12-6] CHARACTERISTICS OF ELECTRICAL-TO-LIGHT TRANSDUCERS 633 



determine the number of pulses necessary to build up to a required bright- 

 ness. The growth of fluorescence is dependent on the rise of brightness 

 with each pulse (representing the radar received pulse), dwell time, the 

 exponential decay of brightness during the beam off time, and the PRF. 

 As is seen from Fig. 12-13 a point is reached where additional pulses do not 

 add further to the brightness (or integration) of the CRT. The number 

 of pulses required to reach this equilibrium condition is dependent on the 

 above-mentioned factors as well as on the type of phosphor. Increase in 

 PRF decreases the number of pulses required to reach equilibrium. 



The visual bias level setting on the indicator is dependent on the average 

 noise power which excites the phosphor, on the ambient light level, on the 

 characteristics of a particular phosphor, and on the discernibility of the 

 human operator. These characteristics are all interrelated. 



Bandwidth. The bandwidth of an intensity-modulated display is 

 proportional to the writing speed and inversely proportional to the spot 

 size. 



writing speed (inches /sec) 



Bandwidth (in cycles) = 



5pot size (inches) 



Writing speed is dependent on the system-required time base sweep speed 

 and the tube size. Further if the display pictures the azimuth trace of a 

 radar antenna (such as in a 'B' or 'C scope) then the bandwidth is also 

 dependent on the speed of antenna travel. Typical numbers would be the 

 display of 20 n.mi. (248 Msec) along a 3.5-inch vertical trace and the display 

 of antenna motion of 100° /sec for 2 seconds along a 3.5-inch horizontal 

 trace. Spot size, as previously discussed, is dependent on beam current and 

 the characteristics of the phosphor. Spot size can vary from 1 to 2 mils at 

 extremely low light levels to approximately 30 mils at very high light levels. 

 It must be mentioned here, however, that some reward is obtained by 

 impression of higher-frequency signal components than the indicator 

 bandwidth. The high light-flash activation of the phosphor incident to the 

 fast rising leading edge of the impressed video pulse accentuates the 

 presence of a signal. 



Tube Size and Form Factor. It is necessary in many radar systems 

 to limit the size of the indicator so as to fit into a confined area of the 

 aircraft. This problem is further complicated by the frequent necessity for 

 installing the indicator in the most desirable location in the aircraft — 

 directly in front of the pilot, for example, in an AI radar. Unfortunately, 

 the form factor of cathode ray tubes is poor. The electron optics needed 

 in the tubes we have been discussing require a large length for a given front 

 face diameter of the tube. The bandwidth and multiple information 

 requirement of most airborne indicators dictate the use of at least a 5-inch- 



