FLIR Navigator II Operator's Manual page 23

ermal energy doesn't pass through much, but it does "transmit" through
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some plastics. When a material is not transparent to infrared radiation,
it is said to be "opaque." Most commonly viewed materials are opaque to
infrared radiation.
Materials that mirror the infrared signatures around them are "reflective."
Everything is reflective to one degree or another, but the most highly reflective
objects are those made of polished, unpainted metal. Painted metals, glass, and
even wood can display greater or lesser degrees of reflectivity, but this becomes
dependent upon myriad factors like their surface coatings, textures, and the
angles from which they are viewed. Reflections can appear hotter or colder
than they really are, based on what they are reflecting. Sun reflecting off of
polished chrome looks quite bright, and a common mistake is to think that this
section of chrome has suddenly become very hot. It hasn't, it is just reflecting
energy from the sun. Look also at the two images on the previous page, and
note the reflections of thermal energy from the bridge and boat off the water,
which can readily reflect thermal energy.
Another reason to care about the weather
e time of day and weather conditions in which you use your Navigator II can
have a significant influence on how objects look on the screen. Remember that
thermal imagers detect and display differences in infrared radiation. If an object
and its background do not display any appreciable temperature difference, that
object will be very difficult to detect.
your Navigator II is used can have a direct impact on your ability to detect and
recognize objects.
When things are exposed to the sun, they absorb infrared radiation. As the
duration of this exposure increases throughout the day, thermal contrast
between targets decreases.
When the sun begins to set, objects begin to cool. In doing so, they radiate
some of this stored thermal energy back into the atmosphere, and a certain
degree of thermal contrast is restored.
the sun comes up the following morning.
cooling is called the "Diurnal Cycle."
Atmospheric conditions can limit the range and imaging performance of your
Navigator II. Under ideal conditions, most of the infrared energy radiated
from an object gets through the atmosphere and to the imager. Under typical
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erefore, the time of day during which
is increase in contrast continues until
is daily sequence of heating and
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