A lighthouse's identity is a duration, not a shape
Every lighthouse on a coast flashes a different rhythm so it can be told apart in total darkness — a building identified by timing, not by what it looks like.
A row of blocks and gaps, timed: 1 second on, 2 off, 1 on, 2 off, 1 on, then 8 seconds of nothing. Fl(3) 15s, printed large. Flash group of three, printed small underneath.
Nobody can see the tower this describes. That's the point of the diagram.
What does Fl(3) 15s actually mean?
Three flashes, then dark, repeating on a fifteen-second cycle — and nothing else about the lighthouse is in that string of characters.
Add up the diagram: 1 second of light, 2 of dark, 1 of light, 2 of dark, 1 of light, then an 8 second eclipse before the group starts again. That's 15 seconds exactly, and it repeats all night, every night, for as long as the light is lit. A sailor doesn't need to see the tower, its colour or its height. They need to watch the sea in one direction for fifteen seconds and count.
Why does every lighthouse need its own rhythm?
Because two lights with the same pattern on the same stretch of coast are a navigation hazard, not a coincidence.
A coastline can have a dozen lights visible from open water on a clear night, and a sailor plotting a position has to know which one they're looking at, not just that a light exists. Assigning each tower its own timed sequence — how many flashes, how long each one lasts, how long the dark gap runs — turns a field of identical white dots into a set of named, distinguishable signals. The building could be repainted or rebuilt and the light would still be the same lighthouse, because the rhythm is the identity, not the structure holding it up.
Why fifteen seconds specifically, and not five or fifty?
Long enough to count with confidence, short enough that a sailor isn't left guessing for a full minute in the dark.
A pattern that repeated every five seconds wouldn't leave time to register three distinct flashes before it started over. One that ran to a minute would make a nervous sailor doubt whether they'd lost count partway through. Fifteen seconds sits in the middle: long enough to watch the whole sequence once, short enough to watch it twice and be certain, which makes this the only kind of architecture whose distinguishing feature is how long something takes rather than what it looks like.
Who owns a light characteristic like this?
The timing convention belongs to nobody. A real lighthouse's actual, currently-in-use pattern is a different matter.
Flash group, duration and eclipse are a public maritime notation — the same kind of shared method that lets a shipping forecast carve open water into named boxes without anybody owning the boxes. A specific working lighthouse's actual published characteristic, tied to its real name and its real stretch of coast, is the sort of detail a maritime authority would need to be asked about if reproducing that exact real light is the point. An invented characteristic on an invented coast does the same job without the paperwork, and it can be built to say precisely what the wearer wants it to.
Does this only work for an actual working lighthouse?
No — the timing diagram is what carries the idea, and it works for any light somebody wants named, real or not.
A structure that means something to one person — a childhood holiday coast, a grandparent's old posting, a folly built to look like one — can be given its own invented Fl(n) sequence the same way a real one has. Nothing about the diagram requires the tower behind it to still be standing, any more than a printed tide table needs a real harbour behind its numbers to still work as an object.
Isn't this basically the same idea as Morse code?
No — Morse assigns a pattern to a letter of the alphabet. A light characteristic assigns a pattern to a single, specific place.
Morse is a general alphabet anyone could use to spell anything. A lighthouse's rhythm spells nothing except itself — it isn't decoded into a word, it's matched against a chart until one entry fits. The two look similar as blocks and gaps, but one is a language and the other is closer to a fingerprint.
Does it print well?
Yes — flat blocks with hard edges are exactly what a timing diagram is already made of.
Keep the light blocks solid white or yellow against black, with no gradient softening the edges of any block — flat colour with a hard edge survives fabric in a way a soft blend never does, the same lesson a painted blaze on a tree trunk has already learned: crude and flat outlasts fine and blended. The numerals along the top are the fine detail here and want setting larger than they'd appear on an actual chart, so they stay legible at a glance rather than requiring the reader to count pixels.
How do I get one made?
Describe your own light at JustOG — the flash count, the timings, and the total cycle. Pick a direction, drag the crop frame, see it composited on the real garment, and it's made to order and shipped.
Designs other people have published are in the shop.
Fifteen seconds of dark and light, on a loop, telling nobody anything except which light this is.