Improving the accuracy of SIF quantified from moderate spectral resolution airborne hyperspectral imager using SCOPE: assessment with sub-nanometer imagery

Hyperspectral imaging of solar-induced chlorophyll fluorescence (SIF) is required for plant phenotyping and stress detection. However, the most accurate instruments for SIF quantification, such as sub-nanometer (≤1-nm full-width at half-maximum, FWHM) airborne hyperspectral imagers, are expensive and uncommon. Previous studies have demonstrated that standard narrow-band hyperspectral imagers (i.e., 4–6-nm FWHM) are more cost-effective and can provide far-red SIF quantified at 760 nm (SIF₇₆₀), which correlates strongly with precise sub-nanometer resolution measurements. Nevertheless, narrow-band SIF₇₆₀ quantifications are subject to systematic overestimation owing to the influence of the spectral resolution (SR). In this study, we propose a modelling approach based on the Soil Canopy Observation, Photochemistry and Energy Fluxes (SCOPE) model with the objective of enhancing the accuracy of absolute SIF₇₆₀ levels derived from standard airborne hyperspectral imagers in practical settings. The performance of the proposed method was evaluated using airborne imagery acquired from two airborne hyperspectral imagers (FWHM ≤ 0.2-nm and 5.8-nm) flown in tandem on board an aircraft that collected data from two different wheat and maize phenotyping trials. Leaf biophysical and biochemical traits were first estimated from airborne narrow-band reflectance imagery and subsequently used as SCOPE model inputs to simulate a range of top-of-canopy (TOC) radiance and SIF spectra at 1-nm FWHM. The SCOPE simulated radiance spectra were then convolved to match the spectral configuration of the narrow-band imager to compute the 5.8-nm FWHM SIF₇₆₀. A site-specific model was constructed by employing the convolved 5.8-nm SR SIF₇₆₀ as the independent variable and the 1-nm SR SIF₇₆₀ directly simulated by SCOPE as the dependent variable. When applied to the airborne dataset, the estimated SIF₇₆₀ at 1-nm SR from the standard narrow-band hyperspectral imager matched the reference sub-nanometer quantified SIF₇₆₀ with root mean square error (RMSE) less than 0.5 mW/m²/nm/sr, yielding R² = 0.93–0.95 from the two experiments. These results suggest that the proposed modelling approach enables the interpretation of SIF₇₆₀ quantified using standard hyperspectral imagers of 4–6 nm FWHM for stress detection and plant physiological condition assessment.