Magic eye tube

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A magic eye (tube or valve), is a thermionic vacuum electron tube that gives a visual indication for audio output, signal strength or other functions. It is also called cat's eye, or tuning eye tube. They were fitted on radio sets from around 1936 onwards, replacing the earlier "Tuneon" neon lamp type tuning indicators, as a novel selling feature. They are a miniature cathode ray tube, in effect.They usually lit up bright green, and the unlit segment on the display would narrow as the station tuned in, thus showing the "peak" signal strength. The purpose of magic eyes in these radio sets were to help tune a station in at its strongest point on the dial. The visual aid of the tube made the signal strength more measurable versus than by ear alone.

When, in the early 1950s FM radio sets were made available on the UK market, many different types of magic eye were made available, with differing displays, but they all worked the same way. Some eyes even had a separate small display to light up showing a multiplexstereo reception on FM.

Magic eyes were also used in reel to reel tape recorders,as their recording level meters.

Magic eye tubes were used in cheaper electronic gear in later years because they were cheaper than mechanical gauges like voltmeters.

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gives a more technical description[1] shows tubes in action[2]