Storage tube

Today they are obsolete, their functions provided by low-cost memory devices and liquid crystal displays.

Depending on the role, the beam of electrons emitted by the gun is steered around the display using magnetic (television) or electrostatic (oscilloscope) means.

In this case the localized image rapidly fades as energy leaving the display through secondary electrons is greater than the rate it is being supplied by the gun.

There were any number of varieties of mechanical layouts used to improve focus or cause the image to be refreshed either internally to the tube or through off board storage.

The easiest example to understand are the early computer memory systems as typified by the Williams tube.

These consisted of World War II surplus radar display CRTs connected to a computer.

To write a value to memory, the address was amplified and sent to the Y deflection plates, such that the beam would be fixed to a horizontal line on the screen.

A time base generator then set the X deflection plate to increasing voltages, causing the beam to be scanned across the selected line.

A list of points[1] stored in main memory was periodically read out to the display to refresh it before the image faded.

For practical use, graphical displays were developed that contained their own memory and an associated very simple computer which offloaded the refreshing task from the mainframe.

Commands that previously caused the terminal to erase its memory and thus clear the display could be emulated by scanning the entire screen at an energy above Vcr2.

In the more common category, they were only capable of storing "binary" images; any given point on the screen was either illuminated or dark.