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