Direct quantification of solar-induced chlorophyll fluorescence using compact solar-blind optical radiometers

Jonas Kuhn , Jochen Stutz

Remote sensing of solar-induced chlorophyll fluorescence (SIF) provides a non-invasive, quantitative measure of plant photosynthetic activity, linking leaf-level physiology to canopy and ecosystem behavior and the global carbon cycle. Current SIF measurements rely on hyperspectral retrievals of the weak fluorescence signal from small changes in Fraunhofer lines or atmospheric absorption features in plant or canopy reflectance spectra. Because this approach is dependent on atmospheric and illumination conditions, it relies on bulky and costly instrumentation, while complex retrieval algorithms demand atmospheric spectroscopy expertise. These limitations restrict widespread proximal SIF remote sensing applications, and contribute to critical observational gaps, highlighting the need for a simplified measurement approach.
We introduce a fundamentally different approach to proximal SIF remote sensing: by measuring the light intensity within a saturated atmospheric O2 line (ca. 10 pm spectral width) we implement a solar-blind radiometer (SBR) that optically isolates SIF from reflected solar radiation. Calculations show that SBR-SIF instruments can be implemented using a Fabry-Pérot interferometer in double-pass configuration. Plant measurements with our prototype confirm the theoretical calculations and provide direct SIF measurements with a precision of 0.1 mW m-2 sr-1 nm-1 in 3 minutes, similar or higher than conventional techniques. SBR-SIF is independent of atmospheric and illumination conditions, requires no spectral retrieval or reference measurement, and enables compact, field-deployable instrumentation. Consequently, SBR-SIF enables scalable proximal SIF measurements that can advance our understanding of physiological processes, support validation of satellite observations, and expand SIF applications in ecosystem monitoring and precision agriculture.