| Launch：2019-07-26 |
* Abstract
The spectral characteristics of ground objects are that any ground objects in nature have their own electromagnetic radiation laws, such as the characteristics of reflecting and absorbing external ultraviolet, visible, infrared and microwave bands, and they all have the characteristics of emitting some infrared and microwave;A few ground features also have the characteristics of transmitting electromagnetic waves, which is called the spectral characteristics of ground features
* Ground object spectral characteristics
Any object in nature has its own electromagnetic radiation law, such as reflecting, absorbing foreign ultraviolet, visible light, infrared and microwave characteristics of certain bands, they also have some infrared, microwave emission characteristics; a few objects also have transmission electromagnetic wave characteristics, this characteristic is called earth. The spectral properties of matter.
When the electromagnetic radiation energy incident to the surface of the object, there will be three processes: part of the incident energy is reflected by the object; part of the incident energy is absorbed by the object, become the internal energy of the object itself or part of the emitted energy, part of the incident energy is transmitted by the object.
* Cassification
1. Reflection spectrum characteristics of ground objects
The reflection ability of different objects to the incoming electromagnetic wave is different, usually using reflectivity. When the electromagnetic radiation energy reaches the interface of two different media, part or all of the incident energy returns to the original medium, which is called reflection. The characteristics of reflection can be expressed by reflectivity, and it is a function of wavelength, so it is called spectral reflectance R (L).
Reflectivity is not only a function of wavelength, but also a function of incident angle, electrical properties (conductivity, dielectric, magnetic properties, etc.), surface roughness, texture, etc. Generally speaking, when the wavelength of the incident electromagnetic wave is fixed, the ground objects with strong reflectivity have large reflectivity, and the hue of the black-and-white remote sensing image is light. On the contrary, the objects with weak reflective ability of incident light have low reflectivity, and the hue of black-and-white remote sensing image is deep. The difference of hue on remote sensing images is an important symbol for interpreting remote sensing images.
2.Emission spectrum characteristics of ground objects
Emissivity any ground object when the temperature is higher than the absolute temperature of 0 K, the composition of matter atoms, molecules and other particles, in constant thermal movement, have the ability to radiate infrared and microwave to the surrounding space. The ability to emit electromagnetic radiation is usually measured by emissivity. The emissivity of ground objects is based on blackbody radiation. The above Stefan-Boltzmann law and Winn's law of displacement only apply to blackbody radiation, but in nature, blackbody radiation does not exist, and generally the radiation energy of ground objects is always smaller than that of blackbody radiation. If we use the formula of blackbody radiation, we need to add a factor, which is the emissivity .
For a certain wavelength, the emissivity is defined as follows:
Emissivity varies according to the dielectric constant of the material, surface roughness, temperature, wavelength, observation direction and other conditions, taking a value between 0 and 1. The difference of emissivity is also the basis and starting point for remote sensing detection.
The emissivity of the object emission spectrum varies with wavelength. It is called the emission spectrum of the object. The curves drawn according to the relationship between the emittance and the wavelength (the abscissa is the wavelength and the ordinate is the emittance) are called the emission spectrum curves.
3.Transmission spectrum characteristics of ground objects
When the electromagnetic wave is incident on the interface between two media, part of the incident energy can cross the interface between the two media. It is called transmission. The energy transmitted through the medium is often absorbed by the medium and converted to heat and then emitted.
The ability of transmitting energy is expressed by transmittance &tau. Transmittance is the percentage of incident light passing through the object and the total energy of the incident. The transmittance of objects varies with the wavelength of the electromagnetic wave and the nature of the objects. For example, the water body has a certain transmittance to 450-560 nm blue-green light wave. Compared with turbid water body, the transmittance depth is 1-2 m, and the transmittance depth of general water body can reach 10-20 M. For example, the microwave with a wavelength greater than 1mm has transmission ability to ice.
Under normal circumstances, most of the objects do not transmit light to visible light. Infrared only has certain transmission ability to the objects with semiconductor characteristics. Microwave has obvious transmission ability to ground objects, which is mainly determined by the wavelength of incident waves. Therefore, in remote sensing technology, according to their characteristics, the appropriate sensors can be selected to detect the information of certain objects under water and ice.
For photographic remote sensing systems, transmittance of film and filter is a key parameter. In nature, people are familiar with the transmission ability of water bodies. This is because people can directly observe the transmission of radiation energy in visible light band. However, transmission beyond visible light, although invisible to the human eye, but it is objective, such as plant leaves, visible light radiation is not transparent, but it can transmit a certain amount of infrared radiation.
