Reducing BRDF Effects on the Estimation of Leaf Biochemical Parameters Using the Nonpolarized Reflectance Factor in the Hemispheric Space

Leaf BRDF (bidirectional reflectance distribution function) is partly controlled by the specular reflection from the leaf surface, which does not convey information about leaf biochemical parameters. Therefore, vegetation remote sensing can benefit from understanding the effects of BRDF on leaf biochemical parameters estimation and attempting to reduce them. In this study, polarimetric measurements, which simultaneously derive intensity and polarization information, were taken on the leaves of four plant species in the hemispheric space. The results showed that there are distinct BRDF effects on leaf biochemical parameters (leaf chlorophyll content (LCC) and equivalent water thickness (EWT)) estimated using spectral indices based on the intensity (IpRF: I parameter reflectance factor) reflected from leaves. The relationships between four types of spectral indices (single reciprocal, difference between two wavelengths or two reciprocal, simple ratio, and normalized difference indices) and leaf biochemical parameters were found to be weak and dependent on the BRDF of leaves, especially in the forward scattering directions, which are dominated by the specular reflection. After separating the bidirectional polarized reflectance factor (BPRF) from the reflectance intensity (IpRF), the non-polarized reflectance factor (Rn-p) significantly reduces the BRDF effects on biochemical parameters estimation near the specular reflection directions, and a relatively improved estimation is found in either individual viewing directions or over all viewing directions. This study demonstrates that polarimetric measurements are able to reduce BRDF effects on leaf biochemical parameters estimation, and therefore are useful to improve the leaf biochemical parameters estimation using spectral indices in the hemispheric space.