红外波段的发射率、透过率和反射率的关系

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.

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 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 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.

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（透过率）+ 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]