Ground surface ultralow background spectrometer

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author: Cily

2022-01-11

In the visible light and near-infrared bands, the radiation of the surface object itself is almost equal to zero. The spectrum emitted by ground objects is mainly reflected solar radiation. After the solar radiation reaches the ground, in addition to the reflection effect, the object also has the absorption effect on the electromagnetic radiation.
Generally speaking, most objects do not have the ability to transmit visible light, and some objects, such as water, have a strong ability to transmit electromagnetic waves of a certain wavelength, especially for the blue-green light band of 0.45~0.56μm. The transmission depth of general water bodies can be Up to 10~20m, clear water can reach a depth of 100m.

For ground objects that generally cannot pass through visible light, electromagnetic waves with a wavelength of 5cm have transmission capabilities. For example, ultra-long waves have strong transmission capabilities and can penetrate ground rocks and soil.

· Several common surface object spectral curves ·

1.Reflection Spectral Characteristics of Rocks

Unlike vegetation, rock reflection spectrum curves have obvious similar characteristics, that is to say, their reflection curves have no uniform characteristics, and their curve characteristics are related to their mineral composition, weathering degree, water content state, particle size, surface roughness, color, etc. Therefore, different rocks have different spectral patterns.

Light-colored minerals and dark-colored minerals have a greater impact on it, light-colored minerals have high reflectance, and dark-colored minerals have low reflectance.
Rocks in nature are mostly planted and covered by soil, so it is also related to the covering

2.Reflectance spectral characteristics of soils

In the natural state, the reflectance of the soil surface has no obvious peaks and valleys. Generally speaking, the finer the soil, the higher the reflectance. The higher the organic matter and water content, the lower the albedo, and the soil type and fertility also have an impact on the albedo.

The reflectance spectrum characteristics of soil are mainly determined by factors such as primary and secondary minerals in the soil, soil moisture content, soil organic matter, iron content, and soil texture. However, due to its smooth spectral curve, the difference in soil brightness is not obvious in remote sensing images of different spectral bands.

3.Reflection Spectral Characteristics of Water Bodies
The reflectivity of the water body is low, less than 10%, far lower than most other ground features. The water body has strong reflection in the blue-green band and strong absorption in other visible light bands.

Pure water is the highest in the blue light band, and the reflectivity decreases as the wavelength increases. The reflectance in the near-infrared band is 0; the reflectance peak of clear water containing chlorophyll is in the green light range, and the more chlorophyll in the water, the higher the peak. This feature allows monitoring and estimation of algae concentrations. The reflectance of turbid water and sediment water is higher than the above, and the peak value appears in the yellow-red area.
The reflection spectrum of water body is related to the state of water, the energy contained in it, organic matter in water, algae, sediment and so on.

4.Reflection Spectral Characteristics of Vegetation
Ground plants have obvious spectral reflection characteristics. Different from soil, water bodies and other typical ground objects, the response of vegetation to electromagnetic waves is determined by its chemical and morphological characteristics, which are related to the development, health and growth of vegetation. conditions are closely related.

400nm-700nm: There is a reflection peak at 550nm, which is formed by chlorophyll a and b inside the leaves and two strong absorption bands centered at 450nm and 640nm-680nm, and carotene lutein at 430nm-480nm There is a strong absorption band, which is why the leaves appear green. In this band range, the spectral characteristics of vegetation are basically controlled by chlorophyll and carotene. The former accounts for 65%-75% of the total absorption, and the latter accounts for 25%-35% of the total absorption.
700nm-800nm: This part forms a strong steep slope, which is formed by the strong reflection caused by the palisade tissue structure of the leaves. It is a unique spectral feature of vegetation, called "red edge". The steepness of the red edge determines the vegetation leaves. health level.
800nm-1100nm: The absorption effect of leaves in this band can be basically ignored, and the scattering effect occupies a dominant position. The light injected into the interior of the leaf has a significant difference in the refractive index between the cell wall and the cell cavity, resulting in more light in the interior of the leaf. Secondary reflection and refraction, upward refraction passes through the upper epidermis to form reflected light, and downward refraction passes through the lower epidermis to form transmitted light. This process has an obvious random nature, so reflectivity and transmittance are similar. A very wide and strong reflection peak in this band is a unique spectral feature of vegetation. When the leaves wither or wither due to lack of water, it means that the cells shrink and the difference in refractive index decreases. Its macroscopic performance is The reflectance value in this band drops significantly.
1100nm-2500nm: The spectral characteristics of this band are basically controlled by the absorption characteristics of liquid water. For visible light, liquid water is quite transparent, but it has two strong absorption peaks in the near-infrared band. The centers are respectively 1420nm and 1960nm, which cause the leaves to have two strong absorption valleys on these two central bands, and the depth of the valleys is related to the content of liquid water. In addition to the two large absorption valleys, there are two small absorption valleys at 980nm and 1140nm, which are also related to the water content of the leaves.

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