Remote sensing of vegetation stress and soil contamination using CO₂ laser reflectance ratios

A tunable CO₂ laser reflectance sensor operating in the 9-11 μm wavelength region of the electromagnetic spectrum was used to measure the mid-infrared active reflectance characteristics of stressed vegetation and contaminated soil. Measurements were performed at various wavelengths, incidence angles, and polarization combinations. The vegetation study was conducted by inducing freezing, chilling, drought, flooding, and heat stress on four different plant species showing varying leaf cover characteristics. The soil study was conducted by adding chemical contaminants, viz., antifreeze, used motor oil (containing gasoline), and unused motor oil to bare soil. Reflectance ratios, i.e., ratios of reflectance at different wavelengths, were investigated as to their relationships with different stresses and contaminations. It was determined that judiciously selected reflectance ratios could be used to identify stressed plants (although the type of stress could not always be identified), as well as the presence and the type of contamination in bare soil. This indicates the potential for CO₂ laser sensors to monitor vegetation stress and soil contamination from standoff platforms.