Angular variation of land surface spectral emissivity in the thermal infrared: laboratory investigations on bare soils

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Abstract
The angular variation of the thermal infrared emissivity of representative samples of naturally occurring materials was studied under laboratory conditions. The angular response of land surface will be of primary importance for thermal infrared remote sensing when two-angle data are available from the Along Track Scanning Radiometer (ATSR) aboard ERS-1. The results show that a decrease of the emissivity with increasing viewing angle does exist. The magnitude of this effect is not simply related to the optical properties of the material but depends also on scattering mechanism, grain size, porosity etc. SiO2 sand has a very small angular variation in the 100.12.0 μm band, and a much larger one in the strong reflectivity bands. Agricultural soils may show a large angular effect, although rather constant with wavelength. The results of this study remain very local; their generalization to more complex media and larger scales is not straightforward.

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