Link z obrazkiem poniżej i tekstem: The tuning indicator tube, or "Magic Eye" as it is was introduced by RCA, was used to display the optimum signal tuning point of a tuner/receiver, or the peak signal level of a preamplifier or tape recorder. The main benefit in tuning reception was for FM tuners where the exact tuning point was harder to determine than for AM. They were cool looking then and still are.