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Title
Remote sensing for resources development and environmental management
Author
Damen, M. C. J.

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
The canopy hot-spot as crop identifier
S.A.W.Gerstl, C.Simmer* & B.J.Powers
Los Alamos National Laboratory, Theoretical Division, N.Mex., USA
* Present address: Institut für Meereskunde an der Universität Kiel, Maritime Meteorologie, FR Germany
ABSTRACT: Illuminating any reflective rough or structured surface by a directional light
source results in an angular reflectance distribution that shows a narrow peak in the
direction of retro-reflection. This is called the Heiligenschein or hot-spot of vegetation
canopies and is caused by the absence of mutual shading of leaves. The angular intensity
distribution of the hot-spot, its brightness and slope, are therefore indicators of the
plant's geometry. We propose the use of hot-spot angular characteristics as crop
identifiers in satellite remote sensing because the canopy hot-spot carries information
about plant stand architecture that is more distinctive for different plant species than for
instance their spectral reflectance characteristics. A simple three-dimensional Monte
Carlo/ray tracing model and an analytic two-dimensional model are developed to estimate the
angular distribution of the hot-spot as a function of thesize of the plant leaves. The
results show that the brightness-distribution and slope of the hot-spot change distinctively
for different leaf sizes indicating a much more peaked maximum for the smaller leaves.
All rough and structured surraces illuminated by a
directional light source with a wavelength
considerably smaller than the size of the
constituents of the surface show a local maximum of
the reflected radiation within a cone around the
direction of retro-reflection. If the sun is
considered as the only radiation source that
illuminates a vegetated surface (e.g. a grass lawn,
an agricultural field, or a forest), and the angular
distribution of the reflected sunlight around the
shadow of the observing instrument is measured, a
narrow intensity peak is observed in the reverse
solar direction when the observer's shadow can be
eliminated. This effect is called
the canopy hot-spot in agricultural remote sensing 1
the Heiligenschein in atmospheric optics 2 , and
the opposition effect in planetary physics 3,4 ,
and is caused by the absence of mutual shading of
the surface's constituents. When observed precisely
in the direction of the incident radiation, only the
illuminated parts of the surface structures (e.g.
leaves) are seen, while in all other view directions
the shadowed parts are also observed which leads to
a reduced reflected light intensity. Since this
effect is purely based on shadowing, no colors are
produced, which is a definite distinction from the
glory that also appears in the solar
retro-direction . Figure 1 shows a photographed
canopy hot-spot, taken from an airplane at about 500
ft. above a coniferous forest. In Figure 2 a
hot-spot is shown of grassland as photographed from
about 2000 ft. with standard panchromatic film. In
Fig. 3 an infrared photograph of a hot-spot over a
coniferous forest is shown. In this figure the
hot-spot is less prevalent because the canopy
reflectance in the near IR is larger than in the
visible, which leads to reduced contrast. The
airplane's shadow in the center of the hot-spot is
no longer visible due to the large distance of the
shadow from the airplane (penumbra effect) and the
competing brightness of the hot-spot
retro-reflection.
The angular intensity distribution of the
hot-spot, its brightness and slope, over unknown
surfaces of planetary bodies, has been used to
estimate the roughness of the moon's surface 3 , the
size of the particles that make Saturn's rings, and
other planetary surface characteristics 4 . However,
the use of observable hot-spot angular
Figure 1. Canopy hot-spot (Heiligenschein) over
coniferous forest in the visible.
characteristics tor crop identification has not yet
been exploited. We propose such use for satellite
remote sensing because the canopy hot-spot carries
information about plant stand architecture that is
more distinctive for different plant species than
for instance their spectral reflectance
characteristics. Bunnik et al 5 have studied the
usefulness of spectral indices measured in the
hot-spot direction (exact retro-reflection) for
biomass determination of agricultural crops.
We developed a simple three-dimensional Monte
Carlo/ray tracing model to estimate the angular
distribution of the canopy hot-spot as a function of
the size and orientation of the plant leaves that
constitute the vegetation canopy. The reflecting
canopy is assumed to be a layered medium with square
or circular horizontally oriented leaves. For
simplicity, take the direction of the incident
radiation to be perpendicular to the layers and
leaves. This may be considered a simple
representation of a planophile and heliotropic