766 BELL SYSTEM TECHNICAL JOURNAL 



Two general types of phosphors have been commonly employed in military 

 radar systems classified according to their phosphorescence characteristics. 

 The medium persistence class of phosphors exhibit decay times of the order 

 of milliseconds and are composed of a single layer of Willemite (zinc ortho- 

 silicate). This type of screen exhibits a green luminous response and is 

 employed extensively in reproducing high-speed wave forms such as encoun- 

 tered in high-speed scanning systems, and for general A-type presentation 

 purposes. The visible hght output decays to the order of 1% of its initial 

 value in approximately 50 milliseconds after electron excitation ceases. 

 For photographic purposes other phosphor compositions of the same per- 

 sistence class whose useful light output has a higher actinic value are com- 

 monly employed. 



The long-persistence phosphors are composed of two layers of screen mate- 

 rial which combination exhibits sustained phosphorescence, the visible light 

 output decaying very slowly after cessation of bombardment. The double 

 layer or cascade screen consists of an innermost layer subject to the direct 

 influence of the electron stream which is composed of a silver activated zinc 

 sulphide and a second layer adjacent to the glass envelope which consists of 

 copper activated zinc cadmium sulphide. The first-named material 

 fluoresces with an extremely brilliant blue light under bombardment and 

 exhibits a rather rapid decay characteristic. The second layer is in turn 

 excited by the blue radiation from the first layer and responds with a yellow 

 visible emission which persists for a matter of several seconds after excita- 

 tion ceases. The initial blue flash is appreciably absorbed by the second 

 phosphor layer, but usually further optical attenuation is required to prevent 

 eyestrain and degradation of night vision of the observer. This is commonly 

 provided by the use of an amber optical filter placed over the screen face. 

 The long-persistence characteristics available in this type of tube have 

 proven invaluable in military radar systems which feature slow antenna 

 scanning. In many of these systems the time between successive scans of 

 the target may be of the order of a second or more and only through the use 

 of the cascade-type long-persistence tube can the image be retained for this 

 period of nonexcitation. Another property of the long persistence class of 

 cathode-ray tube screens which is of advantage for radar purposes is an 

 accumulative increase in brightness with successive scans of the target. 

 Since the target image is usually repetitive as regards position on the screen, 

 the image brightness will increase with successive scans while because of the 

 random character of noise no such increase in noise image will result, and a 

 small but evident signal-to-noise improvement obtains. The long-persist- 

 ence type of screen characteristic is employed in the majority of military 

 radar indicators of the B, C, and PPI-types. 



Another general characteristic of the cathode-ray tube screen which influ- 



