EESEARCH ON VISIBILITY: INTENSITY-MODULATED SCOPES 



125 



him to look at one side of the scope and then 

 "sneaking in" a pip at the opposite side of 

 the scope, he might not notice the presence 

 of the pip until it was as much as 6 or 8 

 db brighter than threshold. The experi- 

 ments quantify this effect. The maximum 

 loss to be expected from attending to the 



point on the graph is a median of at least 

 35 observations per observer. The interest- 

 ing thing is the steady increase in search 

 time as the pip gets weaker and weaker. 

 At least within the range from threshold to 

 10 db above threshold, search time is a con- 

 tinuous monotonic function of pip brightness 

 (signal strength). This would not be no- 

 table were it not for the fact that a 10 db 

 range in such signals is at least four and 



4 



SIGNAL STRENGTH IN DB ABOVE THRESHOLD 

 Fig. 12. Search time and signal strength 

 INCHES Search time was limited to two minutes. Bias, 



VISUAL optimal; no noise. The rotation rate of 30 rpm 

 ANGLE °^6ans that the pip was re-excited every two 

 seconds. 



DISTANCE FROM FIXATION POINT 



Fig. 11. Retinal position and pip size 



The largest pip, 3 ais .x 3°, subtends about 10^' 



X 22p' at the eye. Distances from fixation point 



to pip were measured from center to center. No 



video noise; optimal bias ( — 5.5 volts from VRI). 



wrong part of the scope is about 16 db; aver- 

 age losses would be much less. 



Search Time 



The time required to find a target in a 

 given area seems to be a topic not much in- 

 vestigated. In radar work it is a very prac- 

 tical question. Therefore, Harriman (21) 

 carried out an experiment on the matter, 

 the data of which are shown in Fig. 12. Al- 

 though only two observers were used, each 



probably five times the interval of uncer- 

 tainty which, of course, always brackets any 

 threshold. Due to the inherent variability 

 of threshold data of this kind, which 

 normally fluctuate from one to two db, the 

 near-threshold portion of the curve is es- 

 sentially indeterminate. 



Observer's Confidence and Criteria of Judg- 

 ment 



Different observers no doubt vary from one 

 to another in the criterion used for visibility. 

 Some may report a pip when they think they 

 see "something." Others may wait until 

 they recognize a characteristic form. It 

 would be interesting to know just how much 



