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6 BOUSKY 
detectable by the eye from most photographic film at unity gamma. In 
other words, this bit of detail may have been lost. 
The higher value of ground detail contrast in this example, taken 
at 0.6, represents a ffequency of occurrence of only about one percent 
of that for the value occurring most often. Most ground detail has a 
contrast closer to the .06 value, which occurs in Figure 4 at a favor- 
able brightness value. Had this bit of detail occurred near thc lower 
extreme of brightness, say 200 foot-lamberts. then the .06 contrast at 
ground level woula be as low as .0035 at extreme altitude. This is 
below the capability of being visually read from any present film. 
This is the most critical problem area affecting extreme altitude 
photography. All the detail contrasts are so low that a relatively high 
proportion may be completely lost. The human eye has a contrast 
threshold for directly visible detail at about .006 or .007. The granu- 
larity of most photographic films raises this threshold by about a fac- 
tor of five, so that the eye when viewing even a high quality trans- 
parency under optimnm conditions cannot see detail contrast below 
about .03 on most present photographic materials. 
The foregoing indicates that the detail contrasts encountered in 
aerial photography are extremely low, ranging for the most part from 
values of 0.1 to below the visible threshold even at low altitudes. At 
high altitudes a much smaller proportion of detail occurs at contrasts 
even as high as .05. It must be pointed out that the data presented 
here do not represent the worst conditions, but rather are actually 
close to the most ideal. Contrast is further reduced by several other 
including increased path lengths of view through the atmos- 
factors 
arity such as that caused by haze 
phere and reduced atmospheric cl 
and smoke. 
From this it would seem that the values of contrast usually ac- 
d for the testing of camera systems and photographic elements 
cepte 
f real importance 
may be far removed from the values which are o 
in practice. 
ILLUMINATION RANGE 
tic of the image which should receive mention 
Figure 5 indicates the range of sur- 
face reflectances from a variety of typical gross detail. It may be 
noted that reflectances vary from about 1% to 80%. While the range in 
reflectance in a typical scene may run 7 to 1, ranges as great as 40 to 
Another characteris 
is the scene illumination range. 
1 are not uncommon. 
ation where the scene brightness at low 
Figure 4 indicates a situ 
ge of 40 to 1. At a gamma of unity this 
altitude is taken at such a ran 
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