(2) Because of the interchange of night and day, there are two times daily (early
morning and early evening) when land and water surfaces are equal in IR output. A prime
example is a steel bridge spanning a large body of water. The water absorbs heat during the
day and releases it slowly at night, thus causing a minimal change in its IR output. However,
the initial composition of the bridge causes it to heat rapidly from the sun's energy and cool
rapidly at sunset. As the bridge cools, it will reach the same temperature as the body of water.
At this point, the bridge will not be visible on IR imagery. Eventually it will get colder than the
body of water and reappear. The point at which invisibility occurs is called 'crossover time.' It
also occurs in reverse at sunrise.
Blackbody effects. A blackbody is a theoretical material in that it effects and
radiates the total IR energy received when the IR energy source is removed. In a natural
environment a body of water comes closest to achieving this state. During the day water will
absorb IR energy and at night it will radiate IR energy. This phenomenon is called the
blackbody effect. Water will appear very light on IR imagery taken in the day and very dark on
Emissivity is the ratio of IR emitted by a surface to the IR radiation emitted by a
blackbody at the same temperature and under the same conditions. The emissivity of a body
is dependent on: the material it is made of, its shape, the finish or texture of its surface, the
light or dark color of its body, its temperature and external sources of electromagnetic energy.
The emissivity-of various materials in relation to a blackbody is listed in Table 1-1, starting with
the perfect emissivity, factor of 1.00 (absorbing and radiating the total IR energy received).
The following categories are generalized target descriptions (assuming a target and its
background have the same emissivity:
(1) Warm target: A target that is warmer than its background will image brighter
than its background on IR positive film.
(2) Hot target: A target that is much warmer than its background will image much
brighter than its background on IR positive film.
(3) Cool target: A target that is cooler than its background will image darker than
its background on IR positive film.
(4) Cold target: A target that is much colder than its background will image much
darker than its background on IR positive film.