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ermal energy doesn’t pass through much, but it does “transmit” through
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 “refl ective.”
Everything is refl ective to one degree or another, but the most highly refl ective
objects are those made of polished, unpainted metal. Painted metals, glass,
and even wood can display greater or lesser degrees of refl ectivity, but this
becomes dependent upon myriad factors like their surface coatings, textures,
and the angles from which they are viewed. Refl ections can appear hotter or
colder than they really are, based on what they are refl ecting. Sun refl ecting
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
refl ecting energy from the sun. Look also at the two images on the previous
page, and note the refl ections of thermal energy from the bridge and boat off
the water, which can readily refl ect thermal energy.
Another reason to care about the weather
e time of day and weather conditions in which you use your Voyager can
have a signifi cant infl uence on how objects look on the screen. Remember that
thermal imagers detect and display diff erences in infrared radiation. If an object
and its background do not display any appreciable temperature diff erence, that
object will be very diffi cult to detect. erefore, the time of day during which
your Voyager 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. is increase in contrast continues until
the sun comes up the following morning. is daily sequence of heating and
cooling is called the “Diurnal Cycle.”
Atmospheric conditions can limit the range and imaging performance of
your Voyager. Under ideal conditions, most of the infrared energy radiated
from an object gets through the atmosphere and to the imager. Under typical
conditions however, atmospheric moisture and dust scatter can absorb some of