f.
Materials. The following generalizations on the appearance of metal, pavement,
soil, grass, trees, and water on negative night IR imagery can be made:
(1) Metal surfaces. Under normal conditions on negative night IR imagery
horizontal surfaces of thin unheated bare metal will appear white, because they are cold.
Since the emissivity of metals is lower than other substances they will emit less energy. Metal
surfaces are good reflectors and will strongly reflect the radiation directed on them from the
sky. At night the intensity of sky radiation is quite low (particularly on clear nights), and the
reflected radiation will be weak. It is possible for metal to reflect the radiation from nearly
warm objects and thus appear to be warm. This effect does not occur often enough to be
significant.
(2) Asphalt pavement appears dark gray to black on negative night IR imagery.
This is because pavement has a good emissivity and is in good thermal contact with the earth,
which acts as a constant heat source. The pavement will retain the heat received from the
sun during the day because of its high thermal capacity. This is generally true for all types of
pavement, including concrete, asphalt, and blacktop.
(3) Earth. Under normal conditions, earth which includes various types of soils,
sands, and rocks will appear dark gray on negative night IR imagery. This results from the sun
heating of the earth during the day and its high emissivity and heat capacity.
(4) Grass appears gray to white on negative night IR imagery. Grass is unable to
draw heat from the earth because of its poor thermal contact with the ground and rapidly
becomes cold by radiation. This results in night inversion because temperatures are lower at
night.
(5) Trees appear light-to-medium gray in tone on negative night IR imagery. This
tone is believed to be associated with the convective warming of the trees by the air in
conjunction with the night inversion of air temperatures, although sun heating from the
previous day and natural heat producing reactions resulting from life processes in the trees
play a role. In daytime the same leaves appear colder than the ground because the air
temperature at tree top height is cooler than at ground level. Shadow areas in the foliage of
trees also are cool relative to areas illuminated by the sun.
(6) Under normal circumstances water will range in tone from dark gray to black
on negative night IR imagery. This warm tone is the product of its high emissivity and good
heat transfer properties. The fluid nature of water allows it to transfer heat by convection as
well as by conduction.
REMEMBER: On positive IR imagery (prints) materials will appear in contrast to negative IR
imagery, i.e., black on negative IR imagery will be on positive IR imagery, etc.
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