RESEARCH ON VISIBILITY: INTENSITY-MODULATED SCOPES 



111 



Scott chose four basic parameters: (a) the 

 product of pulse length and i-f bandwidth, 

 (b) the ratio of pulse length times sweep time 

 of the CRT to spot diameter, (c) the ratio of 

 antenna beamwidth to the angle subtended 

 at the eye by the beamwidth, and (d) the 

 ratio of PRF to critical PRF. With these 

 parameters she was able to arrive at an 

 equation which predicted fairly well the data 

 taken in her own experiments, but failed in 

 the case of large beamwidths. It also fails 

 to include a factor for screen brightness, a 

 critical variable, and of course the validity 

 of extending the theory beyond present evi- 

 dence is not known. The equation itself is 

 not reproduced here, because of limitations 

 of space, but the interested reader will be 

 well repaid by examining it. There is no 

 doubt that it represents the best approxima- 

 tion of an integration of radar visibility data 

 that has yet been attempted. 



In spite of the integrative value of the 

 equation, it of necessity includes constants 

 for the individual CRT and the individual 

 observer. Of course, it does not even at- 

 tempt to handle the supra-threshold bright- 

 nesses, multiple signals, or discriminabihty. 

 Whether the equation is applicable to other 

 phosphors and other tubes is not yet known. 



Other Factors 



The electrical parameters are only part of 

 the story, although possibly the largest part. 

 In the remaining pages a variety of factors 

 influencing visibility will be discussed, even 

 including some of the electrical parameters. 

 The reason for the overlapping treatment de- 

 rives from a psychological rather than an 

 engineering orientation. Different electrical 

 combinations often produce equivalent 

 visual effects. The discussion follows a clas- 

 sification of what is seen on the scope rather 

 than of what circuits are required for visibile 

 operation. Payne-Scott's analysis is char- 

 acteristically a radar-engineering approach; 

 the following is a human-engineering one. 

 Most of it is based on recent research by the 



writer and his colleagues at the Johns Hop- 

 kins laboratory. 



Screen Brightness 



Screen brightness is mainly controlled by 

 the bias voltage on the control grid of the 

 CRT; a secondary control is the gain of the 

 video amplifier which amplifies noise as well 

 as signal. There is a very definite optimal 

 bias in a given tube, as is shown in Fig. 2. 

 The legend to this figure explains the visi- 

 bility measure used in this and subsequent 

 graphs. The optimal bias in a typical 7BP7 

 tube was found to be about 5-6 volts less 

 than cut-off, which corresponds to a bright- 

 ness of the screen of about 0.1 foot-lambert in 

 in the region adjacent to the pip. This bias 

 is apparently less than that reported by 

 Nottingham (40) ; the difference may be due 

 to different experimental methods. At any 

 rate, for objects of the size of the radar pip, 

 the background brightness needs to be 

 greater than this if visual sensitivity is to be 

 maximal. (See Blackwell's data, 6.) The 

 level for maximal sensitivity varies with size 

 of detail to be seen, but in general should be 

 from 1 to 10 ft.-L on the scope. These levels 

 are attainable, but only by reducing the re- 

 sponsiveness of the phosphor to incremental 

 excitation, due to saturation effects. The 

 brightness-voltage function is not linear at 

 lower biases, i.e., brighter screens (cf. Fig. 3). 

 The P7 phosphor does not, therefore, permit 

 maximal use of visual sensitivity. Whether 

 metal-backed screens (12) are better in this 

 respect is not kno^Ti; they have never been 

 visually tested. The PIO phosphor also re- 

 veals an optimal bias (15). 



Screen brightness is thus critical; and bias 

 is perhaps the most important of all electrical 

 parameters. Optimal bias is in turn depend- 

 ent on video gain and on noise, for these can 

 directly affect screen brightness. PRF and 

 rotation rate of the antenna can also affect 

 screen brightness. The effect of any circuit 

 parameter ought first to be evaluated in re- 

 lationship to brightness. If, for example, 



