12-9] SPECIAL DISPLAY DEVICES 657 



beam. A replica of this electric charge pattern is thereby produced as a 

 light pattern on the phosphor screen. Fig. 12-36 shows the arrangement. 



The storage tube has the capability of retaining information for periods 

 comparable to the radar frame time because of the charge control developed 

 by secondary emission action on the surface of the insulator material. The 

 problem of writing or placing a charge on the insulator surface has been 

 approached in different ways. One solution, which is used in most direct- 

 view tubes, charges a unit element of the insulator off the floating potential 

 equilibrium curve and then shifts the potential toward the equilibrium 

 point by a controlled amount of secondary emission action dependent on 

 the input signal. Equilibrium potential of the insulator is never reached 

 in this method of writing and therefore it is called nonequilibrium writing. 



The direct-view storage tube makes use of transmission modulation or 

 grid control reading. This is achieved by the established electrostatic fields 

 on the insulator surface. These electrostatic fields modulate a low-velocity 

 reading beam (flood beam) as it passes through the field. The insulator 

 target for this type of reading is usually made of fine wire mesh (250 mesh 

 to the inch) over which a thin layer of insulator material is placed. Erasing 

 of the charge information in the transmission modulation type storage 

 tube is mechanized by discharging the entire surface of the insulator by 

 purposely causing slow-velocity electrons to land on the insulator. This is 

 accomplished by pulsing the backing electrode of the insulator material 

 with a positive pulse such that the entire insulator surface, by capacitive 

 effect, is made positive with respect to the flood gun. With flood electron 

 landing, the potential of the cathode and the insulator is made the same. 

 When the positive pulse is removed, the negative-going waveform develops 

 a negative potential on the insulator with respect to the flood cathode to 

 prevent primary electrons from the flood gun from landing. 



The technical disadvantages of the available direct-view storage tubes 

 are limitations in resolution, shades of gray, and duration of viewable 

 storage without degradation. Resolutions of 30 photographic lines to the 

 inch at a brightness level of 1000 foot lamberts are obtainable today. The 

 resolution limitations are caused by the coplanar grid effect and by the 

 electron lens characteristic of the insulator mesh grid holes. The tangential 

 direction imparted to the low-velocity flood beam, the distance to the 

 phosphor screen and the potential of the phosphor screen become important 

 resolution considerations. Shades of gray, where each shade is defined in 

 3-db brightness steps, are limited by the background brightness of the tube. 

 The direct-view storage tube screen, unlike conventional cathode ray tubes, 

 has a uniformly illuminated brightness area. This illuminated area (which 

 is the flood-gun illuminated area) is the integrated brightness caused by the 

 flood-beam excitation of the phosphor when the erase pulses are present. 

 During the erase pulse, flood electrons are not biased and therefore can 



