An electron tube that is used to store information for a determined and controllable time and from which the information may be extracted as required. Various principles are used to operate storage tubes and there are many different types of tube. The most common types of tube in general use are charge-storage tubes in which the information is stored as a pattern of electrostatic charges. The information may be extracted as a visual display, as in the storage cathode-ray tube, or as an electronic signal.

Charge-storage tubes contain a target plate on which the information is stored. The information to be stored is used to modulate the intensity of an electron beam and the beam is made to scan the target. Secondary emission of electrons from the target plate occurs and an electrostatic charge pattern is left on the target. Each small area of the target that retains information and is distinguishable from neighbouring small areas forms a storage element. The number of secondary electrons emitted from each storage element is a function of the energy of the electron beam, the tube design, and the intensity of the beam. If the unmodulated beam produces a secondary-emission ratio greater than unity a positive-charge image is produced; if it is less than unity a negative-charge image results. The information-modulated electron beam is called the writing beam; the rate at which information can be written to successive elements is the writing speed.

Storage cathode-ray tubes produce a visual display of controllable duration. The tube has two electron guns – the writing gun and the flooding gun; it also has a phosphor viewing screen and two fine mesh metal screens. One of the metal screens, the storage screen, is coated with a thin dielectric material to form the target and the other serves as an electron collector. A positive charge image is produced on the storage screen by scanning with a high-resolution intensity-modulated writing beam from the writing gun. It remains until it decays or is erased. Information is extracted by flooding the storage screen with an electron beam from the flooding gun. Each storage element effectively forms an elemental electron gun with each mesh hole forming a control element of one of the guns. The value of positive charge deposited at each aperture determines the amount of current (from the flooding beam) that can pass through to the phosphor viewing screen. At the viewing screen a light output is produced that is a function of the original information. The stored charges are erased by flooding the storage surface with low-velocity electrons, thereby depositing negative charge on each element, until the pattern is erased and the surface prepared for storing a new image.

Photoconductive storage tubes depend for their operation on photoconductivity or the related electron-bombardment conductivity, in which the conductivity of a material is temporarily increased when exposed to bombardment by light photons or electrons. The target consists of a back-plate electrode coated with a thin layer of photoconductive material. The information is deposited on the target either by exposing it to a light image or by scanning with a high-resolution intensity-modulated light beam or electron beam. The information is extracted by scanning the target with an unmodulated reading beam; the numbers of electrons, i.e. the signal intensity, reaching the back-plate electrode depends on the conductivity of each small element. The output is produced as a current in a load resistor in series with the back-plate electrode.