Sensitivity of Sun-Induced Fluorescence to Spectral Stray Light in High Resolution Imaging Spectroscopy

Abstract:

Remote sensing of solar-induced chlorophyll fluorescence (SIF) is advancing our capacity to monitor vegetation productivity. Because the solar-induced fluorescence (SIF) emission is usually < 5 % of recorded canopy radiance, sources of measurement error must be reduced for accurate SIF retrieval. Stray light—the unwanted scattered light from the optical and mechanical elements of the instrument—is a source of error in all gratings-based spectrometers. The effect of stray light on SIF estimation (from retrieval algorithms) is unknown for most retrieval methods. We derived the FLD solution in the presence of stray light under the simplifying assumptions that SIF, reflectance, and stray light are constant across the region of the spectrum used in the retrieval. We show that stray light contributions from the canopy and reference spectra are present in the standard FLD solution. In practice, the simplifying assumptions are unlikely to hold because SIF and reflectance vary across regions of the spectrum commonly exploited by the FLD retrieval. To quantify the impact of spectral stray light on SIF retrieval without relying on simplifying assumptions, we performed a sensitivity analysis. We tested how varying magnitudes of spectral stray light affected multiple retrieval algorithms using simulated spectra derived from real measurements and an empirically derived stray light distribution function. When spectral stray light was one order of magnitude smaller than the radiance signal, mean bias was 7.3 % for 3FLD retrievals applied to Fraunhofer lines, and < 3 % for O₂-A band retrievals. However, when spectral stray light was three orders of magnitude smaller than the radiance signal, mean bias was < 2% across all FLD and 3FLD retrievals. We compare the magnitude of bias due to stray light with the magnitude of the biological signal in previous SIF studies, and discuss considerations regarding acceptable levels of stray light.