Terminology in thermal infrared remote sensing of natural surfaces

Abstract

A consistent nomenclature for thermal infrared remote sensing of natural surfaces is outlined in this paper. Terms such as ‘canopy temperature’ and ‘surface temperature’ should be confined to general, qualitative descriptions and terms such as directional radiometric temperature, hemispherical emissivity, aerodynamic temperature, or hemispherical-directional reflectance used for technical precision. Some discussion also is included on the relation between hemispherical-directional thermal reflectance and directional thermal emissivity. A distinction is made between two kinds of emissivity; e-emissivity and r-emissivity. Through use of detailed models of directional thermal emissivity and bidirectional reflectance, we suggest that the influence of temperature gradients in vegetation on directional emissivity usually will cause uncertainties of less than 0.005. In addition, uncertainties that occur in estimating surface temperature, which arise because an ensemble of black bodies each at a different temperature does not have a black body radiance distribution with temperature, will not always be negligible for purposes of remote sensing of natural surfaces. Differences between mean thermodynamic and ensemble radiometric temperatures of 1 K or larger are possible even though differences in ensemble radiometric temperatures for different wavelength bands in the atmospheric window (8–14 μm) usually will be less than 0.1 K.