An Aldis lamp is a visual signalling device, essentially a focussed lamp which can produce a pulse of light. 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.

Aldis lamps were pioneered by the British Royal Navy in the late 19th Century, and were used up until the end of the 20th Century on board naval vessels. They provided handy, secure communications during periods of radio silence, and were particularly useful for convoys operating during the Battle of the Atlantic. Lamps were normally mounted on the mastheads of vessels; smaller, handheld versions were also used. Power was usually provided by the vessel's emergency generator, and the lamps were powerful enough to be used during daylight hours. They had a secondary function as simple spotlights.

The lamp emits flashing light of varying duration, used to form a code which could be used to communicate specific messages. From 1867 to 1874 the Royal Navy used a proprietary code devised by a Vice Admiral Philip Colom, after which Morse code was adopted as a universal standard.

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.

See also

• heliograph

External link

• An Aldis lamp in operation (http://www.airwaysmuseum.com/Aldis%20lamp%20&%20Very%20pistol.htm)
• Selection of signalling lamps (http://www.morsecode.freeserve.co.uk/lamps.htm)