A New Method to Estimate the Leaf Chlorophyll Content From Multiangular Measurements: Anisotropy Index

Anisotropy index (ANIX), which is defined as the ratio between the maximum and minimum reflectance factors in the principal plane, has been applied in characterizing the optical properties of vegetation, but it is seldom used to estimate the chlorophyll content in leaf level. In this study, we found the leaf spectral ANIX depended on the variation of leaf chlorophyll content (LCC). Newly proposed indices, [ASRI:
$A_{750}/A_{720}$
and anisotropy normalized difference index (ANDI): (
$A_{750} - A_{720}$
)/(
$A_{750}$
+
$A_{720}$
), A is the modified ANIX (mANIX = ANIX −1)], were derived from the format of the existing reflectance-based vegetation indices: simple ratio (SR) and ND indices. The diffuse reflection, closely related to LCC, can be extracted from the two new indices [or directly obtained as the minimum bidirectional reflectance factor (BRF)]. They have strong linear relationships with LCC (
$R^{2}=0.88$
and 0.89) and have good estimation accuracy using an independent dataset (RMSE = 6.39 and
$6.10 \mu \text{g}$
/cm2). We also found that the ASRI (ANDI) had similar LCC estimation accuracy as the range-ASRIs or range-ANDIs (RANDIs or RANDIs), which were calculated by the random combination of reflectance factors in the forward scattering directions (ranged from −30° to −50°) and near the backward scattering directions (ranged from 0° to 20°). The wide and effective ranges of viewing angles strengthen the usability of ASRI and ANDI to estimate LCC in the practical measurements.