Any object whose temperature is higher than absolute zero (0K=-273. 16℃) has molecular thermal motion, which can produce electromagnetic radiation in the middle and far infrared. This kind of electromagnetic radiation produced by the thermal motion of particles inside an object is called thermal radiation. A large number of facts prove that; The intensity of electromagnetic radiation emitted by objects at different temperatures and its distribution according to wavelength are different. Therefore, it is a very convenient method to use temperature as an absolute measure of thermal radiation energy.
In order to discuss the thermal radiation properties of objects conveniently, it is necessary to have an ideal standard thermal radiation body as a reference source, that is, an absolute blackbody. Absolute blackbody is defined as: at any temperature, the absorption coefficient (rate) α (λ, t) of incident radiation of any wavelength is always equal to 1, that is, an object with α (λ, T)= 1 is called absolute blackbody for short. Obviously, the reflectivity γ=0 and the transmittance c=0 of the blackbody.
Black body is the object with the strongest radiation ability. Generally speaking, the radiation of an object is smaller than that of a black body. Therefore, what is expressed based on boldface is emissivity, also known as specific emissivity. Its definition is the ratio of the radiation of the target object to the radiation of the blackbody at the same temperature, which is often expressed as ε. Spectral emissivity refers to the ratio of radiation per unit wavelength width at a specific wavelength.
1860, Kirchhoff found in experiments that at the same temperature, the ability of any object to emit electromagnetic waves of a certain wavelength is directly proportional to its ability to absorb electromagnetic waves of that wavelength. Black body has the strongest ability to absorb electromagnetic waves, so it has the strongest ability to emit electromagnetic waves.
1900, Planck deduced the thermal radiation theorem with the concept of quantum theory, and its analytical formula is:
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Where c is the speed of light in vacuum; H is Planck constant, and its value is 6.626× 10-34 Joule seconds; K is a Boltzmann constant with a value of 1.3806× 102-3 Joules/kn.
Planck's formula is in good agreement with the experimental blackbody radiation spectrum curves at various temperatures (Figure 2-2). Three characteristics of blackbody radiation can be seen intuitively from the figure: ① The total radiation flux density is directly proportional to the area under the curve and increases rapidly with the increase of temperature. In the wavelength range from zero to infinity, the Planck formula is integrated, and the total radiation flux of blackbody radiation per unit area to hemispherical space is obtained as follows:
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Figure 2-2 blackbody spectral curves at different temperatures
There it is called the Stephen Boltzmann constant. It can be seen that the increase of blackbody radiation flux density is proportional to the fourth power of absolute temperature, which is the so-called Stephen Boltzmann's law. ② The peak wavelength λmax of spectral radiation flux density moves to the short wave direction with the increase of temperature, which is called Wien's displacement law. The dashed line in Figure 2-2 is the trajectory of these peaks. (3) The curves do not intersect with each other, so the higher the temperature, the greater the spectral radiation flux density of each wavelength, that is, it increases with the increase of t..
Objects are generally divided into three categories according to their radiation characteristics: the first category is blackbody, which emits the largest radiation (emissivity is 1), and the emissivity has nothing to do with wavelength. The second is that the emissivity of gray body has nothing to do with wavelength (Figure 2-3), but its emitted radiation is smaller than that of black body. For example, the emissivity of soil at 20℃ is 0.92-0.95, and that of water at the same temperature is 0.96, both of which are quite close to blackbody. The third type is selective radiator, whose emissivity changes with wavelength, and it is an object with strong radiation absorption effect on atoms and molecules, such as mercury lamp and xenon lamp.
The radiation flux density spectra of blackbody, gray body and selective radiator are different according to the wavelength distribution. It should be pointed out that the black body is not necessarily black, and the gray body is not necessarily gray. For example, marble is white in visible light, but its emissivity in the infrared band of 8 14μm is 0.95, which is almost blackbody.
Figure 2-3 Three Kinds of Radiation from Objects
(2) Thermal inertia of objects
Thermal inertia (P) is a measure of the thermal response sensitivity of an object to environmental temperature changes. The greater the thermal inertia, the slower the thermal response to environmental temperature changes. It is a macroscopic physical quantity that describes the thermal characteristics of an object. The relationship with the density and thermal parameters of the object is as follows:
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The unit is Joule/cm 2 s 1/2 degrees. Where k is the thermal diffusion coefficient in cm2/s, indicating the temperature change rate of the object. ρ is the density in grams per cubic centimeter. C is the specific heat in joule/gram.
Newton's cooling law and heat conduction equation can prove that when the heat energy absorbed or lost by objects is the same, their temperature change amplitude is inversely proportional to thermal inertia. The temperature of the object with large thermal inertia changes slightly, while the temperature of the object with small thermal inertia changes greatly. Table 7-6 gives some thermal parameter values of rocks and water.
Figure 2-4 shows the temperature change curves of dolomite, limestone and granite in a solar daily cycle. According to the thermal inertia values given in Table 7-6, the thermal inertia order of the three rocks is: P dolomite > P limestone > P granite. Therefore, there are obvious differences in the temperature variation amplitude of the three rocks in a solar daily cycle, and the order of the largest temperature difference between day and night is δ t dolomite.
Fig. 2-4 Daily Temperature Variation Curves of Three Kinds of Rocks with Different Thermal Inertia
The best detection time of thermal infrared remote sensing is around dawn and noon. Among them, the thermal infrared images before and after dawn reflect the thermal characteristics of various objects, and the objects with large thermal inertia show "warm" characteristics in the images, while the objects with small thermal inertia show "cold" characteristics; The image at noon reflects the reflection characteristics of ground objects and shows the terrain characteristics. Using the information characteristics of these two images, we can distinguish and distinguish the ground objects.
Thermal infrared remote sensing has been widely used in environmental monitoring, such as monitoring volcanic activity, forest fires, spontaneous combustion of underground coal, pollution of rivers, lakes and seas, and resource investigation. For example, finding water sources, geological mapping, exploring geothermal, uranium and sulfide deposits, etc.