12-9] 



SPECIAL DISPLAY DEVICES 



669 



materials have the proper impedance necessary to make a good electro- 

 luminescent control device. Single crystal CdS has the proper impedance, 

 but techniques for producing large single crystals have not been developed. 

 Vapor deposition of CdS has met with limited success because of the 

 difficulty of obtaining a thick enough layer to get a proper impedance 

 matching. 



Consideration has also been given to utilizing the combination of 

 photoconductor and electroluminescent phosphor as direct display devices. 

 Although an adequate resolution of about 50 lines per inch is obtainable 

 and will probably be further improved, the limitation of rise and fall-off 

 is the greatest deterrent to developing this control element into a useful 

 radar display device. If a powder with a short time constant is developed, 

 another limitation is encountered. The short persistence would not allow 

 enough light storage at one unit area (assuming a display with moving 

 information which is made up of many bits) as the radiation source is 

 sequentially moved to write a pattern. Since human intelligence requires 

 at least a 1 /30-second frame time, the integrated light level from any 

 particular unit area is extremely small and becomes smaller as the bit rate 

 is increased. What is required, as in all indicators, is methods of (1) placing 

 information on a given unit area in as fast a time as is demanded by the 

 system, (2) retaining this information for a full frame rate until a second bit 

 of information needs to be placed on the same unit area, (3) erasing the first 

 bit information at a fast rate, and (4) writing the new information. 



Development of such a solid-state device by E. A. Sack^^ contemplates 

 using ferroelectric capacitors in a matrix array to control the electro- 

 luminescent phosphor. A simple schematic of one element of this array 

 appears in Fig. 12-44. The ferroelectric and electroluminescent capacitors 



Light 

 Power 

 Supply 



Electroluminescent 

 Capacitor Ytx 



Ferroelectric 

 Capacitor 



— o o — ^A^^^^■ 



Control Signal 



Input 



" ~| Control Signal 

 "j" Supply 



Fig. 12-44 Schematic of ELF Screen. 



are connected in series with a source of a-c power. The voltage drop across 

 the electroluminescent phosphor is controlled by the impedance of the 

 ferroelectric capacitor. As shown in Fig. 12-45, a ferroelectric material such 

 as barium strontium titanate has the property of changing its capacitance 

 value as a function of change of potential across the dielectric. The 

 impedance change which is caused by the d-c potential bias and input signal 



15E. A. Sack, "ELF — A New Electroluminescent Display," Proc. IRE 46, 1694-99 (1958). 



