LOIS GRANT, Purdue University


This study is based upon the hypothesis that, through measurement of polarized reflectance, leaf reflectance may be separated into two components: (1) a non-polarized, diffuse component which emanates from the interior of the leaf; and (2) a polarized component which is derived from the surface of the leaf. In order to test this hypothesis, leaf reflectance of a number of plant species was measured with a portable polarization photometer which allows non-destructive measurements of leaf reflectance at the Brewster angle in six wavelength bands. It was found that all leaves had at least some polarized reflectance. The polarized component of reflectance was independent of wavelength demonstrating that polarized reflectance can be attributed to reflectance at the leaf surface. Differences in polarized reflectance among species could be explained by variation in surface features. There appeared to be two mechanisms responsible for the polarized reflectance: (1) specular reflectance and (2) Mie scattering. Attributing the polarized reflectance to either specular reflectance or Mie scattering required knowledge of the optical dimensions of features on the leaf surface. It was also found that there is no one, constant value of polarized reflectance for a species. But the polarized reflectance may vary from leaf to leaf throughout the growing season. These differences in polarized reflectance may be attributed to variations in details of the features of the leaf surface caused by ontogenic development and environmental conditions. The non-polarized component of reflectance was dependent on wavelength and can be attributed to reflectance from the internal structure of the leaf and, in some cases, to light scattered by Rayleigh-sized particles of the leaf surface. The polarized and non-polarized components of leaf reflectance may be viewed as independent sources of information about the identity and condition of the leaf. Information relevant to the leaf surface will be contained in the polarized component of reflectance; information related to the internal structure of the leaf is manifest in the non-polarized component of reflectance.



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