Red, green, and white handheld signal lights have been used since the 1930s. But because of their infrequent use, pilots struggle with memorizing the meaning of the color-coded signal messages.
Most pilots have a light gun signal card on a kneeboard or in an electronic flight bag. One problem with using a light gun is that it is difficult for a tower controller to handhold the pencil beam of light steady on an airplane. This results in an intermittent on-and-off light confusing the pilot, who might not be able to discern if the light is meant to be a steady or a flashing light.
Aside from light guns, we also have new technology available such as an ordinary cellphone. This is a great option to have for communicating with ATC when an airplane’s radios malfunction. It is easier to follow cellphone clearance instructions than to receive light gun signals.
That said, FAA rules and policies have not been updated for the use of cellphones as a communications backup. However, FAR 91.3—Responsibility and authority of the pilot in command—essentially overrides everything in case of an emergency. There are an increasing number of incidents where cellphones have been used, and I am not aware of pilots getting in trouble for doing so. This does mean that you should always travel with a charged cellphone and record the phone numbers for every control tower you might need.
One example of using a cellphone for lost communications occurred during an aircraft checkout in a G1000-equipped Diamond DA40. The pilot and flight instructor were about 30 miles from their home airport when they noticed an alternator failure. The flight instructor called the tower on the radio and reported that the airplane’s avionics were operating solely on battery power and that they did not know how long the battery would last. The controller asked if they had a cellphone on board and provided the tower’s phone number. They were to call on the radio 10 miles out, and if the radios were not operable, to use their cellphone.
So, always be prepared and have ATC phone numbers in your cell phone.