5. Filtered daytime IR energy is reflected from surfaces in almost the same manner as
visible light. In daytime it is possible for an imaged target to be formed solely by reflected
near-IR energy. Most surfaces become poor reflectors as wavelengths or reflected energy
becomes longer in the IR region of the frequency spectrum. If a filter is used which eliminates
not only the visible light energy but also part of the near-IR energy, more nearly pure thermal
images can be formed. With appropriate filters it is possible to eliminate energy contributions
light and the near-IR frequencies, a specific micron cutoff filter is used. To eliminate energy
contributions in the micron region, another type of cutoff filter is used. Filtered daytime IR
imagery gives a thermal presentation in which heat sources, such as fires or warm waste
water from a power plant, or objects such as housetops and operating vehicles, stand out
particularly well.
6. Mission planning summarization. During mission planning make sure you plan the
mission for a specific time period that takes into account all factors needed for an optimum
mission; low altitude, clear weather, and hours of darkness. This does not mean the aircraft
cannot fly at a higher altitude and during hours of daylight or during marginal weather. The
imagery will not be as good as that obtained from a mission flown under optimum conditions.
7. Comparison of IR imagery with conventional photography. In Figure 1-10 "A" shows a
typical example of the thermal image of an industrial target produced by an IR sensor. For
comparison a photograph of the same target is shown in Figure 1-10 "B". It is immediately
apparent the size and shape of the large structures are determinable from either image.
a.
Smaller targets, such as parked automobiles, are detected in the IR image but clues
for identification must be obtained from parking patterns and proximity to roadways. The IR
image is a positive print; thus, hot targets appear light and cold targets appear black. The
large white structure is an active source, containing a number of furnaces contributing
significantly to the IR radiation. Note the row of white dots parallel to the hot building. In the
aerial photo these may be identified as smokestacks. The IR image indicates the activity of
each stack. The largest stack (below the row of smaller stacks) has no smoke coming from it,
yet the IR image shows it to be quite hot.
b.
The images shown in Figure 1-10 were obtained in late afternoon. Long shadows
appear in the photograph. The residential buildings east of the plant are emitting significant IR
from roof surfaces exposed to the sun. Trees behind these dwellings are poor emitters. The
line structure of the IR image is very apparent. Each line corresponds to one scan of the IR
scanner. The lines are most noticeable in the gray areas which represent intermediate values
of IR radiation.
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