Signal lamp 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 ) . Naval usage . Aldis lamps were pioneered by the British Royal Navy in the late 19th century , and continue to be used through the present day on naval vessels . They provide handy , secure communications during periods of radio silence as well as other times and were particularly useful for convoys operating during the Battle of the Atlantic . There were several types . Some signal lamps were mounted on the mastheads of ships , some small hand - held versions were also used ( ' The Aldis lamp ' ) and other more powerful ones were mounted on pedestals . These larger ones used a carbon arc lamp as their light source with a diameter of 20 inches ( 50 cm ) . These could be used to signal to the horizon , even in conditions of bright sunlight . Although it was originally thought that it was only possible to communicate by line - of - sight , in practice it was possible to illuminate cloud bases both during the night and day which allowed for communication beyond the horizon . A maximum transmission rate possible by using flashing lights is no more than 14 wpm . They have a secondary function as simple spotlights . Early history . The idea of flashing dots and dashes from a lantern was first put into practice by Captain , later Vice Admiral , Philip Colomb , of the British Royal Navy , in 1867 . His original code , which the Navy used for seven years , was not identical with Morse , but Morse code was eventually adopted with the addition of several special signals . Another signalling lamp was the Begbie lamp , a kerosene lamp with a lens to focus the light over a long distance . Flashing lights were the second generation of signalling in the Royal Navy , after the flag signals most famously used to spread Nelson ' s rallying - cry before the Battle of Trafalgar . Modern use . The Commonwealth Navies and NATO forces use signal lamps when radio communications need to be silent or electronic " spoofing " is likely . Also , given the prevalence of night vision equipment in today ' s armed forces , signaling at night is usually done with lights that operate in the infrared ( IR ) spectrum making them less likely to be detected . All modern forces have followed suit due to technological advances in digital communications . Air traffic control usage . In air traffic control towers , signal lamps are still used today , as a backup device in case of a complete failure of an aircraft ' s radio . Light signals can be red , green or white , and steady or flashing . Messages are limited to a handful of basic instructions ( e . g . " land " , " stop " etc . ) ; they are not intended to be used for transmitting messages in Morse code . Aircraft can acknowledge signals by rocking their wings or flashing their landing lights .