Signal lamp

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Signal lamp training during World War II.

Signal lamp, also called Aldis lamp, is a visual signaling device for optical communication (typically using Morse code) – essentially a focused lamp which can produce a pulse of light. It is named after its inventor ACW Aldis. This pulse is achieved by opening and closing shutters mounted in front of the lamp, either via a manually-operated pressure switch or, in later versions, automatically. The lamps were usually equipped with some form of optical sight, and were most commonly used on naval vessels and in airport control towers (using color signals for stop or clearance).

Aldis lamps were pioneered by the British Royal Navy in the late 19th Century, and were used until the end of the 20th Century on naval vessels. They provided handy, secure communications during periods of radio silence as well as other times and were particularly useful for convoys operating during the Battle of the Atlantic. There were several types. Some signal lamps were mounted on the mastheads of ships, some small handheld versions were also used ('The Aldis lamp') and other more powerful ones were mounted on pedestals These larger ones used a carbon arc lamp as their light source with a diameter of 20 inches (50 cm). These could be used to signal to the horizon in optimum conditions. They had a secondary function as simple spotlights.

The idea of flashing dots and dashes from a lantern was first put in to practice by Captain, later Vice Admiral, Philip Colomb in 1867. His original code, which the Navy used for seven years, was not identical with Morse, but Morse code was eventually adopted with the addition of several special signals. Flashing lights were the second generation of signalling in the Royal Navy, after the flag signals most famously used to spread Nelson's rallying-cry before the Battle of Trafalgar.

The Royal Navy phased out the use of Aldis lamps in 1997, although by that time they were largely ceremonial. Other modern forces have followed suit as technological advances in digital communications have made the device obsolete.