RESEARCH ON VISIBILITY: INTENSITY-MODULATED SCOPES 



117 



experimental comparison of scope sizes under 

 these conditions, with every other feature 

 of the CRT held constant, has yet been 

 reported. 



Shape and Contour Variables 

 Pulse Shape 



The pulse transmitted and received ordi- 

 narily is rectangular. Deviations from rec- 

 tilinearity can be expected if the receiver 

 band pass is too narrow; this results in re- 

 duction of pulse energy and lower visibility. 

 Even with energy equated, two pips differ- 

 ing in temporal distribution of energy may be 



ment has not yet been perfected. The ex- 

 periments are needed so that we may know 

 how to distribute a given quantity of signal 

 energy most efficiently. The sharp, rectan- 

 gular pip pictured in Fig. 1 is definitely not 

 representative of operationally encountered 

 pips. 



Focus 



In lieu of precise information on the varia- 

 tion of energy in the separate dimensions of 

 the pip, the effect of defocussing or blurring 

 it may be of interest. Focus is established 

 by a special focus coil, whose voltage can 



TABLE I 

 Focus, Bias, and Visibility 

 Columns to the right show data for different bias voltages. These vary from high to low in order 

 to effect scope brightnesses from dim to bright. Optimum detecting bias is between — 4v. and — 6v. 



"Best focus" means the sharpest, least blurred sweep line as judged by direct visual inspection, no 

 pip being present. Whether to focus on sweep line or on pip makes little difference in visibility, as 

 shown by line c. On the other hand, extreme de-focussing does have a serious effect, as shown in line 

 /, which varies with scope brightness. 



-8v. 

 (Bright) 



(a) "Best focus" voltage set by operator 



(b) Focus voltage giving best visibility of pip 



(c) Visibility advantage of (b) over (a) 



(d) Visibility score at (b) in db signal voltage 



(e) Visibility score with extreme defocussing 



(f) Visibility loss due to defocussing (d) minus (e) 



215 



215 



none 

 42 



24.5 

 17.5 



unequally visible because of different contour 

 gradients. That is, a pip which has no 

 sharply defined contours in the radial dimen- 

 sion would probably be much less visible 

 than one with sharp borders. This prob- 

 lem has not yet been investigated on scopes, 

 but needs to be. 



Azimuth Gating of Pip 



The angular or beamwidth dimension of 

 the ordinary radar pip is of irregular bright- 

 ness; usually the pip is brighter in the center 

 than at the leading or trailing edges, thus 

 producing a roughly triangular rather than 

 rectangular distribution. Plans have been 

 made by the writer to investigate the effects 

 of azimuth gating methods, but the equip- 



be varied over a wide range — in a typical 

 7BP7 tube, from 168 to 322 volts. The 

 best focus is between these extremes. In 

 an unpublished study by the writer, a trained 

 operator set the "best focus" of the sweep 

 line at each of several bias levels. The 

 threshold intensity was then determined for 

 a typical pip (| microsecond x 1°) at the 

 ''best focus" and at each of six other focus 

 coil voltages (three greater and three less) 

 from 168 to 322 volts. The results are 

 shown in Table I. 



In defocussing, virtually no energy is lost; 

 it is simply spread out more, so that the 

 pip becomes very fuzzy. Very interestingly, 

 this profound change appears to have no 

 significant effect on a dim scope, where the 



