Sensitivity of solar-induced fluorescence to spectral stray light in high resolution imaging spectroscopy

Remote sensing of solar-induced chlorophyll fluorescence (SIF) advances our ability to monitor gross primary productivity. Because SIF is usually <5% of recorded canopy radiance, sources of measurement error must be reduced for accurate SIF retrieval. Here we quantify the impact of spectral stray light on SIF retrieval using a high-resolution imaging spectrometer. We first derived the FLD retrieval 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 could cancel out if obtained from the same instrument. In practice however, the simplifying assumptions are unlikely to hold because SIF and reflectance vary across regions of the spectrum commonly exploited by the FLD retrieval, and stray light can also vary across the spectrum, as we demonstrate with an empirical example. To quantify the impact of spectral stray light on SIF retrievals in more realistic scenarios, we performed a sensitivity analysis. We used a stray light signal distribution function and measurements of spectral radiance and SIF to generate spectra with known quantities of SIF and stray light over four orders of magnitude of stray light. We then quantified the bias in retrieved SIF using four SIF retrieval approaches-standard FLD, 3FLD, a spectral fitting method (SFM), and a method based on singular value decomposition (SVD)-applied to the red and/or far-red spectral domains. We found that spectral stray light can cause either a positive or negative bias in SIF estimates. In the highest stray light scenario, with spectral stray light one order of magnitude smaller than the radiance signal, the stray light error ranged from < +/- 1% of SIF (for O2-A band retrievals with FLD, 3FLD, and SFM methods, and for far-red retrievals in the 748 nm to 756 nm domain with SVD-based and 3FLD methods applied to Fraunhofer lines) to >30% of SIF (O2-B band retrieval with SFM, and 3FLD applied to Fraunhofer lines in the red domain). For context, a bias of 27% of SIF is a comparable magnitude to seasonal variation in SIF observed in the Amazon basin, and to SIF differences among plant functional types. However, when spectral stray light was three orders of magnitude smaller than the radiance signal, mean bias was < +/- 2% for all retrieval methods. Our analysis informs when spectral stray light corrections must be performed for SIF remote sensing.