Spectral Reflectance of Crops and Soils
(book excerpts)Spectral reflectance refers to the percentage of light reflected by a surface at different wavelengths. The reflectance properties of an object depend on the material and its physical and chemical state, the surface roughness, and the geometric circumstances (e.g., the incidence angle of the sunlight). The reflectance of a material also varies with the wavelength of the electromagnetic energy. These differences in reflectance make it possible to identify crops and soils by analyzing their spectral reflectance signatures. These signatures are visualized in spectral reflectance curves that show the percent reflectance as a function of wavelengths. Spectral imaging techniques like RGB, multispectral, and hyperspectral differ in the number of spectral bands they capture, with RGB capturing the fewest and hyperspectral the most. RGB imaging uses three bands (red, green, and blue) to create images. Multispectral imaging captures data across a broader range of broader, but still limited, spectral bands, including some outside the visible spectrum, like near-infrared. Hyperspectral imaging takes this further, capturing hundreds or even thousands of narrow, contiguous spectral bands, providing detailed spectral information for each pixel. In general, healthy vegetation is a very good absorber of electromagnetic energy in the visible region. Chlorophyll strongly absorbs light at wavelengths around 0.45 (blue) and 0.67 µm (red) and reflects strongly in the green light. Therefore, our eyes perceive healthy vegetation as green. Healthy plants have a high reflectance in the near-infrared between 0.7 and 1.3 µm. This is primarily due to the internal structure of plant leaves. As this internal structure varies amongst different plant species, the near-infrared wavelengths can be used to discriminate between different plant species. Spectral reflectance of vegetation is also influenced by leaf and canopy structure, leaf water content, and the maturity of the vegetation, as well as plant species. Bare soil generally has an increasing reflectance, with greater reflectance in near-infrared and shortwave infrared. Some of the factors affecting soil reflectance are moisture content, soil texture (proportion of sand, silt, and clay), surface roughness, presence of iron oxide, and organic matter content. Different spectral imaging technologies, both active and passive, along with various platforms like satellites, drones, tractors, and robots, are employed depending on the application.
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Topics Within This Chapter:
- RGB, Multispectral, and Hyperspectral Spectral Imaging
- RGB Imaging in Agriculture
- Applications of RGB Imaging
- Multispectral Imaging in Agriculture
- Applications of Multispectral Imaging
- Hyperspectral Imaging in Agriculture
- Applications of Hyperspectral Imaging
- Spectral Reflectance of Crops
- Vegetation Indices
- Greenness and Biomass Indices
- Chlorophyll and Nutrient Indices
- Water Content and Stress Indices
- Canopy Structure and Leaf Area Indices
- Spectral Reflectance of Soils
- Soil Indices
- Normalized Soil Moisture Index (NSMI)
- Salinity Index (SI)
- Soil Color Index (SCI)
- Clay Index (CI)
- Organic Carbon Index (OCI)
- Bare Soil Index (BSI)
- Soil Adjusted Vegetation Index (SAVI)
- Normalized Difference Tillage Index (NDTI)
