46 Lecture 2 
-18 
Oo ' 
yo sb 
10 LOG (CONTRAST THRESHOLD) 
10 100 1000 
AREA IN SQ. MINS. 
Fig. 2.16. Area/contrast threshold relationship at background luminance of 0,1 ft-L. 
(A) Blackwell's data (B) With noise background—circular patch (C) With noise-free 
background—circular patch (D) With noise-free background—line patch. 
seen by the eye); this is the same rate as for a line (or slit) target patch. 
With larger areas, the rate falls off. 
b. With fairly low background luminance (0.1 ft-L), the threshold is almost 
the same whether the background is noise or merely a uniform bright - 
ness, but at higher background luminance (say, 1 ft-L) the threshold is 
much poorer for noise than for uniform brightness. This is consistent 
with the conception of internal noise in the human optical system. 
Figure 2.16 shows the results for a background of 0.1 ft-L, and also shows 
that these results agree well with those obtained with different experimental 
methods by Blackwell [23]. 
2.5. ECHO-FORMATION AND RANDOMIZATION 
One of the most serious difficulties in sonar development is that, however 
good the array and system generally, there is always a limitation on performance 
imposed by the medium through which the system operates. In the medium, the 
sea, there are variations in temperature, salinity, aeration, etc., as well as 
currents, turbulence, and inhomogeneities generally, which affect to a greater 
or lesser extent the transmission of signals. There are reflections and scatter- 
ing at the bottom and surface of the sea which produce multipath transmission 
as well as an interfering background. The environmental conditions which have 
an adverse effect on sonar performance include the instability of transducer 
position when the equipment is shipborne. These various factors are very hard 
