A
signal lamp (sometimes called an
Aldis lamp, named after
Arthur Cyril Webb Aldis who invented a widely used design) is a visual signaling device for
optical communication (typically using
Morse code). Modern signal lamps are a focused lamp which can produce a pulse of light. 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. 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).