A visual signal is a message sent by showing something the receiver can see across open ground: a flag or panel by day, a mirror flash in sunshine, a fire or smoke column, or a shuttered lamp at night. Every one of them is a broadcast, because anyone with a line of sight to the signal can see it, so the real choice is not just how far the signal must go but who else could see it on the way.
Part of Signal Systems and Communication
Visual signalling is the use of flags, reflected sunlight, flame and lamp light to pass short coded messages between points that can see each other.
Every Visual Signal Is A Broadcast With A Few Exceptions
A horn call or a runner reaches the person you aim it at. A flag does not: it is visible to everyone in the arc it faces, friend or stranger, for as long as it is shown. The only visual method with a narrow audience is the mirror, whose beam spreads by roughly 9 m for every kilometre, so a watcher a few tens of metres off the line sees nothing (see Mirror and Light Signals for the derivation). Fire and lamp light go in every direction unless something blocks them.
The same fact is turned round in Camouflage and Concealment: a single candle is visible at night from about 1.5 km and a campfire from 5 to 10 km, which is the other side of “a signal that carries far enough to be useful”. Plan the signal and the concealment together.
The Four Methods Side By Side
| Method | Works when | Audience | Main hazard |
|---|---|---|---|
| Flag or panel (Flag Semaphore) | Daylight | Everyone facing the flag | Misread colours; seen by strangers |
| Mirror (Mirror Heliograph) | Direct sun | A narrow beam only | Eye dazzle; cloud ends the message |
| Fire and smoke (Fire and Smoke Signals) | Night (flame), day (smoke) | Everyone in sight, for kilometres | Starts grass fires; marks your position |
| Shuttered lamp (Lantern Signals) | Night | Everyone in the open arc | Lamp fire; ruins the signaller’s night sight |
What Limits The Range
Three things decide how far a visual signal can be read, and the smallest of them wins.
- The curve of the Earth. For an eye at height h metres the horizon is about 3.57 x the square root of h kilometres, and for two elevated points you add their separate ranges. This is the geometric ceiling and the same formula the observation-post article derives.
- The size of the signal against the eye. A flag or lamp that subtends too small an angle cannot be read however clear the air.
- Terrain and air. A hill or tree line between the points ends the signal outright; haze shortens it; refraction extends it slightly.
Worked Example: A Tower Signaller And A Ground-Level Watcher
A signaller on a 10 m tower has eyes at 10 + 1.7 = 11.7 m. A watcher on the ground has eyes at 1.7 m.
- Signaller’s horizon: 3.57 x √11.7 = 3.57 x 3.42 = 12.2 km.
- Watcher’s horizon: 3.57 x √1.7 = 3.57 x 1.30 = 4.65 km.
- Together: 12.2 + 4.65 = 16.9 km on flat ground in perfect air.
That is a ceiling. The flag, lamp or fire must also be big and bright enough to read at that distance, and no hill may stand between. The geometric figure says the Earth will not stop the signal before 16.9 km, nothing more.
Choosing A Method
Step 1: List who else can see the signal point. Walk the likely approach routes and look back. Anyone who can see the signal point can read the signal.
Step 2: Decide what the message must carry. One alarm bit goes by a fire or flag; a sentence needs a code and a trained operator (Code Systems).
Step 3: Pick the method by hour and weather. Sun gives a mirror; dusk and dark give a lamp or fire; overcast daytime gives flags.
Step 4: Have a second method ready. A mirror is useless under cloud, a flag at night. The pairing is in the audio methods for when sight fails.
Common Mistakes
- Choosing by range alone. A signal that reaches the next settlement also reaches the road beside it.
- Using a method nobody has tested at the real distance. Test from the receiving end, by day and by night.
- One method only. Weather takes any single visual method away.
Safety
Flame, glare and fall hazards
Fire signals start grass and woodland fires (see Fire and Smoke Signals); a mirror flash can damage eyes if aimed at a face or looked into; a signaller on a tower or tree platform needs a rail and a fixed way down.
What’s Next
- Learn next: Audio Signal Methods — what carries when sight is blocked
- Combine with: Code Systems — turning a flash or a flag into a message