红外波段的发射率、透过率和反射率的关系
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.
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Absorption
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
Transmission is the degree to which thermal energy pass 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 ud frequently as lens material in thermal imaging systems.
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Reflection
Reflection is the degree to which infrared energy reflects off a material. Polished metals such as aluminum, gold and nickel are very good reflectors.
Conrvation of energy implies that the amount of incident energy is equal to the sum of the absorbed, reflected, and transmitted energy.
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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. Becau 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 increas 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]切换快捷键
Therefore, objects that are good absorbers are good emitters and objects that are poor a
bsorbers are poor emitters. Applying equation 2, Equation 1 can be restated as follows:
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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]
Becau 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,
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100% = Emissivity + Transmissivity（透过率） + Reflectivity
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According to equation 6, there is a balance between emissivity, transmissivity, and reflectivity. Increasing the value of one of the parameters requires a decrea in the sum of the other two parameters. If the emissivity of an object increas, the sum of its transmissivity and reflectivity must decrea. Likewi, if the reflectivity of an object increas, the sum of its emissivity and trasmissivity must decrea.
Most solid objects exhibit very low transmission of infrared energy - the majority of incident energy is either absorbed or reflected. By tting 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 increas, reflectivity must decrea. If reflectivity increas, emissivity must decrea. 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.
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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.
