658 DISPLAY SYSTEM DESIGN PROBLEMS 



traverse the distance to the phosphor screen. Fortunately, the duty cycle 

 of erase to store can be kept small. The number of gray shades has been 

 measured, for a particular background brightness, as four or five. This 

 implies a maximum brightness contrast ratio of 32 to 1. 



For definition purposes the duration of storage and the duration oj viewable 

 storage must be considered separately. The duration of storage is the 

 retention time of information on the insulator material if writing and 

 reading are separated. With presently used materials this storage duration 

 is many hours, if not days. The duration of viewable storage is the retention 

 of the insulator information for continuous display. This implies that the 

 flood beam is continuously illuminating the phosphor. In this sense the 

 storage time in present tubes is not much longer than 10 seconds. The 

 reason for this short retention time is that gas molecules are present in the 

 imperfect vacuum. Collision between the flood electrons and these gas 

 molecules cause positive ion landing on the insulator surface so that stored 

 information is obliterated. With this type of tube, ion bombardment is 

 expecially severe, since the insulator is the most negative element, being 

 sandwiched between a positive collector and a positive phosphor, and is 

 therefore most susceptible to positive ion bombardment. Viewable storage 

 time can be increased at the expense of brightness by pulsing the flood beam 

 to lower its duty rate and thereby cause fewer collisions with gas molecules. 



Flat Tubes. One of the biggest limitations with all the tubes we have 

 discussed is the form factor of the tube. To obtain a usable deflection 

 sensitivity the tubes must be long. Electrostatic deflecting tubes, in almost 

 all cases, are longer than magnetic deflecting tubes. Tubes using the latter 

 deflecting method have been built with deflection angles up to 110 deg. 

 Major development eff'ort is being conducted to further reduce the length 

 of the tubes. Aiken^" has placed the electron gun and accelerating and 

 deflection elements in an arrangement other than the in-line method used 

 in the tubes discussed above. As Fig. 12-37 shows, the electron beam is 

 made to move along an edge of the tube. A set of deflection plates arranged 

 along this edge deflects the beam if the voltage is lowered on any of the 

 plates. In the absence of an electrical field the beam would continue in its 

 initial path. By placing the proper sequence of the voltages on the plates, 

 the beam can be made to move up to any point on the tube. A second series 

 of deflection plates which extends across the tube deflects the beam to the 

 phosphor front face. By choosing the proper time sequence on the two sets 

 of deflection plates, the electron beam can be made to strike any point on 

 the phosphor front face. 



Focusing is simplified in this tube. The electron beam which initially is 

 circular becomes elliptical as the beam is deflected up from its normal travel 



'OW. R. Aiken, "A Thin Cathode Ray Tube," Proc. IREA5, No. 12 (December 1957). 



