Antique radio "Tuning eye"

This is a very short (14 second) video showing the operation of a tuning indicator on an antique radio. They are often called tuning eye for the resemblance.

The video is very dark because otherwise the brightness of the indicator washes itself out and you can't see it.
This is on the 1939 Zenith 7S363 tuning on the shortwave band.
The tuning eye is not a light bulb or LED.
Electrons strike a surface coated with phosphorous causing it to glow, similar to how the face of a TV picture tube glows.
These eyes come in many different shapes and colors.
This round green is the most popular of the period.
It is not neon technology but I consider the color and luminescence very close to a green neon light.
Radio manufacturers used these eye tubes because mechanical meters were very expensive at the time.
How did it work?
Radios have an automatic volume control (AVC).
It automatically turns the radio up to full sensitivity on weak stations, and turns the sensitivity down on strong stations to keep it from blasting and distorting.
The tuning eye reacts to the level of AVC.
The stronger the station, the more AVC is applied which closes the tuning eye.
More technical:
A constant high voltage is attracted to the circular plate that is coated with the phosphorous, causing the whole round plate to glow.
A wire is present inside the tube that causes a shadow on the bottom of the round display. It's the shadow you see on the bottom of the display.
The stronger the signal is, the more negative that shadow wire becomes, making the shadow smaller.