A HOTSPOT MODEL FOR LEAF CANOPIES

The interaction between radiation and structured land surfaces is a key element in remote sensing. A fundamental property of this interaction is the way the observed radiance of a remotely-sensed scene changes with the source and viewer geometry. This variation depends on land surface type and structure and may provide a means for mapping and monitoring the structure and its changes through remote sensing. Among the effects which occur as source and observer geometry change is the hotspot effect. The canopy hotspot occurs when the source and viewer are aligned so that there is no visible shadow, and thus the scene appears bright. The magnitude and behavior of this phenomenon as viewing and illumination positions change provides important information about canopy structure. In particular, the angular change in brightness is a function of the relative size and spacing of objects making up the canopy. In this paper, the hotspot effect is modeled using general principles of environmental physics as driven by parameters of interest in remote sensing, such as leaf size, leaf shape, leaf area index, and leaf angle distribution. Specific examples are derived for canopies of horizontal leaves. Finally, the hotspot effect is implemented in the framework of the Suits model for a canopy of leaves to illustrate what might occur in an agricultural crop. Because the hotspot effect arises from very basic geometrical principles and is scale-free, it occurs similarly in woodlands, forests, crops, rough soil surfaces, and clouds.