Development of an automated hyperspectral system to monitor hyperspectral reflectance and sun-induced chlorophyll fluorescence with directional and hemispheric view geometries simultaneously

Simultaneous observations of directional and hemispherical reflectance and emission are valuable to understand canopy structural and physiological dynamics. However, employing both observation geometries concurrently to measure either reflectance or sun-induced chlorophyll fluorescence (SIF) remains challenging and therefore has rarely been explored with continuous observations. Here, we designed and implemented the enhanced RotaPrism system, an automated instrument that integrates a hyperspectral sensor for reflectance measurements and a hyperspectral sensor with much higher spectral resolution for SIF retrieval, enabling monitoring of both hemispherical and directional viewing geometries. We installed this system at a rice paddy and a mixed forest site, collecting spectral data continuously throughout the growing season. We assessed instrumental uncertainty through quality control protocols designed for our rotating system and evaluated radiometric uncertainty by analyzing radiometric consistency between the two sensors. Quality control metrics showed high pass rates, demonstrating the reliability of our system. The two spectrometers maintained high consistency in spectral data throughout the growing season. Our system revealed notable differences between the two viewing geometries at both sites regarding the seasonal variations of spectral reflectance and SIF, likely influenced by differences in canopy structure and spatial heterogeneity of spectral signals. Specifically, hemispherical measurements captured integrated canopy-scale responses including both sunlit and shaded components, while directional observations were more sensitive to the sunlit portion of the canopy. The relative difference between hemispherical and directional measurements varied systematically with vegetation phenology and canopy density. Our findings underscore the reliability and stability of the system for continuous monitoring of vegetation spectral variations using the two geometric view configurations, supporting potential applications in canopy photosynthesis assessment and hyperspectral satellite validations. • Rotating prism system enables SIF and reflectance sensing in two view geometries • Quality control and radiometric tests confirm robust system performance • Seasonal SIF and reflectance differ by view geometries in rice and forest sites