104 



RADAR SCOPES 



be watched. Anj^ of these can affect visi- 

 bihty and wide variations may exist among 

 screens of identical code name. There is, 

 therefore, no sure way of diipHcating most 

 screens for exacting research purposes. For 

 this reason, no complete listing of common 

 phosphors will be here attempted. Lever- 

 enz (29) has a compendium of the properties 

 of 23 high efficiency and 22 low efficiency 

 phosphors which have been studied in the 

 laboratory and, in other articles (30, 31), pre- 

 sents the properties of the commonly manu- 

 factured screens. Of these, only about five 

 are much used in radar work. They are 

 divided into two major classes with respect 

 to persistence of afterglow. 



The short persistence class has one out- 

 standing member: 



PI. Used on most laboratory oscilloscopes, 

 in some rapid-scan radar. Usually used in 

 A-scan. Light green; decays to 1 per cent in 

 about 50 milliseconds. Zinc-silicate activated 

 with manganese (ZnSi04:Mn). Very stable, 

 often used as a reference phosphor for relative 

 energy and efficiency. Narrow emission band 

 with well-defined peak at 523 m^- 



The long persistence class are called 

 ''memory" screens; they not only preserve 

 strong echoes for many seconds but help to 

 integrate weak, recurring signals (see Sutro, 

 46, and Brill, 8). Examples are: 



P7. The most widely used long persistence 

 phosphor today, especially on PPI's. Consists 

 of two phosphor layers, hence a "cascade" 

 screen. The first layer (ZnS:Ag) fluoresces 

 blue on electronic excitation with a peak emis- 

 sion at about 435 mju- The bright blue itself 

 decaj^s in a few microseconds, but its light 

 energy excites the second layer (ZnCdS:Cu) to 

 a whitish yellow with a peak emission at about 

 570 mpi. Very variable. (See Nottingham, 41.) 



P14 Similar to P7 except that second layer 

 (ZnCdS:Ag) glows orange rather than yellow, 

 with peak emission at about 600 m/n (See 

 Innes, 25.) 



PIO. Sometimes called a scotophor (KCl) 

 rather than a phosphor, because it darkens on 

 excitation rather than brightens. Extremely 

 long-persistent; requires heat for erasing. 



In addition, a medium persistence screen, 

 the P12, may be mentioned. It is a single 

 layer screen (ZnF2i]Mn) Avhich glows orange, 

 has a peak emission at about 589 m/x, and is 

 often used in fire-control radar. 



Methods of Research on Visibility 



Equipment 



Either real or simulated radar scopes have 

 been used. An entire real radar system has 

 only the advantage of realism ; usually it is a 

 poor research device. 



Real Radar 



Naval fleet units have made comparisons 

 between complete radar systems installed in 

 adjacent ships. A clear superiority of one 

 system over another can be established by 

 such means, though the reasons for it often 

 have to be sought in the laboratory. Or, 

 statistical selection of radar contacts can be 

 drawn from operational records and often it 

 is possible to make useful comparisons of 

 systems, operators, or scopes. The method 

 is laborious and expensive. 



Artificial Radar 



Still retaining visual realism, several 

 groups of scientists, (notably those at the 

 Radiation Laboratory, Massachusetts Insti- 

 tute of Technology; Naval Research Labora- 

 tory; Division of Radiophysics of the Aus- 

 tralian Council for Scientific and Industrial 

 Research; and Systems Research Labora- 

 tory of the Johns Hopkins L^niversity) have 

 employed an artificial radar set with simu- 

 lated signals but real scopes with radar scans. 

 This method is costly, but signal generators, 

 oscillators, and meters cost less than ships. 

 Even so it cannot be undertaken by a small 

 laboratory. In spite of the expense, the 

 method is probably the best compromise be- 

 tween scientific control and realism. 



CR tubes but No Simulation of Radar Scan 



Another method makes use of CR tubes 

 but makes no attempt to simulate scarming 

 or real targets. The signal is a ''raster," a 



