Among direct and diffuse components that compose photosynthetically active radiation (PAR), diffuse component of sunlight is important to evaluate fraction absorbed PAR (FAPAR) of plants because diffuse flux penetrates more deeply than direct flux and the peak of photosynthetic photon flux density (PPFD) use efficiency occurs at low to medium PPFD. Shading distribution in leaf-scale aerial images of plants by low-altitude measurement via UAVs contains sunlight information as well as plant structure and leaf pigments. In this work, we investigate the relationship between statistical properties of leaf-scale images, share of diffuse flux (SDF) in global radiation and solar zenith angles (SZA). Higher-order statistics (HOSs) were calculated from ground-based close-range images of wheat leaves under various sunlight conditions. SDF were measured based on two field spectrometers. We confirmed that (1) images under clear sky can be distinguished from those under cloudy sky based on HOSs, and (2) it is possible to estimate SZA based on HOSs of leaf-scale images under clear sky.
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