1. A small cathode-ray tube having a fluorescent screen on which the pattern varies with the voltage applied to the grid that is used chiefly in radio receivers to indicate accuracy of tuning.
2. A special-purpose electron tube in which electrons are accelerated by high-voltage anodes, formed into a beam by focusing electrodes, and projected toward a phosphorescent screen that forms one face of the tube.

To form a display, or image, on the screen, the electron beam is deflected in the vertical and horizontal directions either by the electrostatic effect of electrodes within the tube or by magnetic fields produced by coils located around the neck of the tube.

3. A type of recording-level indicator using a luminous display in a special tube.

The display is usually like an "eye" with a keyhole in the middle, and the maximum recording level corresponds to the closing-up of a slot at the bottom of the keyhole which is superseded by meters in current-model recorders.