A lantern signal is a steady lamp whose light is shown and hidden by a hinged shutter to send a coded flash at night. Put the lamp inside a hood or box open only toward the receiver and the audience shrinks from every direction to a sector, but even a hooded lamp is seen by anyone inside that sector, so use it for the few messages that are worth that exposure.
Part of Signal Systems and Communication
Night lamp signalling shows and hides a steady light with a shutter to send dots and dashes between points that can see each other.
Hide The Lamp, Do Not Wave It
The parent article describes a board hinged in front of a lantern. Keep the flame steady and move only the shutter: a swung lamp smears, flickers and shows from every side. The sender’s task has two halves: send the message toward the receiver, and keep the light away from everyone else, including the sender’s own eyes. Eyes need about 20 to 30 minutes in the dark to adapt, as Night Observation states, and a bare flame near the sender’s face undoes that in seconds.
What You Need
- A lamp with a steady flame, protected from wind. Oil lamp designs are in Oil Lamp Construction.
- A box or hood with the front open and the other sides closed, lined to reduce stray light.
- A shutter, a flat board or tin plate on a hinge or handle, that covers the whole opening.
- A fire-safe stand and a lid or dry sand within reach, from Lamp Fire Prevention.
- A code and a second operator to read the reply.
Worked Example: How Much A Hood Narrows The Audience
The sizes are example values, chosen to show the method.
- A flame 2 cm wide behind an opening 4 cm wide, 20 cm in front of the flame.
- The whole flame is visible through the opening within an angle of plus or minus (4 - 2) / (2 x 20) = 0.05 radians, about 2.9 degrees.
- Any part of the flame is visible within plus or minus (4 + 2) / (2 x 20) = 0.15 radians, about 8.6 degrees, a sector about 17 degrees wide.
- At 4 km that sector is about 2 x 0.15 x 4,000 = 1,200 m wide.
By comparison a mirror beam at 4 km is about 4 x 9.3 = 37 m wide. A hood cuts the audience from all directions to about 17 degrees, but anyone standing in that sector 4 km out still sees the lamp, and it is roughly 30 times wider than a mirror at the same distance. A narrower opening or a longer hood narrows it further but also dims it.
Range: Test It, Do Not Quote It
The parent article gives 3 to 5 km on a dark night with good air. Camouflage and Concealment gives a single candle as visible at about 1.5 km. The gap between them is the lamp’s brightness and any reflector, which this corpus has not measured. Treat 1.5 km as the figure for a small flame, treat 3 to 5 km as what a bright lamp might reach, and test at the real distance with the real lamp.
Method: Sending A Night Message
Step 1: Light the lamp and let the receiver’s eyes settle. Both operators must be dark-adapted; do not look into the lamp.
Step 2: Aim the opening at the receiver and block the sides with the hood.
Step 3: Attention first. Send the agreed attention code and wait for the reply flash. No reply means no message.
Step 4: Send in short groups. Keep dots and dashes in fixed ratios (a dash three dots long, as in Morse Code System) and send slowly enough for the slower operator.
Step 5: Repeat back and close. The receiver sends the message back; the sender confirms; the shutter stays shut.
Common Mistakes
- Waving the lamp. It shows in every direction and cannot carry a code.
- A bright opening in the sender’s own eyes. The sender is night-blind for the reply.
- Signalling from a skyline. The lamp and the signaller are silhouetted.
- Using the lamp when a horn would do. A lamp in the dark tells every watcher where you are.
Safety
Lamp fuel and shutter fires
A hot lamp in a closed hood can scorch wood, spill fuel or start a fire; keep it on a stable fireproof stand with a metal lid or dry sand at hand. Never refuel a lit lamp. Fuel that is not fully burned gives off carbon monoxide in a closed room, so signal from the open.
What’s Next
- Learn next: Audio Signal Methods — signals that need no line of sight
- Combine with: Simple Code Tables — the meanings the flashes carry