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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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