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 21communicationcommunication]] (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 [[warship|naval vessels]] and in [[airport]] [[Air traffic control|control tower]]s (using color signals for stop or clearance).