shaded area and upon prevailing meteorological conditions. Thermal shadows can often be
detected for hours. An additional type of thermal shadow can be noted on the lee side of
projecting objects during windy conditions. This occurs because the wind does not disturb the
surface in the area behind the object (lee). Consequently, this area may be warmer or cooler
than the surrounding area, depending upon whether the breeze is warming or cooling the
surface over which it is blowing.
(2) Ghosting can best be explained by example. Before taking off, a jet pilot
usually warms up the aircraft's engines. This causes the air, runway, or apron surface behind
the engines to heat. After the plane has departed, the area will remain hotter than the
surrounding area. In addition, the shadow area cast by the aircraft will be cooler than the
surrounding area. An IR mission flown over this area soon after the aircraft has departed will
easily detect the outline of the aircraft, cool area, and the area of warm jet exhaust (Annotation
C, Figure 1-9).
Surroundings. A knowledge of the surrounding area and pattern forms is vital to the
identification of targets or extremely hot objects. This may require maps or ground data for
accurate identification. One important fact to note when interpreting surroundings is that
disturbed soil tends to show up very clearly on IR imagery. Disturbed soil has a different
temperature value than hard packed soil; therefore, a missile site and its configuration will
normally show up on IR imagery because of the disturbed soil and thus reflect actual returns.
The radar associated with the site will show up as a warm spot at its location within the site.
The passing of a vehicle across an open field will be easy to detect because of the disturbed
soil. This may be helpful in locating hidden targets.
IR can detect the difference in normal soil and disturbed soil for months and even
years. Currently archeologists are using IR to help detect ruins. Consequently,
great care must be taken when reading IR imagery to assure any disruption of
surroundings is current and/or pertinent.
Shade or tone. The temperature difference of objects determines the tone of IR
imagery. This is a key factor to interpretation because the tone differentiation provides clues
to the composition or activity of objects. For example, a decoy would produce a lighter tone
than an actual object (Annotation A versus Annotation B, Figure 1-9); a campfire would
produce a different shade of black than would an operating generator or a running vehicle.
This stress on tone is the major difference between the interpretation of IR imagery and
conventional imagery. The gray tones on IR imagery result from the IR reflectiveness of
objects and not their color.
3. Materials versus film tone. fable 1-2 consists of a list of materials and their film tones
which normally may be expected under specific conditions on IR imagery.