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Archives 6 
PHOTOGRAPHIC INTERPRETABILITY 5 
  
  
  
  
  
  
  
  
  
rom 
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tered to produce atmospheric illumination or skylight (indicated by the wait 
arrows at C) which in turn contributes an additional 2,000 foot-candles, -ove 
or So, to illuminate the ground. Thus a total of about 10,000 foot- teps 
candles may illuminate the ground area, the 
Typical ground detail is taken here, for example, as having a vith 
reflectance ranging from 1% to 40% so that the brightness of ground the 
detail at ground level will range from 100 to 4,000 foot-lamberts (in- 
dicated by the arrows at D). This light, in passing through the atmos- 
phere toward the camera, will again be attenuated and to it will again sir 
be added the dilution of skylight. Thus the light reaching the camera n 
(indicated by the arrows at E) will range from just under 2,100 foot- 2 a 
lamberts to about 4,400 foot-lamberts. Notice that the total range of ge 
photographic detail brightness which covered a range of 40 to 1 at the ken 
ground has been reduced to a range of 2.1 at 1 at the camera. Also, the ble 
average brightness which may have been about 650 foot-lamberts at the old 
ground is raised to about 2,400 foot-lamberts at the camera. nd 
What does this mean in terms of the ability to photographically ore 
record ground detail. Figure 4 indicates what the actual variations of 
detail brightness might be in a typical case, traced across a scene Eo 
O- 
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TS 4 his 
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ATMOSPHERIC CONTRAST ATTENUATION n- 
pious 4 les 
he 
area. The lower graph shows the 40 to 1 brightness range at the ground, D- 
while the upper graph shows the same detail signal as it would appear 
at the camera reduced to just over 2 to 1 total range. The arrows at TS 
the right have selected a high contrast point of 0.6 (brightness ratio of he 
4:1 between object and background), which at the camera has been ae 
reduced to a contrast of only 0.1 (brightness ratio of 1.26:1). The 
arrows at the left have selected a lower contrast point. This at the 
ground shows a contrast of 0.06 while at the camera it has been re- he 
duced to an apparent contrast of 0.025, a value less than the minimum B 
al 
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D = TT Sa SZ OS D VUE VE VRAI VAT VE T - 
there questions pertaining to Mr acteristic of the particular emulsion. Does this 
answer your question?