314 RADIO WAVE PROPAGATION EXPERIMENTS 
low-frequency noise in the video to change proportion- 
ally. Video narrowing, however, does not change the 
low-frequency video noise components. Therefore, the 
reduction in signal visibility with video narrowing is 
less pronounced than with i-f narrowing. 
The human eye cannot distinguish between two ob- 
jects which are closer together than about 1 minute of 
are. If the light intensity contrast is limited, two ob- 
jects cannot be resolved even at a much greater angular 
separation. When the separation approaches approxi- 
mately 14 of a degree, the best visibility will be ob- 
tained for the smallest contrast. Thus, the action of 
the human eye can be regarded as that of a filter which 
preferentially selects those frequencies having a period 
of the order of 44 of a degree on the scope. At normal 
viewing distances this value of angular separation is 
of the order of 1 mm linear separation. Since the 
screen behaves like a linear transformation between 
the video signal and the light transmitted to the eye, 
this filter action of the eye is exactly equivalent to a 
video. filter whose maximum pass frequency corre- 
sponds to 1 mm divided by the sweep speed s. For most 
presentations, where the pulse length is considerably 
shorter than 1 mm on the scope, this effective video 
narrowing action of the eye is usually much more im- 
portant than the effect of video bandwidth in the re- 
ceiver. It is to be noted, however, that video bandwidth 
effects in the receiver can be observed when the sweep 
speed is sufficiently fast for proper delineation of the 
pulse. There is now a considerable amount of experi- 
mental evidence to support this rather simple picture 
of the combined effect of video bandwidth and the 
resolution properties of the eye. 
Because of this property of the eye, if the viewing 
distance is maintained constant, a large diameter PPI 
will be more sensitive in the detection of signals than 
a small one. A magnifying glass will produce an effec- 
tive increase in sensitivity on the small scope at the 
expense, however, of a restricted searching area. 
The focus on the PPI or A scope also acts like a 
video narrowing device. If the tube is defocused along 
the range scale, equivalent video narrowing will take 
place by an amount which is dependent upon the spot 
size. However, because of the effect on the human eye 
a loss in signal visibility will not occur until the de- 
focused spot is larger than approximately 1 mm. 
Defocusing to this extent is certainly disadvantageous 
in the ultimate discrimination. of two close radar tar- 
gets, and for this reason good spot focus must be 
maintained. 
Tn signal detection it is clearly necessary that the 
average signal deflection voltage be as large as the 
average noise fluctuation in the absence of signal. This 
is a purely statistical problem susceptible to theoretical 
analysis. Calculations show that the quantities which 
determine signal visibility, apart from the geometrical 
factors just described, are the total number of sweeps 
on which the signal is visible and the total number of 
sweeps on which only noise is visible. It is assumed 
that for these sweeps integration or averaging takes 
place. This result is confirmed experimentally with 
two restrictions. The total number of signal pulses is 
given by T X PRF (pulse repetition frequency), and 
the signal threshold power.is found to vary inversely 
with both PRF and T. While this holds for all values 
of PRF under investigation (12.5 to 3,200 c) it holds 
only for a limited region in T (approximately 0.05 to 
3 sec). The reason why the integration is not satis- 
factory outside these limits of T are related to the 
maximum flicker frequency detectable by the eye. For 
times shorter than perhaps 0.05 sec additional sweeps 
containing only noise will be integrated. Likewise, for 
T greater than 3 sec the eye and brain do not appear 
to integrate properly all the individual voltages. In 
other words, the system has incomplete memory. It 
has been found that the maximum system integration 
time (usually of the order of 6 sec) can be increased 
appreciably by operator practice. With a considerable 
amount of experience a good radar operator can eftfec- 
tively integrate for times as long as 4% min. It is to 
be noticed that because of this integration in the eye 
and brain of a radar operator other methods for pro- 
viding integration, such as P-7 screens or photographic 
integration, will fail to provide substantial benefit 
unless their effective integration time exceeds several 
seconds. This conclusion is borne out experimentally. 
In the radar scanning problem the same factors 
must be considered as have already been discussed, 
but, in addition, one must investigate factors peculiar 
to scanning. Among these are the rotation speed of the 
antenna, the beam width of the set, etc. It has been 
found, however, that the statisical problem met with 
in scanning is quite similar to that encountered in the 
absence of scanning. The complete system integration 
depends on two factors: the number of pulses inter- 
cepted by the radar beam during one traversal of the 
target, and scan-to-scan integration. If the scanning 
tate is sufficiently rapid (faster than 10 rpm) the 
signal visibility will be independent of the antenna 
rotation rate. Faster rotation rates intercept a smaller 
number of pulses for each revolution,’but there are a 
greater number of scan-to-scan integrations which just 
make up for the deficit. However, below the critical 
speed of about 10 rpm, scan-to-scan integration will 
not take place, and the signal threshold power will be 
proportional to the square root of the antenna rotation 
tate. This improvement in signal visibility at slower 
scan rates will continue until the antenna is on the 
target, during each revolution for approximately 6 
sec, whereupon the visibility is essentially that of a 
“searchlighting” set. Thus the total scanning loss is 
given by the rather simple formula 
1 
LOS = ) 
