Experimental Study of Brightness Surface Temperature Angular Variations of Maritime Pine (Pinus pinaster) Stands

An experimental design based on an airborne TIR camera equipped with wide-angle lenses for measuring directional brightness temperature over forest canopies is described. A particular care is given to the analysis of possible sources of errors (related to instruments and atmosphere) and to their corrections. The protocol of data acquisition is adapted to provide TIR measurements in a −60°–+60° range of zenith view angles and in all azimuthal directions. The results obtained over maritime pine (Pinus pinaster) stands in the Landes forest in the southwest of France during summers 1995 and 1996 reveal important hot spot effects and differences of brightness temperatures reaching 4 K between vertical and oblique measurements. Acquisitions performed over stands of different ages, between 5 and 48 years old, also illustrate the dependence of the structure of stands on the hot spot.