What is field spectrum?

The spectral feature of earth surface features is that any ground object in nature has its own law of electromagnetic radiation, such as reflection and absorption of external ultraviolet, visible light, infrared and microwave. They all have the characteristics of emitting some infrared rays and microwaves; a few ground objects also have the characteristics of transmitting electromagnetic waves, which is called the spectral characteristics of the ground objects.

When electromagnetic radiation energy is incident on the surface of a ground object, three processes will occur: Part of the incident energy is reflected by the ground object; part of the incident energy is absorbed by the ground object and becomes the internal energy of the ground object or partly re-emitted, and part of the incident energy is transmitted by the ground object. 

See the following formula for details: In the energy conservation formula: E0 is the total incident energy; Eρ is the reflected energy of the ground object; Eα is the absorbed energy of the ground object; Eτ is the transmitted energy of the ground object. 

    Let ρ=Eρ/Eο×100%, that is, the percentage of the reflected energy of the ground object to the total incident energy, which is called the reflectance; α=Eα/Eο×100%, that is, the percentage of the energy absorbed by the ground feature to the total incident energy, called the absorption rate; τ=Eτ/Eο×100%, that is, the percentage of the energy transmitted by the ground object to the total incident energy, which is called the transmittance.The formula is changed to: ρ+α+τ=1

It can be seen that: for a ground object with high reflectivity in a certain band, its absorption rate is low, which is a weak radiator; on the contrary, a ground object with high absorption rate has a low reflectivity.

Reflectance spectrum characteristics of filed spectrum:

The reflectivity of different ground objects to incident electromagnetic waves is different, and it is usually expressed by reflectivity. When electromagnetic radiation reaches the interface between two different media, the phenomenon that part or all of the incident energy returns to the original medium is called reflection. The characteristics of reflection can be expressed by reflectance, which is a function of wavelength, so it is called spectral reflectance r(l). Reflectance is not only a function of wavelength, but also a function of the angle of incidence, the electrical properties of the object (conductivity, dielectric, magnetic properties, etc.), surface roughness, texture, etc. Generally speaking, when the wavelength of the incident electromagnetic wave is constant, the reflectivity of the ground object with strong reflection ability is high, and the color tone presented in the black and white remote sensing image is light. On the contrary, the ground objects with weak ability to reflect incident light have low reflectivity, and the color tone presented in the black and white remote sensing image is dark. The difference in tones on remote sensing images is an important sign for the interpretation of remote sensing images.

Emission spectrum characteristics of filed spectrum：

Emissivity When the temperature of any ground object is higher than the absolute temperature of 0 K, the atoms, molecules and other particles that make up the substance are constantly in thermal motion and have the ability to radiate infrared rays and microwaves to the surrounding space. Usually, the ability of ground objects to emit electromagnetic radiation is measured by emissivity.  The emissivity of ground features is based on blackbody radiation. The above-mentioned Stefan-Boltzmann law and Wien displacement law only apply to black body radiation, but in nature, black body radiation does not exist. Generally, the radiation energy of ground objects is always smaller than that of black body radiation.  If you use the blackbody radiation related formula, you need to add a factor, this factor is the emissivity (ελ), or "specific emissivity".  For a certain wavelength, the emissivity is defined as follows: the emissivity changes according to the dielectric constant of the substance, the surface roughness, temperature, wavelength, observation direction and other conditions, and takes a value between 0 and 1. The difference in ground object emissivity is also the basis and starting point of remote sensing detection. Ground feature emission spectrum The law that the emissivity of the ground feature changes with the wavelength is called the emission spectrum of the ground feature. The curve drawn according to the relationship between the emissivity of the ground feature and the wavelength (the abscissa is the wavelength, the ordinate is the emissivity) is called the ground emission spectrum curve.

Transmission spectrum characteristics of filed spectrum:

When electromagnetic waves are incident on the interface of two media, part of the incident energy can pass through the interface of the two media, which is called transmission. When the transmitted energy passes through the medium, it is often partially absorbed by the medium and converted into heat energy to be emitted. The ability to mediate the transmission energy is expressed by the transmittance τ. Transmittance is the percentage of the incident light transmitted through the ground object to the total incident energy.  The transmittance of features varies with the wavelength of electromagnetic waves and the nature of the features. 

For example, water has a certain transmission ability for blue and green light waves of 0.45～0.56μm, and the transmission depth of more turbid water is 1～2m. Generally, the transmission depth of water can reach 10～20m. For another example, microwaves with a wavelength greater than 1 mm have the ability to transmit ice. 

Under normal circumstances, most ground objects have no ability to transmit visible light. Infrared rays only have a certain transmittance ability to objects with semiconductor characteristics. Microwave has obvious transmission ability to ground objects, and this transmission ability is mainly determined by the wavelength of the incident wave. Therefore, in remote sensing technology, suitable sensors can be selected according to their characteristics to detect the information of some ground objects under water and ice.

For photographic remote sensing systems, the transmittance of film and filters is a very critical parameter. In nature, people are most familiar with the transmission ability of water bodies. This is because people can directly observe the transmission of radiant energy in the visible light band. It is invisible, but it exists objectively, such as plant leaves, which are opaque to visible light radiation, but it can transmit a certain amount of infrared radiation.