Communications can be accomplished visually using, for example, and Aldis Lamp to flash messages between aircraft and ground stations in Morse Code or through standard conventions associated with emergency situations.
Light signals and pyrotechnics can be used to issue simple instructions to aircraft in flight or on the manoeuvring area. This is sometimes used by aerodrome control towers as a backup in case of radiocommunication failure. For example, a steady green light means "Cleared to land" for traffic in the air and "Cleared for take-off" for traffic on the ground. A red pyrotechnic would mean "Notwithstanding any previous instructions, do not land for the time being".
^ A signal lamp (also called an Aldis lamp, named for its inventor Arthur C. W. Aldis) is a visual signaling device for optical communication (typically using Morse code) - essentially a focused lamp which can produce a pulse of light.