Abstract
Reflectance anisotropy is a signal that contains information on the optical and structural properties of a surface and can be studied by performing multi-angular reflectance measurements that are often done using cumbersome goniometric measurements. In this paper we describe an innovative and fast method where we use a hyperspectral pushbroom spectrometer mounted on a multirotor unmanned aerial vehicle (UAV) to perform such multi-angular measurements. By hovering the UAV above a surface while rotating it around its vertical axis, we were able to sample the reflectance anisotropy within the field of view of the spectrometer, covering all view azimuth directions up to a 30° view zenith angle. We used this method to study the reflectance anisotropy of barley, potato, and winter wheat at different growth stages. The reflectance anisotropy patterns of the crops were interpreted by analysis of the parameters obtained by fitting of the Rahman-Pinty-Verstraete (RPV) model at a 5-nm interval in the 450–915 nm range. To demonstrate the results of our method, we firstly present measurements of barley and winter wheat at two different growth stages. On the first measuring day, barley and winter wheat had structurally comparable canopies and displayed similar anisotropic reflectance patterns. On the second measuring day the anisotropy of crops differed significantly due to the crop-specific development of grain heads in the top layer of their canopies. Secondly, we show how the anisotropy is reduced for a potato canopy when it grows from an open row structure to a closed canopy. In this case, especially the backward scattering intensity was strongly diminished due to the decrease in shadowing effects that were caused by the potato rows that were still present on the first measuring day. The results of this study indicate that the presented method is capable of retrieving anisotropic reflectance characteristics of vegetation canopies and that it is a feasible alternative for field goniometer measurements.
Highlights
Natural surfaces reflect light anisotropically, which means that the reflected radiance varies with viewing and illumination geometry
In the red band (650 nm), where shadows are darkestasdue to absorption of radiance by chlorophyll, the lowest anisotropy factor (ANIF) were observed in the forward darkest due to absorption of radiance by chlorophyll, the lowest ANIFs were observed in the forward scattering direction and the highest ANIFs in the backward scattering direction (Figure 4b,e)
We presented an innovative and fast method in which we captured reflectance anisotropy by extracting the multi-angular views that are collected by the HYMSY system
Summary
Natural surfaces reflect light anisotropically, which means that the reflected radiance varies with viewing and illumination geometry. Reflectance anisotropy is commonly studied by performing multi-angular reflectance measurements using goniometers in laboratories under controlled conditions e.g., [12,13,14,15] or in the field under natural conditions e.g., [16,17,18,19,20,21]. Both laboratory and field goniometer measurements have their advantages and disadvantages [22]. A drawback of both laboratory and field goniometer measurements is that they are cumbersome and time consuming
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