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[1][2]) to direct the aircraft. ICAO regulations require air traffic control towers to possess such signal lamps.[3][4] The signal lamp has a focused bright beam and is capable of emitting three different colours: red, white and green.[5][6] These colors may be flashed or steady, and have different meanings to aircraft in flight or on the ground.[5][2][7] 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.[6][7] Air Traffic Control signal light guns are typically specified with a (white) center beam brightness of > 180,000 - 200,000 candela,[1][8][9] and are visible for roughly 4 miles in clear daylight conditions.[9][10][11] The table below describes the meaning of the signals.[5][2][7] The use of handheld combination red/green/white signal lamps for air traffic control dates back to at least the 1930s.[12]