Table 1-2. Materials Versus Film Tone.
4.
The following detail indicates the various factors that affect tone.
a.
Weather. Although the IR surveillance system is capable of providing usable results
in about 75 percent of all weather conditions, varying weather conditions definitely affect data
results, and therefore, influence mission planning.
(1) Heavy snow and rainstorms greatly reduce the R1 energy available for
detection, making it nearly impossible to obtain usable data.
(2) Heavy thick fog has a somewhat similar effect to that of rain and snow
because of its high water vapor content.
(3) Although the IR detecting set has the ability to see through clouds of
thicknesses up to approximately 20 ft, clouds have rather highly reflective surfaces. It may be
necessary to filter out the visible light region. High altitude clouds cast shadows on the ground
being cooler than the areas casting the shadows. The shadowed areas appear lighter when
clouds prevent the solar energy from striking the earth. The cloud density and the type of
material in the shadowed area affect the data received from that area.
(4) After long periods of continuous overcast or rainfall, the terrain may lack
sufficient 1R radiation contrast to provide detailed data. Since little additional solar energy is
available, materials which normally heat and cool at different rates have had time to reach the
same temperature levels, thus providing little contrast. In the water-saturated condition, all
materials tend to assume the same emissivity level of the water itself.
(5) Bright sunny weather provides large amounts of energy to be absorbed by the
terrain, and the different heating and cooling rates of materials provide better contrasts in the
IR region.
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