666 DISPLAY SYSTEM DESIGN PROBLEMS 



The best known materials which exhibit these electroluminescent 

 quantities are ZnS and ZnSe. Other materials which are being investigated 

 are Zn2Si04, BaTiOg, and some organic compounds. 



The electroluminescent "cells" are made by one of three methods: 



1. Suspending the powder in a plastic layer which is sandwiched between 

 two conductive layers, the first layer being a glass coated with tin oxide 

 (NESA) and the second layer vaporized aluminum. The a-c voltage is 

 applied between the conductive layers. 



2. Embedding the phosphor in a coating of low-melting-point glass or 

 enamel, which is then fired onto a metal or glass base. A second electrode 

 of transparent conductive film completes the sandwich. This type of 

 construction develops a very rugged "cell." 



3. Suspending the phosphor in a plastic medium and sandwiching this 

 between a conducting glass cloth or metal mesh and (second electrode) 

 vaporized aluminum to develop flexible electroluminescent cells. 



The electrical properties (of the electroluminescent "cells") of time 

 constancy, brightness, and frequency characteristics must be considered. 

 If the phosphor has been completely de-excited (by heat, for example) and 

 an electrical field is placed across it, then it requires approximately 5 cycles 

 to reach 50 per cent of the light output. Although the actual time of 

 excitation can be decreased by increasing the frequency of the alternating 

 field, the number of cycles cannot be decreased. On the contrary, the higher 

 the frequency, the greater the number of cycles that are required to reach 

 the required light level. If the phosphor has not been de-excited (has some 

 of its traps filled from previous electroluminescent excitation) the build up 

 to 50 per cent of maximum output would require one or two cycles. 



Electroluminescent phosphors show rapid decay after excitation. A 

 typical decay of 0.5 time constant is 0.5 msec for ZnS:Mn. This figure 

 represents an exponential decay reaching a 10 per cent output level. 

 Temperature does not noticeably affect either the time constant of excita- 

 tion or decay. 



Electroluminescent brightness increases nonlinearly and is dependent on 

 the applied field. It may be expressed as follows: 



L = ^:/e--(EoiE)^r. ^j2-6) 



where L = brightness, E = field strength, and ^ and Eg are constants 

 characteristic of a particular material at a particular frequency. It is 

 typical of electroluminescent phosphors that the brightness increases 

 linearly with frequency. However, the voltage drop incident to the 

 impedance of the cell electrodes begins to limit the output at high fre- 

 quencies. 



The cells can be operated at any voltage by varying the thickness of the 

 phosphor, since it is the electric field intensity which is of paramount 



