Review and improvement of an algorithm for determining emissivity of a heterogeneous cavity in thermal infrared remote sensing

The algorithm for determination of effective emissivity of a cavity developed by Caselles and Sobrino (Remote Sensing of Environment, 29, 135-146) has been proved to violate Kirchhoff's law, and to give low values of emissivity. In this paper a Kirchhoff's law-observing algorithm is developed, and comparison of the two algorithms is performed. Emissivity retrieved by our algorithm can be 1.5 times larger than emissivity calculated with the algorithm of Caselles and Sobrino. Although no significant difference is found between them for almost blackbodies (vegetated surfaces with large values of ground and vegetation emissivity), for other kind of terrestrial surfaces (bare soils and rocks) difference between the two algorithms can be up to 0.05 for a typical cavity.