Light characteristic: decoding a chart abbreviation and IALA marks

An online tool for reading the characteristic of a navigational light. It decodes chart abbreviations — Fl(3)WR.15s12M, Q(9)15s, VQ(6)+LFl.10s, Al.Bu.Y.3s — into class, number of flashes, colors, period and range, and then plays the rhythm in real time. Beside it sits an IALA buoyage reference with the topmarks drawn properly, and a calculation of the range at which the light will rise for you.

How to use it

Paste the entry from your chart

For example Fl(3)WR.15s12M. Spaces and dots can go wherever is convenient — the parser is not fussy about them.

Watch the rhythm

The lamp flashes as written, and the strip below shows the same period unrolled in time.

Check the range

Move the height of eye and you will see from how far this light will rise for you in particular.

Paste the abbreviation from your chart and watch how that light actually flashes

Characteristic from the chart

The lamp flashes with the real period

A reference and a calculation, not a navigational publication. The characteristics of particular lights, their sectors and their positions come only from a corrected List of Lights and a corrected chart. The rhythm here is a reconstruction from the entry, not a measurement of a real light.

Features

A live rhythm, not a line in a table

A lighthouse is identified by its rhythm: three flashes, a pause, three again. A reference that prints "Fl(3)15s" and stops explains everything except the one thing you need at night. Here the light flashes on screen with exactly the period written down.

Decodes an arbitrary entry, not a fixed list

Class, group, composite rhythms like Q(6)+LFl, composite groups like Fl(2+1), Morse, alternating and sectored lights, period, range and elevation. Verified against twenty-four real chart entries and the characteristics of every IALA mark.

A rhythm strip with a seconds scale

One period unrolled into a bar: where it burns and in what color, where it is dark, with ticks by the second. It shows what the text cannot — that an occulting light is lit more than it is dark, and an isophase light exactly half.

Geographic and luminous range are different things

The geographic range follows from the height of the light and your own height of eye; the luminous range from the power of the light and the visibility. You will see it at the smaller of the two, and the tool says plainly which one is the limit.

Topmarks are drawn, not labeled

Two cones point up on a north mark and down on a south mark — by day that is the only difference between them. They are drawn properly here, because replacing them with an icon throws away the very subject of the reference.

Region A and Region B switch over

Lateral marks are colored the opposite way round in the two halves of the world: in Region B a red buoy marks the starboard side of the channel, not the port side. Cardinal, isolated danger and safe water marks are the same everywhere — the tool marks which is which.

What you can do

Decode any light characteristic from a nautical chart
See the rhythm: period, number of flashes, colors
Work out the range of first sighting from your height of eye
Compare cardinal marks by their flash counts
Switch between IALA Region A and Region B
Test yourself: name the mark from its rhythm alone

Why the flash count matters more than the color

At night color reads badly. Near the limit of range, white, yellow and a weak green run together, and against shore lights the color is lost altogether. The rhythm, on the other hand, always reads: you count it against a watch, and it depends on neither haze nor distance. The whole system is built on that. On cardinal marks the rhythm carries the entire meaning — three flashes mean east, six with a long flash mean south, nine mean west, and continuous means north. Counting flashes against a second hand is easier than arguing with yourself about a color. Which is why a reference that shows a mark as a picture but not as a rhythm is explaining the secondary thing instead of the main one.

Frequently asked questions

Is the rhythm on screen exactly the same as a real lighthouse?

The period and the number of flashes are exact — they are written in the characteristic itself. The duration of a single flash is never written on a chart, so it is taken from the IALA recommendations: a flash of 0.5 seconds, an eclipse of 1 second within a group, a long flash of not less than 2 seconds. The rhythm here is therefore a faithful reconstruction with the right period, not a copy of one particular light. The tool says so directly under the result.

What is the difference between Fl, Oc and Iso?

The proportion of light in the period. A flashing light (Fl) is lit LESS than it is dark. An occulting light (Oc) is lit MORE: it burns and is briefly covered. An isophase light (Iso) is exactly half and half. The rhythm strip shows this at a glance, whereas in prose the difference has to be remembered.

What does Fl(2+1) mean?

A composite group: two flashes, a short pause, one more, then a long eclipse. It marks a preferred-channel buoy — a lateral mark carrying a band of the other side's color. A plus sign outside the brackets means something else: Q(6)+LFl is six quick flashes followed by one long one, the characteristic of a south cardinal mark.

Why does the tool ask for the IALA region?

Because lateral marks are colored the opposite way in the two halves of the world. Region A — Europe, Africa, Australia, New Zealand, the Gulf and part of Asia — has the port side of the channel red. Region B — the Americas, Japan, Korea, the Philippines — has it green. Cardinal marks, isolated danger, safe water and special marks do not depend on the region, and the tool notes that on each mark.

What is the range of first sighting?

The distance at which the light rises above the horizon. It depends only on geometry — the height of the light and your height of eye — and comes to 2.08 × (√height of eye + √height of light) in nautical miles. It does not depend on the power of the light at all. Separately there is the luminous range: how far the light itself carries in the given transparency of the air. You will see the light from the smaller of the two.

Can this replace a List of Lights?

No. It decodes the entry you bring to it, but it holds not one real light: no positions, no sectors, no corrections. The characteristics of particular lights come only from a corrected List of Lights and a corrected chart.

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Who is this tool for?

⚓ Sailors and yacht crews

Published 01 Sep 2026 Updated 06 Sep 2026 Author: The Supreme