An Approach to Improve Leaf Pigment Content Retrieval by Removing Specular Reflectance Through Polarization Measurements

Specular reflectance is an important error source in the remote retrieval of leaf pigment content. However, removing this disturbance is challenging because the specular component cannot be directly separated from the leaf reflectance. In this paper, we removed the specular reflectance through polarization measurements at single leaf scale. First, polarization measurements were taken on leaves in the nadir and an oblique viewing direction. Specular reflectance in these two directions was estimated. Second, leaf specular reflectance was subtracted from the total reflectance. Based on the consequent diffuse reflectance, three common types of vegetation indices (VIs), simple ratio (SR), normalized difference (ND), and red edge position (REP) were built to retrieve pigment content. The results show that: 1) specular reflectance interfered with the retrieval. The greater this component, the more serious it caused the disturbance. The retrieval accuracy in forward 35° was much lower than that in the nadir. 2) After specular reflectance removal, the retrieval in both directions was generally improved for the SR and the ND VIs. The improvement in forward 35° was remarkable, with accuracy reaching or approaching the corresponding nadir level. On the two pigments, improvement for chlorophylls was more apparent than for carotenoids. 3) Of all VIs investigated, the REP was insensitive to specular reflectance and had the best resistance capability. Overall, the proposed methods can effectively eliminate specular interference and improve pigment content retrieval. Even under disadvantageous circumstances with strong specular reflectance, the improvement was still reliable.