In the case of a radio failure or aircraft not equipped with a radio, or in the case of a deaf pilot, air traffic control may use a signal lamp (called a "signal light gun" or "light gun" by the FAA<ref name="FAA2214" /><ref name="FAATbl431">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>) to direct the aircraft. ICAO regulations require air traffic control towers to possess such signal lamps.<ref name="ICAOannex14V1513">Template:Cite book</ref><ref name="Bahrain">Template:Cite book</ref> The signal lamp has a focused bright beam and is capable of emitting three different colors: red, white and green.<ref name="ICAO2" /><ref name="FAA4313">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> These colors may be flashed or steady, and have different meanings to aircraft in flight or on the ground.<ref name="ICAO2" /><ref name="FAATbl431" /><ref name="EURlex">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Planes can acknowledge the instruction by rocking their wings, moving the ailerons if on the ground, or by flashing their landing or navigation lights during hours of darkness.<ref name="FAA4313" /><ref name="EURlex" /> Air traffic control signal light guns are typically specified with a (white) center beam brightness of > 180,000 - 200,000 candela,<ref name="FAA2214">Template:Cite book</ref><ref name="MIL25971D">Template:Cite book</ref><ref name="RuckerRFP">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and are visible for roughly 4 miles in clear daylight conditions.<ref name="RuckerRFP" /><ref name="ATI">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="PPSTech">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The table below describes the meaning of the signals.<ref name="ICAO2">Template:Cite book</ref><ref name="FAATbl431" /><ref name="EURlex" /> The use of handheld combination red/green/white signal lamps for air traffic control dates back to at least the 1930s.<ref name="US2085020">Template:Cite book</ref>
| Signal | Aircraft in flight | Aircraft on the ground | Ground vehicles or personnel |
|---|---|---|---|
| Flashing white | ICAO – Land at this airport and proceed to apron (this is not a clearance to either land or taxi. Clearances to land and taxi will follow.) FAA – Not applicable |
Return to starting point on airport | Return to starting point on airport |
| Steady green | Cleared to land | Cleared for takeoff | Cleared to cross, proceed or go |
| Flashing green | Return for landing | Cleared to taxi | Not applicable |
| Steady red | Give way to other aircraft and continue circling | STOP | STOP |
| Flashing red | Airport unsafe, do not land | Taxi clear of the runway in use | Clear the taxiway/runway |
| Alternating red and green | Exercise extreme caution | Exercise extreme caution | Exercise extreme caution |