A mirror signal reflects sunlight in a narrow beam that widens by only about 9 m for every kilometre of distance, so only a watcher standing in that beam sees the flash, which makes it more private than a fire. It needs direct sunshine, so use a shaded lamp at night, remembering that a lamp can be seen by everyone in line of sight.
Part of Early Warning Systems
Mirror and light signals send short coded messages by flashing sunlight from a mirror or by showing and hiding a lamp, covering distances beyond the range of the voice or a horn.
Narrow Beam, Narrow Audience
The sun is not a point. Seen from the Earth it spans about half a degree, a figure I am recalling at about 0.53 degrees, which is 0.0093 radians. A flat mirror cannot make a beam tighter than the sun’s own width, so the beam it throws spreads by about 0.0093 m for every metre of distance, about 9.3 m per kilometre (roughly 9 m). The flash is bright because it is a second sun and visible to someone standing in it from very far. A person 50 m off the line of the beam sees nothing at 5 km.
That makes the mirror a signal with a built-in direction. The same property means the signaller must aim it carefully. The signalling method itself, with the sighting hand, the V of two fingers and the codes, is explained in Signal Systems. The mirror material and how to make one are in Glass, Mirrors and Lights.
What You Need
- A flat mirror, glass or polished metal, 7-15 cm across (the signal article’s size).
- A way to aim: fingers in a V, a sighting hole or a stick.
- Direct sunshine and a clear line to the watcher.
- A shuttered lamp for night use, with a shutter you can work quickly.
- A code agreed in advance, short and simple.
Method: Sun, Aim, Short Message
Step 1: Find the sun. The mirror is tilted until the reflected sunlight falls on the watcher. It will not work on an overcast day, and a patch of cloud over the sun cuts the signal in the middle of a message.
Step 2: Aim with a sighting aid, not by guessing. Hold the mirror near the face, bring the reflected sunlight onto your extended fingers, then tilt slightly to move it to the target.
Step 3: Send a short message. A few flashes with pauses is plenty; long messages are for trained operators.
Step 4: Wait for the answer, which the watcher gives with a flash or a flag.
Step 5: At night use a shuttered lamp. Work it from behind a screen, so the light shows toward the watcher and not toward your own post.
Worked Example: How Wide Is The Beam
Beam width at distance d is about d × 0.0093 (ignoring the mirror’s own width, which is small beside it).
- At 1 km (1,000 m): 1,000 × 0.0093 = 9.3 m.
- At 5 km: 5,000 × 0.0093 = 46.5 m.
- At 10 km: 10,000 × 0.0093 = 93 m.
A watcher at 5 km has to be within about 23 m either side of the line from the mirror to see the flash, and so does anyone else. The signaller’s aim has to be right to within the beam width at the target, which is why you aim with a sighting aid and not by eye. A lamp at night has no such directionality and can be seen all round, which is why the signal article and the camouflage article say to shield it.
Common Mistakes
- Signalling on a hazy or overcast day. The flash is weak or absent; fall back on a horn or runner.
- Aiming without a sighting aid. The beam is narrow; a small tilt moves it off.
- A lamp lit in your own watcher’s eyes. It ruins night vision. See Night Observation.
- Leaving a mirror or glass uncovered in the sun. Accidental flashes reveal your position, as the camouflage article warns.
Safety
A mirror flash is a second sun, and the sun burns
The reflected flash is bright enough to dazzle and, if stared at, to damage eyes; never aim it at someone’s face at close range and never look into it. A curved or lens-shaped reflector concentrates the heat and can ignite tinder, as the salvage article explains. Night lamps need the usual care: see Lamp Fire Prevention.
What’s Next
- Learn next: Passive Detection — warning that needs no watcher or signal at all
- Combine with: Signal Fires and Beacons — the signal for the hours when there is no sun