequently asked questions
What does Fl(3)WR 15s 21m 18M mean?
A light that flashes three times, shows white and red sectors, repeats the whole pattern every fifteen seconds, stands twenty-one metres above chart datum, and is nominally visible for eighteen miles.
What is the difference between m and M?
Lower case m is metres of elevation; upper case M is nautical miles of range. They appear next to each other in the same line and mean entirely different things, which is why charts are strict about the case even when everything else is abbreviated.
How do I tell a south cardinal from a west one?
Count the flashes like a clock face. Three is east, six is south, nine is west, and continuous quick flashing is north. The south mark adds a long flash on the end so you know you counted six rather than losing your place in nine.
What does it mean when a light shows two colours?
They are sectors, and which one you see depends on your bearing from the light. That is the entire point of a sectored light — the colour changing as you drift out of the safe white sector is the warning.
Is nominal range how far I will see it?
Not necessarily. Nominal range is luminous range in clear weather. What you actually get is that or the geographic range — where the curve of the earth stops hiding it — whichever is smaller, and haze cuts the luminous one hard.
Does red mean the same thing everywhere?
No. In IALA Region A — Europe, Africa, most of Asia and Australasia — red is on your left coming in from seaward. In Region B — the Americas, Japan, Korea and the Philippines — red is on your right. The shapes match; only the colour convention flips.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.