CO2 laser light scattering by bare soils for emissivity measurements: absolute calibration and correlation with backscattering and composition

Measurements of the scattering cross section of a number of bare soils have been made with CO₂ laser illumination at 10.59 μm. The primary focus was on absolute calibration of the measurements. First, comparison of emissivity values resulting from the application of Kirchhoff´s relation after angular integration of the bidirectional measurements, with emissivity values obtained from the analysis of the emitted radiation show excellent agreement to within less than 0.3%. Second, it was found that a simple formula holds for a relationship between the emissivity and co- and cross-polarized backscattering cross section at an angle of 30°. Third, a clear correlation was observed between emissivity and composition (in this case % Al+Fe oxides; % SiO₂) for a homogeneous series of samples from the same area in Niger. These results emphasize the importance of calibrated experimental data. The implications of the research give evidence of the advantage of obtaining emissivity from remote reflectivity measurements and possibly only backscattering measurements, and remotely estimating mineral composition