570 
amenable to description by simple models when possible. Finally, we will address the question of polarization 
measurements from space and examine what should be the interest of such measurements for remote sensing purposes. 
II-POLARIZATION BY VEGETATION CANOPIES 
II.A. Polarization by Leaves: 
II.A.l. Leaf Surface Morphology. The light reflecting and polarizing properties of the leaf surface are 
determined by the index of refraction and optical roughness properties of the plant cuticle, an extracellular, multi 
layered membrane of pectin, cellulose, cutin, and wax which forms a continuous and protective skin at the air-plant 
interface (Martin and Juniper 1970). Trichomes, outgrowths such as hairs from epidermal cells, are covered by the 
cuticle and affect the surface morphological properties. 
The outermost portion of all cuticles consists of epicuticular wax. The wax may be amorphous, semicrystalline, or 
crystalline in form and exhibits geometric configurations ranging from simple rods to complex dendrites (Hall et al. 
1965). Electron micrographs of a leaf surface often reveal small, crystalline wax structures such as found on Sorghum 
bicolor L. (sorghum), small flakes 0.1-0.16 mm thick and 0.21-1.58 mm in diameter and wax filaments 0.5-1.25 mm 
in diameter and up to 140 mm long (Atkin and Hamilton 1982a, b). These semicrystalline or crystalline wax 
microscale structures, having dimensions sometimes smaller than the wavelength of light, are arrayed on a 
comparatively smooth amorphous wax substrate which undulates on a scale of perhaps 1000 wavelengths (Hull et al. 
1978; Sargeant 1983). 
The morphology of trichomes and both the cuticle and the epicuticular wax provide most of the surface detail of plant 
tissue on a microscopic and optical scale, making the leaf surface an optically rough surface. The light reflecting and 
light polarizing properties of this surface depend on its roughness properties. 
II.A.2. Surface Reflectance of Leaves. Leaf reflectance increases with increasing angles of incidence (Gates 
and Tantaporn 1952), possibly due to the effects of specular reflection (Shul'gin and Khazanov 1961). Leaf reflectance 
is intermediate between that of a perfectly diffuse and a perfectly specular reflector (Breece and Holmes 1971, Brakkeet 
al. 1989, Sarto et al. 1989, Waiter-Shea and Norman 1989), which suggests it is the sum of diffuse and specular 
components. The diffuse component is non-polarized, varies little with changing angles and emanates from the 
interior of the leaf (Shul'gin and Khazanov 1961, Shul'gin and Moldau 1964). The non-diffuse componentis 
polarized, emanates from the leaf surface and is spread about the specular direction (Rvachev and Guminetskii 1966, 
Vanderbilt et al. 1985a). The leaf surface roughness determines the angular spread of the polarized, specular reflectance 
lobe because leaves appear capable of specularly reflecting light in all directions (Rvachev and Guminetskii 1966, 
Woolley 1971, Vanderbilt et al. 1985a). Qinglin et al. (1990) showed the surface reflection of the leaves of three 
species could be described using Kirchhoff rough surface scattering theory. 
The amount of light specularly reflected by a leaf varies with species (Grant et al. 1983, McClendon 1984, Grant et al. 
1987a, Grant et al. 1987b, Grant et al. 1993). Visibly shiny leaves tend to have higher specular reflectance than matte 
leaves (Shul'gin and Khazanov 1961), though leaves which appear to have no shiny appearance can still specularly 
reflect light (McClendon 1984). Leaves with sparsely distributed hairs can specularly reflect more light than glabrous 
leaves and some highly pubescent leaves may be strong specular reflectors (McClendon 1984). Thus the presence or 
absence of hairs does not serve as a predictor of specular reflectance. 
II.A.3. Polarized Reflection: Leaf Surface vs. Leaf Interior. One might assume that the various optical 
properties of a leaf should depend upon both its surface and interior, probably not upon the leaf surface exclusively. 
Yet there may be an exception. In a survey of 18 plant species which included agronomic and weed species, fores! 
species, vegetable species and potted plant species, the linear polarization of the light reflected by individual leaves, 
measured in situ , 55° from normal, was found to depend only on the leaf surface properties (Grant et al. 1987a, Grant 
et al. 1993). 
Unlike previous investigations which emphasized angular effects, Grant et al. (1987a and 1993) investigated the 
polarized reflecting properties of leaves as a function of wavelength. Their results (Figs. 1A and IB) show the linearly 
polarized part Rp of the leaf reflectance factor in the visible and near-infrared wavelength regions appears unaffected by 
cellular pigments in each leaf. Statistical tests show there is no significant change in Rp with wavelength; spectrally, 
Rp is flat. The results (Figs. 1C and ID) show the non-polarized part Rpq of leaf reflectance varies spectrally 
according to the constituent pigments in each leaf. How can light reflected by a leaf not display at least some evidence 
of interaction with leaf pigments, especially pigments with a spectral presence so evident in Rn? The answer appears 
to be that the polarized part of the reflected light never entered the leaf tissue to interact with the leaf pigments. These 
results support the hypothesis that Rp represents light reflected at the leaf surface. 
The degree of polarization, Rp/(Rp+R>j), includes information from two dissimilar sources, a fact which complicates 
data interpretation. The numerator Rp contains information about the leaf surface and is a leaf-dependent constant 
(Figs. 2A and 
leaf surface, ai 
simplify data 
polarization. 
Fig. 1. 
adaxial 
approx 
variabl 
The results (F 
absorption sp 
spectral regioi 
Grant conclud 
appears to be 
reflected by a 
originates at t 
primarily, but 
absorbing me 
properties of t 
1I.A.4. Othe\ 
the semicryst; 
Grant et al. (1 
no evidence t 
measured spe 
a high densit’ 
bluish surface 
II.B. Polar 
II.B.1. Intro 
borne sensor« 
the plant can 
polarizing pr