Signal Lamp / Aldis Lamp

What sets a signal lamp apart

Almost everything else on a modern bridge depends on electronics, satellites or at least a functioning power bus. A signal lamp, still commonly called an Aldis lamp after its early-20th-century design, is the deliberate exception: a focused beam of light switched on and off by a trigger to send Morse code, readable by eye at several miles with no receiving equipment needed on the other end beyond a lookout. It survives on the bridge not out of nostalgia but because it is the one visual signalling method that works when radios are silenced, jammed, or simply not fitted on the vessel being signalled — small craft, naval vessels under certain conditions, or another ship confirming identity visually during close-quarters situations.

Main components

Lamp head and lens

A high-intensity bulb or, on modern units, an LED array behind a focusing lens that concentrates the beam into a narrow, bright cone rather than the wide spread of a searchlight, which is what makes the flashes readable as distinct Morse characters at distance.

Trigger mechanism

A pistol-grip trigger that operates a shutter or directly switches the lamp circuit, letting the operator key Morse code by hand with the same feel regardless of how the beam itself is produced.

Power supply

Battery-powered handheld units are standard for portability to the bridge wings; some vessels retain a fixed, ship-power lamp on the monkey island for longer signalling sessions where battery endurance would be a limitation.

Sighting device

A simple sight or scope aligned with the beam axis so the operator can aim the lamp precisely at a distant receiving vessel rather than sweeping the beam across the horizon.

Regulations and class

SOLAS Chapter V requires ships to carry a daylight signalling lamp not dependent solely on the ship's main power supply, as part of the mandatory navigational equipment carriage requirements. The lamp does not need type approval in the way GMDSS equipment does, but it must be operational and its battery condition maintained, and it is a standard item checked during flag state and port state control inspections of bridge equipment. Officers are still required to demonstrate basic Morse code proficiency during certification, which keeps the lamp a functioning piece of equipment rather than a certificate-only fixture.

Typical faults

• Battery left uncharged or corroded terminals — the most common finding at port state control, since the lamp is used rarely enough to be forgotten between drills.
• Lens cracked or fogged with condensation, scattering the beam and making Morse characters unreadable at any useful range.
• Trigger mechanism stiff or sticking from lack of use, producing ragged, unreadable dots and dashes even when the lamp itself works.
• LED retrofit units with a beam angle too wide for the lens housing, reducing effective signalling range compared with the original bulb design.

What to look for in a supplier

• A beam intensity and focus rating suited to the ranges the vessel actually needs, not just a bright light.
• Battery or power options matched to how the lamp will actually be stowed and used — handheld on the bridge wing versus fixed on the monkey island.
• Robust housing, since this is deck equipment that sits in a bracket exposed to weather between the rare occasions it is used.

Test the lamp and its battery at every navigation equipment check, not only before drills — it is the backup for the day every other method of signalling has failed.