668 



DISPLAY SYSTEM DESIGN PROBLEMS 



halftones. There are limitations to this device: (1) this type of cell has 

 energy gain only in the ultraviolet part of the spectrum, while our require- 

 ment for intensification is in the visible part of the spectrum; (2) the 

 response time for changes in the ultraviolet radiation is a few seconds. 

 This single layer type has been described here simply for its historical 

 importance. 



Much work has been done in a double-layer type of intensifier, where the 

 control of the electrical field across the electroluminescence is developed 

 across a photoconductor by incident visible light radiation. A double-layer 

 image intensifier^'' is illustrated in Fig. 12-43. A thin photoconductor, whose 



Photoconductor 



Opaque Layer 



Electroluminescent 

 / Phosphor 



Glass 



Object 



Transparent 

 Conducting Layer 



AC Voltage 



Emitted Light From 

 intensified Image 



Fig. 12-43 Double Layer Image Intensifier. 



impedance per unit area is very great compared with the impedance of the 

 electroluminescent layer per unit area, is placed electrically in series with 

 the light emitter material. An a-c voltage is placed across the sandwich. 

 If incident light is made to fall on the localized area of the photoconductor, 

 thereby changing its resistivity, then the increased a-c voltage (or increased 

 dielectric field) across the electroluminescent cell causes light emission. 

 The ability to obtain gain in this device is dependent on the change in 

 resistivity of the photoconductor and the light efficiency of the electro- 

 luminescent phosphor. 



The major limitations presently encountered in the development of 

 photoconductors are (1) inability to obtain uniform photoconductor layer 

 materials, (2) difficulty of proper impedance matching to the electro- 

 luminescent layer and (3) poor response time. Very few photoconductive 



14F. H. Nicoll and B. Kagan, J. Opt. Soc. Am. 45, 657 (1955). 



