12-6] CHARACTERISTICS OF ELECTRICAL-TO-LIGHT TRANSDUCERS 627 



are excited by light. To obtain both flash highlight intensity and long 

 persistence, some phosphors such as the P7 are composed of two materials 

 in cascade, i.e., two layers. The layer nearer the electron gun (ZnS:Ag) 

 fluoresces with a high, blue light intensity. This light in turn excites the 

 longer-persistence yellow phosphor, thereby giving long persistence. If the 

 yellow layer (ZnCdS:Cu) were bombarded by the electron beam directly, 

 it would give a very bright flash but would have shorter persistence. 



12-6 IMPORTANT CHARACTERISTICS OF ELECTRICAL- 

 TO-LIGHT TRANSDUCERS 



The most important characteristics of a display device which aff"ect the 

 radar system design are: 



(a) Resolution and display size 



{/?) Indicator and background brightness 



(f) Signal integration properties of the tube 



(d) Tube size and form factor 



(e) Susceptibility to physical environment 



Resolution and Display Size. Previous chapters have indicated the 

 high price paid for range and angle resolution in terms of system bandwidth 

 and antenna aperture size. It is often important, therefore, that the 

 resolution of the display device be such that it is not the limiting factor in 

 determining overall system resolution. Several simple examples will 

 illustrate the possible nature of the resolution problem. 



Example 1. Assume a side-looking ground-mapping system designed to 

 provide a high resolution map of a strip twenty miles wide on each side of 

 the aircraft (see Paragraphs 1-4 and 14-3 for more details concerning this 

 type of radar system). Assume the beamwidth and pulselength of this 

 radar are such as to produce an average resolution element of 100 ft on a 

 side. The instantaneous output of each side of such a radar system may be 

 presented as an intensity-modulated line on the face of a cathode ray tube 

 as shown in Fig. 12-11. The line may be photographed on a moving film 

 strip which is synchronized with aircraft motion to produce a continuous 

 map. 



From the data given, it can be seen that the line will contain (20) (6020) / 

 100 = 1200 resolvable range elements. If the line is 6 inches long, there are 

 200 resolvable range elements per inch, provided the indicator does not 

 limit resolution. To achieve this latter condition, the indicator must have 

 400 resolvable elements per inch,^ i.e. a spot size of 0.0025 inch or 2.5 mils. 



^The display may be viewed as a sampling device. In accordance with the sampling theorem 

 outlined in Paragraph 3-3, the effective sampling frequency must be twice the input band- 

 width. Hence, the size of each sampling element must be half the size of the input elements. 



