RESEARCH ON VISIBILITY: INTENSITY-MODULATED SCOPES 



113 



by each operator every time he uses his scope 

 (55) ; this could feasibly be done with the aid 

 of an attenuated artificial signal which could 

 be built into the radar indicator. The sug- 

 gestion has not thus far been acted upon. 



A very interesting implication of the fore- 

 going analysis is that a signal which reduces 



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CRT BIAS IN VOLTS 



Fig. 3. Screen brightness and CRT bias 

 A brightness-voltage function for a typical 

 7BP7 tube, measured by a photomultiplier tube. 

 Actual voltage values vary with the tube and with 

 its age. The visual reference for this tube was 

 slightly less than 22 volts, optimal visibility was at 

 17 volts, or about 0.1 ml for |th second decay. 

 Reprinted from Williams and Bartlett (54). 



screen excitation should present a more visi- 

 ble pip than one which increases it, provided 

 the bias is optimal or less. Because at me- 

 dium and high excitation levels the phosphor 

 screen becomes less and less responsive to 

 new excitation (Fig. 3), the signal is, in effect, 

 "chpped" by the screen. Therefore, a given 

 signal voltage quantity will produce less incre- 

 ment than decrement of brightness; the re- 

 sult will be a greater visual contrast in the 

 decremental pip. To test this hypothesis, 



Harriman and Wilhams (22) reversed the 

 polarity of the signal lead-in wires to the 

 CRT: instead of putting the signal on the 

 control grid, they put it on the cathode. 

 The result was an intensity modulation in 

 the opposite direction: the pip appeared 

 darker, rather than hghter than the back- 

 ground. Threshold measurements made 

 with the two types of pip are graphically 

 shown in Fig. 4. Two things are to be 

 noted: (a) with brighter scopes, the ultimate 

 visibiUty of the "dark" pip is better than for 

 the "bright" pip, and (b) the optimal bias is 

 lower (i.e., screen brightness higher) for the 

 "dark" pip, at least for small pips. The 

 latter means that the screen can be operated 

 at a brightness of about 0.5 ft.-L for optimal 

 detection rather than at the 0.1 ft-.L neces- 

 sary for the bright pip. Very likely, some 

 part of the increased visibihty of the dark pip 

 is due to the higher adaptation level of the 

 eye. 



It is to be remembered, of course, that the 

 bias which is optimal for small, weak signals 

 may not be so for other operational purposes. 

 For example, spot size increases in some un- 

 predictable manner with beam current; in 

 fact, at very high beam currents it may in- 

 crease very rapidly and even exhibit "bloom- 

 ing," a pecuhar phenomenon of light spread- 

 ing which apparently is not well understood 

 (2). Whether, short of blooming, the in- 

 creased spot size at moderately higher cur- 

 rents (brighter scopes) is sufficient to impair 

 resolving power, is not yet known. 



Pip Size 



The size of the perceived pip is a crucial 

 variable in its visibility. Radar pips may be 

 as short in duration as | microsecond or as 

 long as several microseconds; pulse duration 

 determines the radial dimension on a PPL 

 Pips may be as narrow in beamwidth as one 

 or two degrees or as wide as 30 or 40 degrees, 

 depending on the lobe pattern of the antenna 

 transmission. The effective beamwidth of a 

 pip from a single target may vary consider- 

 ably as the distance from the antenna, be- 



