What was an Aldis lamp?

The Aldis lamp was named after Arthur Cyril Webb Aldis (1878–1953), who invented a widely used design of signal lamp. An Aldis lamp is a focused lamp that can produce a pulse of light and so can serve as a visual signalling device for optical communication, typically using Morse code. In large versions, 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. With hand-held lamps, a concave mirror is tilted by a trigger to focus the light into pulses. They were pioneered by the British Royal Navy and continue to be used through to the present day, on naval vessels. They provide secure communications, which is especially useful during periods of radio silence, and were particularly valuable for convoys operating during the Battle of the Atlantic.