Tuning Indicators (Magic Eye)

The magic eye or tuning indicator (also called magic eye or cat's eye or tuning eye in North America), in the technical literature called "electronic ray indicator tube", is a vacuum tube that provides a visual indication ( side or on the dome of the tube itself) of the amplitude of an electronic signal, such as an audio output, the strength of the radio frequency signal or other functions. Among its main uses on radio receivers it showed the accuracy of the tuning and in tape recorders the recording level. Essentially composed of an amplifier triode and a small cathode ray tube which are projected onto a surface covered with phosphors, visible from the outside.