Evaluation of Prospect Inversion Approaches Using Multiangular Spectral Reflectance Factor of Leaves

PROSPECT is an effective method to retrieve leaf biochemical constituents, such as chlorophyll ( $\text{C}_{\text {ab}}$ ), carotenoid ( $\text{C}_{\text {xc}}$ ), water ( $\text{C}_{\text {w}}$ ), and dry matter ( $\text{C}_{\text {m}}$ ). Several published PROSPECT inversion approaches have been performed using spectral bidirectional reflectance factor (BRF) measured at fixed illumination-viewing geometry. However, considering random leaf orientation, varied solar illumination angles, and wide field of view of the sensor, angular reflection is actually used to retrieve leaf biochemical constituents, which may affect the performance of these inversion approaches. Based on multiangular spectral BRFs of 497 leaves from 13 plant species in the hemispheric space or the principal plane, we evaluated the performance of seven PROSPECT inversion approaches, including PROSPECT, PROREF, sPROCOSINE, PROCWT-S4, PROCWT-S5, PROFED, and PROSED. Significant angular effects were found on the original PROSPECT inversion, which showed low and angle-dependent retrieval accuracy. PROREF, sPROCOSINE, PROCWT-S4, and PROSED improved certain leaf biochemical constituent retrievals compared with PROSPECT. Interestingly, PROCWT-S5 and PROFED improved the retrievals of $\text{C}_{\text {ab}}$ , $\text{C}_{\text {xc}}$ , $\text{C}_{\text {w}}$ , and $\text{C}_{\text {m}}$ . Among seven inversion approaches, PROFED not only produced the best accuracy in retrieving $\text{C}_{\text {ab}}$ ( $R^{{2}}$ = 0.92, RMSEadj = $5.92 \mu \text{g}$ /cm2), $\text{C}_{\text {xc}}$ ( $R^{{2}}$ = 0.46, RMSEadj = $2.86 \mu \text{g}$ /cm2), $\text{C}_{\text {w}}$ ( $R^{{2}}$ = 0.91, RMSEadj = 0.0031 g/cm2), and $\text{C}_{\text {m}}$ ( $R^{{2}}$ = 0.25, RMSEadj = 0.0017 g/cm2) but also was insensitive to viewing angles. These results suggest that PROFED could effectively retrieve leaf biochemical constituents across a wide range of plant species at random illumination-viewing geometry, which is beneficial for ecology and botany studies.