Emissivity in the Infrared
Physics of Emissivity
Infrared (thermal) energy, when incident upon matter, be it solid, liquid or gas, will exhibit the properties of absorption, reflection, and transmission to varying degrees.
Absorption is the degree to which infrared energy is absorbed by a material. Materials such as plastic, ceramic, and textiles are good absorbers. Thermal energy absorbed by real-world objects is generally retransferred to their surroundings by conduction, convection, or radiation.
Transmission is the degree to which thermal 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 is the degree to which infrared 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.
(1) Incident Energy = Absorbed Energy + Transmitted Energy + Reflected Energy
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.
(2) Emitted Energy = Absorbed Energy
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:
(3) Incident Energy = Emitted Energy + Transmitted Energy + Reflected Energt
Setting the incident energy equal to 100%, the equation 3 becomes:
(4) 100% = %Emitted Energy + %Transmitted Energy + %Reflected Energy
Because emissivity equals the efficiency with which a material radiates energy, equation 4 can be restated as follows:
(5) 100% = Emissivity + %Transmitted Energy + %Reflected Energy
Applying similar terms to %Transmitted Energy and %Reflected Energy,
(6) 100% = Emissivity + Transmissivity + Reflectivity
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:
(7) 100% = Emissivity + Reflectivity
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.