Signal lamp, also calle 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 Authur C W 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).