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 Affects of Emissivity
Thermal imaging cameras detect and
measure the sum of infrared energy over a range of wavelengths
determined by the sensitivity of the camera’s detector. Thermal imagers
cannot discriminate energy at 7µm from energy at 14µm the way the human
eye can distinguish various wavelengths of light as colors. They
calculate the temperature objects by detecting and quantifying the
emitted energy over the operational wavelength range of the detector.
Temperature is then calculated by relating the measured energy to the
temperature of a blackbody radiating an equivalent amount of energy
according to Planck’s Blackbody Law.
Because the emissivity of an object affects
how much energy an object emits, emissivity also influences a thermal
imager’s temperature calculation. Consider the case of two objects at the
same temperature, one having high emissivity and the other low. Even though
the two objects have the same temperature, the one with the low emissivity
will radiate less energy. Consequently, the temperature calculated by the
thermal imager will be lower than that calculated for the high emissivity
object.
Apparent Temperature
Thermal imaging cameras cannot detect the
emissivity of objects in order to calculate their true temperature. They
can only calculate the “apparent” temperature of objects. The apparent
temperature of an object is a function of both its temperature and
emissivity. Given two objects with the same true temperature but different
emissivity, a higher apparent temperature will be calculated for the object
with higher emissivity (see figure below). Given two objects with the same
emissivity but different true temperature, a higher apparent temperature
will be calculated for the object with higher true temperature. The
apparent temperature of an object may be substantially different from its
true temperature. Only when the emissivity of objects is known can thermal
imagers compensate for emissivity and calculate true temperature.
Reflectivity
Objects with high reflectivity can reflect
energy radiated by other objects. For example, polished aluminum reflects
about 90% of the energy incident upon its surface. Just as thermal imagers
cannot detect the emissivity of
objects in order to calculate their true temperature, they also can’t detect
the reflectivity of objects. Therefore, when calculating the apparent
temperature of an object, thermal imagers detect and quantify energy emitted
from the object, as well as, energy reflected from the surface of the
object. If an object reflects energy from another radiating source with a
higher temperature, the apparent temperature that is calculated for the
object will be higher than its true temperature. Likewise, if an object
reflects energy from another radiating source with a lower temperature, the
apparent temperature that is calculated for the object will be lower than
its true temperature.
Given two objects with the same true
temperature but different emissivity, a higher apparent temperature will be
calculated for the object with higher emissivity (see figure below). Given two
objects with the same emissivity but different true temperature, a higher
apparent temperature will be calculated for the object with higher true
temperature. The apparent temperature of an object may be substantially
different from its true temperature. Only when the emissivity of objects is
known can thermal imagers compensate for emissivity and calculate true
temperature.
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