42 



HOW WE SEE 



made with varying luminances, spots of 

 various sizes, and with background lumi- 

 nances varying from full daylight to slightly 

 less than starlight levels. In all, more than 

 two million observations were recorded. 

 Some 450,000 of these have now been 

 analyzed, and they constitute what is 

 probably the largest single study of human 

 vision which has been reported to date. 



There are so many ways in which the 

 data of this study can be exhibited that it is 

 necessary to select a few charts from it. 

 Fig. 34 shows the least perceptible luminance 

 contrast which can be seen by the normal 

 eye against various background luminances. 



.6 -5 -4 -3-2-1 I 2 



BACKGROUND LUMINANCE IN LOG FT. -L. 



Fig. 34. Visibility of objects as a function of 

 background luminance and size of object. The 

 object sizes, A, B, C, D, and E, are in minutes of 

 visual angle. (After Blackwell, 2) 



The different curves represent the data for 

 test objects of various sizes. Two relation- 

 ships are shown very clearly here: (a) as 

 luminance decreases, the luminance contrast 

 of the just perceptible object increases, and 

 (b) at any luminance level, small objects 

 have to have more contrast to be seen. 

 The latter relationship is demonstrated more 

 clearly in Fig. 35. There the size of the 

 object is plotted against the least perceptible 

 luminance contrast. In this case, the 

 different curves represent the data obtained 

 at the various luminance levels. 



Applications of Contrast Data. The whole 

 art of camouflage depends on methods 



whereby low color and brightness contrast 

 are used to conceal objects by decreasing 

 their visibility. Standard camouflage for 

 naval aircraft consists in painting the 

 underside a light color so as to present low 

 contrast against the sky when seen from 



^ 5 

 CC CO 



^ UJ -25-2 0-15-10-5 5 10 15 2 2 5 30 



O X 



-" •- LOGARITHM OF LEAST VISIBLE CONTRAST 



Fig. 35. These are the same data as in Fig. 34, 

 replotted in another way. The different curves 

 are for various luminance levels in footlamberts 

 (After Blackwell, 2) 



100% 



z 



60 2 



40 O 



z 



ANGLE BETWEEN LINE OF 

 SIGHT AND WINDOW 



Fig. 36. The importance of luminance contrast 

 in a practical situation. (After Olenski and 

 Goodden, 71) 



below, and the top side blue to match the 

 sea when seen from above. When aircraft 

 operate predominantly in one type of 

 combat environment, as do naval aircraft, 

 satisfactory camouflage can be achieved 

 easily. If, on the other hand, the aircraft 

 must operate under a wide variety of 



