118 



RADAR SCOPES 



sweep line itself is barely visible (VRI or 

 Visual Reference Intensity), but has a 

 marked effect on a bright scope. Although 

 the loss at the brighter levels can be attrib- 

 uted to contour mutilation, the size of the 

 effect is magnified somewhat by the db unit, 

 whose light-output equivalent varies with. 

 grid bias. The explanation of the major 

 effect no doubt Hes in the eye; it integrates 

 total light in an area (A X I = C) at low 

 luminances, but requires brightness contrast 

 more than area at high luminances and re- 

 quires it in adjacent retinal regions (contour 

 or border effect). (See Lamar, Hecht, 

 Shlaer, and Hendley, 27.) The defocussed 

 pip on a bright background evidently does 

 not supply the needed brightness contrast. 

 More detailed and analytic study of this 

 matter is desperately needed. 



Orientation Factors 



Range of Target 



With ordinary-sized pips and normal oper- 

 ating conditions there appears to be no 

 marked effect of range position on visibility, 

 as stated previously. This presumably is 

 due to an approximate compensation of 

 brightness and area. Unfortunately, no di- 

 rect physical measure of luminosity of the 

 pip at various ranges has ever been made. 

 This needs to be done. From visibiUty 

 studies we can conclude that the middle 

 ranges are about equally favorable for pip 

 detection and that there may or may not 

 be a fall-off in visibiUty near the center or 

 near the periphery depending on size and 

 brightness of the pip and the condition of 

 the scope. 



Bearing Position 



In looking at a scope of the PPI type, will 

 it make any difference whether the pip ap- 

 pears at north, east, south, or west? The 

 answer is, apparently not, except under cer- 

 tain pecuHar circumstances. In the course 

 of thousands of observations of pips on PPI's 

 during experiments at Johns Hopkins, no 

 effect of bearing position has ever been 

 noted, with one exception reported by 



Garner and Hamburger (15). This special 

 case appears to be due to a glare factor 

 uniquely associated with the large projec- 

 tion PPI. 



Sweep Direction 



An unpublished study by the writer 

 showed no significant difference between 

 clockwise and counter-clockwise direction of 

 movement of the sweep line. 



Scope Position 



Similarly, unpublished data of the writer's 

 showed no effect of scope position provided 

 the observer's normal hne of sight was main- 

 tained. 



Angle of Viewing Scope 



Tilting the whole scope is quite another 

 matter. Considering scope curvature, it is 

 by no means obvious how much tilt would 

 be required to impair visibility. Therefore, 

 another experiment was undertaken by the 

 writer in which visibiUty thresholds were 

 tested with the scope tilted at 90, 80, 70, 

 60, 45, and 30°, the observer's head remain- 

 ing fixed. The threshold for a single pip 

 was unimpaired from 90° through 60° of 

 viewing angle. At 45° there was a drop-off 

 of 3 to 4 db and a further drop-off of about 

 3 db at 30°. No thresholds were taken at 

 angles less than 30° because it was evident 

 that the observer would soon be unable to 

 see anything. Incidentally, the same trend 

 was shown both for conditions of darkness 

 and 'with a very slight room illumination 

 (about 0.1 ft.-c) insufficient to produce glare 

 on the scope face. Unquestionably, view- 

 ing angle would become a more critical fac- 

 tor in the presence of glare. (For these 

 tests an uncovered 7BP7 cathode-ray tube 

 was used; the observers looked directly at 

 the face of the tube without intervening 

 cover glasses or filters.) 



Decay Time {Signal Age) 



Sweet and Bartlett (48) measured visi- 

 biUty thresholds for a single pip at several 

 periods in its decay history. They did this 



