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markedly less scatter than those of the headed canopy. 
Asimulh View Direction (degree«) 
Angl* of Incidence (degrees) 
Fig. 3. (a) The polarized portion, Rq, of the reflectance factor was determined from measurements of a 
wheat canopy at a wavelength of 640 nm in 32 view directions, (b) The single variable, the angle of 
incidence of the sunlight on the leaf, which equals one half the phase angle, explains much of the 
variation in the values of Rq with view direction. During data collection, the solar zenith angle varied 
between 25° and 37°. The variable Rq in this figure corresponds to Rp in the text. 
II.B.4. Magnitude of Polarization Results suggest that the magnitude of the polarized portion of the 
reflectance factor of plant canopies is less than 1.5% when view and illumination directions are less than 60° from 
vertical. Values tend to be small toward the hot spot, increasing toward the solar azimuth direction. Values larger than 
I. 5% have been reported when view and/or illumination directions are larger than 60° (Rondeaux and Vanderbilt, 1993). 
For soils, polarized reflectances of 10% have been reported (Bréon and Tanré, 1994). 
II. C. Conclusions 
The polarization of the light reflected by a leaf measured in visible and near-infrared wavelengths appears to be a surface 
phenomena, unaffected by cellular pigments, metabolites and structure. The amount of polarized, reflected light varies 
with species and may be separated in two components, one polarized, originating at the leaf surface and containing no 
significant information about leaf pigments and the other non-polarized and emanating primarily, but not entirely, 
from the leaf interior. 
For plant canopies, simple, single scattering models explain the primary variation of polarized reflectance with angle. 
The models use the Fresnel equations and include three measurables 1) leaf optical properties (index of refraction, 
surface roughness), 2) canopy architecture (probability of gap, leaf area with height) and 3) directions of view and 
illumination. 
Just as for individual leaves, measurements of the polarized component of canopy reflectance Rq show no significant 
evidence of leaf pigment absorption. Estimates of the Degree of Linear Polarization (Rq/ Rl) do vary with wavelength 
only because Rj varies with wavelength. The half phase angle, the angle of incidence of the sunlight on the leaf, 
explains much of the variation of the canopy polarized reflectance as a function of angle. The magnitude of the 
polarized reflectance (but not the degree of linear polarization) is generally less than 1.5%. 
Polarized reflectance is potentially informative for classifying plant species and variety, detecting heading and flowering 
plant development stages, estimating leaf area index and number of plants per m2 and improving estimates of NDVI 
III-ATMOSPHERIC EFFECTS IN POLARIZED LIGHT 
III.A. Generality 
Let us consider now the polarization properties of the atmosphere, which result from scattering by molecules, aerosols 
and cloud particles. 
Molecular scattering is the main mechanism which explains the polarization of skylight observed in the blue spectral 
region during clear conditions. As the molecular scattering process is well understood, its influence may be taken into 
account when interpreting polarization measurements. Moreover, because the molecular scattering efficiency varies 
according to the law, the magnitude of the resulting polarized light, although large at blue wavelengths, decreases 
significantly toward near infrared wavelengths where the dominant polarization effects are due to aerosols and/or clouds.