12-9] SPECIAL DISPLAY DEVICES 66S 



Power loss in the inductors and the grid capacitance requires an oscillator 

 with a power output of approximately 20 watts. Since each color phosphor 

 has a different efficiency, it is necessary to compensate the video informa- 

 tion of each color to obtain good hues. Actually, it is easier to accomplish 

 this on the single gun chromatron tube by electronic bias control than on 

 the three-gun shadow mask tube by compensating the three transfer 

 characteristics. 



Two-Color Tubes. In many radar displays only two colors are required. 

 For example a two-color presentation might be used to show moving targets 

 among stationary targets. Work in the transparent phosphors, the Aiken 

 deflection system, and electroluminescence indicates that satisfactory 

 progress is being made to realize dual color presentations. With respect to 

 transparent phosphor development, it has been demonstrated that two 

 phosphors can be placed in tandem. One beam excites the first phosphor 

 while a second beam (of much higher energy) penetrates the first phosphor 

 and excites the second phosphor. As can be expected, the color purity is 

 not as good as it is in the tubes discussed above. 



Electroluminescence. The control circuitry of this solid-state display 

 is still in the laboratory stage. However, the potentials of this display 

 medium, which offers the multiple advantages of good form factor, storage, 

 no vacuum, no high voltage, high light output, and color capability, are 

 discussed to point out some of the goals of indicator improvement in the 

 future. 



Light is generated in electroluminescent cells in basically the same way 

 as it is generated by cathode luminescent methods. An a-c field is applied 

 across a photoluminescent phosphor to change the light level of the screen. 

 The alternating electrical field, although high — of the order of 10^ — 10* 

 volts /cm — is not enough to excite the phosphor. There are, however, 

 regions of higher electrical stress inside the phosphor particles. These local 

 fields may be as high as 10^ volts /cm. In these areas of high local fields 

 there are present electron-trapping centers as well as luminescent centers. 

 (These electron-trapping centers lie below the conductive band by only a 

 small amount of energy.) When the intensity of the electric field reaches 

 a high point, electrons in these traps are freed and enter the conductive 

 band. These electrons are accelerated by the high electric fields and thereby 

 attain considerable energy above the bottom of the conductive band. Some 

 of these accelerated electrons collide with filled luminescence centers, 

 liberating electrons in the center. Eventually the avalanche of free electrons 

 will reach regions of lower field strength and will be terminated. When the 

 applied field is reversed (a-c voltage) the electrons will return to the region 

 containing empty luminescence centers and recombine with them and 

 thereby emit light. 



