FLIR Navigator II Operator's Manual page 16

emissivity. Emissivity is the measure of an object's effi ciency at radiating thermal energy. For
example, shiny metals are poor emitters. Instead of radiating their own thermal energy, they
tend to refl ect radiation from their surroundings.
Infrared, from theory to practical application
Infrared imagers operate by detecting the relative intensities of thermal energy radiated from the
surfaces of objects, and displaying these intensities in black and white video as shades of gray.
ey do not show a "heat picture." Even if an object is very hot, it may not display well if there
is little or no temperature contrast between the object and its surroundings.
ermal imagers primarily detect thermal energy radiated from an object's surface; thermal
imagers can't "see through" much of anything.
As you look at the thermal images created with your Navigator II, you will see multiple sources
of thermal energy in addition to your main object of interest. When looking at a scene with
a large number of heat sources, it can get confusing trying to sort it all out. Kirchhoff 's Law
is an easy way to account for the diff erent sources of thermal radiation you see in your images.
Kirchhoff says that all of the thermal radiation in an image has been Emitted (given off by the
object), Transmitted (passed through the object), or Refl ected (bounced off the object).
Most of the strong energy sources you will see in a given scene are from "emitted" energy.
is, they are giving off heat energy. Examples of strong emitters of thermal energy include people
and boat engines.
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 dependant 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.
Another reason to care about the weather
e time of day and weather conditions in which you use your Navigator II 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
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