Basics of radiation physics for remote sensing of vegetation UNIVERSIDADE DE SÃO PAULO ESCOLA SUPERIOR DE AGRICULTURA “LUIZ DE QUEIROZ” DEPARTAMENTO DE GENÉTICA LGN5831 Tópicos Especiais de Genética e Melhoramento Basics of radiation physics for remote sensing of vegetation Prof. Roberto Fritsche-Neto roberto.neto@usp.br Piracicaba, August 28, 2017

Absorption and reflectance

Remote sensing Remote sensing is the collection of information (sensing) from an object or area from a distance (remote) Source and detector are both remote

Types of remote sensing a radar (or e.g., lidar) transmits and receives radio (or light) signals reflected back to itself a microwave imager (or e.g., spectrometer) measures natural thermal radiation (or reflected sunlight)

Electromagnetic spectrum The most useful in remote sensing of vegetation: Visible to SWIR 300 nm to 15 μm

Thermal remote sensing of the crop canopy The Stefan-Boltzmann law states: The emittance of radiation in the thermal infrared region of the electromagnetic spectrum by the crop canopy is proportional to its absolute surface temperature raised to the fourth power

Lambert’s Cosine Law - irradiance The radiant flux received (irradiance) is related to: The cosine of the angle z (θ = zenith angle) between the beam and a line normal to the surface

Incoming solar radiation The illumination of a surface and radiation emitted from the surface may vary with time of day, season, wavelength, location, and atmospheric conditions

Incoming solar radiation

Radiation interacting with a material Scattered radiation need correction for this noise

Radiation is scattered by particles scattering particle is much smaller same order as the wavelength several times larger than the wavelength Remote sensing of VIS/NIR performs poorly under cloud cover because of atmospheric scattering

Spectral reflection The reflectance spectrum can be measured in the laboratory or in the field using a spectrometer The spectral signature potentially varying over time (as vegetation develops or senesces) or space (field effects) If the ratio or difference of specific reflectivities are very sensitive to plant species or health, then they can be used as a diagnostic tool

Remote sensing of vegetation Large difference between % reflectance of Red and NIR light by a healthy plant Feature that is central to remote sensing of plants

Normalized Difference Vegetation Index (NDVI) Healthy green leaves: strongly absorb red light (i.e., low reflectance) weakly absorb light in the NIR (i.e., high reflectance) Non-green vegetation or soil background is opposite Normalized Difference Vegetation Index (NDVI) Function of vegetation cover, biomass, leaf area index (LAI), and leaf chlorophyll concentration