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 Physics of Emissivity
Electromagnetic energy,
when incident upon matter, be it solid, liquid or gas, will exhibit the
properties of absorption, reflection, and transmission to varying
degrees.
Absorption
Absorption is the degree to which electromagnetic energy is absorbed by a
material. Materials such as plastic, ceramic, and textiles are good
absorbers. Energy absorbed by real-world objects is generally retransferred
to their surroundings by conduction, convection, or radiation.
Transmission
Transmission is the degree to which electromagnetic energy passes through a
material. There are few materials that transmit energy efficiently in the
infrared region between 7 and 14µm. Germanium is one of the few good
transmitters of infrared energy and thus it is used frequently as lens
material in thermal imaging systems.
Reflection
Reflection is the degree to which electromagnetic energy reflects off a
material. Polished metals such as aluminum, gold and nickel are very good
reflectors.
Conservation of energy implies that the amount of incident energy is equal
to the sum of the absorbed, reflected, and transmitted energy.
Incident Energy = Absorbed Energy + Transmitted Energy + Reflected
Energy
[1]
Emitted Energy = Absorbed Energy
Consider equation 1 for an object in a vacuum at a constant temperature.
Because it is in a vacuum, there are no other sources of energy input to the
object or output from the object. The absorbed energy by the object
increases its thermal energy - the transmitted and reflected energy does
not. In order for the temperature of the object to remain constant, the
object must radiate the same amount of energy as it absorbs.
Emitted Energy = Absorbed Energy
[2]
Therefore, objects that are good absorbers are good emitters and objects
that are poor absorbers are poor emitters. Applying equation 2, Equation 1
can be restated as follows:
Incident Energy = Emitted Energy + Transmitted Energy + Reflected
Energy
[3]
Setting the incident energy equal to 100%, the equation 3 becomes:
100% = %Emitted Energy + %Transmitted Energy + %Reflected Energy
[4]
Because emissivity equals the efficiency with which a material radiates
energy, equation 4 can be restated as follows:
100% = Emissivity + %Transmitted Energy + %Reflected Energy
[5]
Applying similar terms to %Transmitted Energy and %Reflected Energy,
100% = Emissivity + Transmissivity + Reflectivity
[6]
According to equation 6, there is a balance between emissivity,
transmissivity, and reflectivity. Increasing the value of one of these
parameters requires a decrease in the sum of the other two parameters. If
the emissivity of an object increases, the sum of its transmissivity and
reflectivity must decrease. Likewise, if the reflectivity of an object
increases, the sum of its emissivity and trasmissivity must decrease.
Most solid objects exhibit very low transmission of infrared energy - the
majority of incident energy is either absorbed or reflected. By setting
transmissivity equal to zero, equation 6 can be restated as follows:
100% = Emissivity + Reflectivity
[7]
For
objects that do not transmit energy, there is a simple balance between
emissivity and reflectivity. If emissivity increases, reflectivity must
decrease. If reflectivity increases, emissivity must decrease. For
example, a plastic material with emissivity = 0.92 has reflectivity = 0.08.
A polished aluminum surface with emissivity = 0.12 has reflectivity = 0.88.
The
emissive and reflective behavior of most materials is similar in the visible
and infrared regions of the electromagnetic spectrum. Polished metals, for
example, have low emissivity and high reflectivity in both the visible and
infrared. It is important to understand, however, that some materials that
are good absorbers, transmitters, or reflectors in the visible, may exhibit
completely different characteristics in the infrared.
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