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1. A METHOD FOR UNCERTAINTY ASSESSMENT OF PASSIVE
SUN-INDUCED CHLOROPHYLL FLUORESCENCE RETRIEVAL
USING AN INFRARED REFERENCE LIGHT
ABSTRACT
Measurements of sun-induced chlorophyll fluorescence (SIF) over plant canopies
provide a proxy for plant photosynthetic capacity and are of high interest for plant research.
Together with spectral reflectance, SIF has the potential to act as a noninvasive approach to
quantify photosynthetic plant traits from field to air and spaceborne scales. However, SIF is a
small signal contribution to the reflected sunlight and often not distinguishable from sensor
noise. SIF estimation is, therefore, affected by an unquantified uncertainty, making it difficult to
estimate accurately how much SIF is truly emitted from the plant. To investigate and overcome
this, we designed a device based on a spectrometer covering the visible range and equipped it
with an LED emitting at the wavelength of SIF. Using this as a reference and applying thorough
calibrations, we present consistent evidence of the instrument’s capability of SIF retrieval and
accuracy estimations. The LED’s intensity was measured under sunlight with 1.27±0.27
mW×sr−1m−2nm−1 stable over the day. The large increase of SIF due to the Kautsky effect was
measured spectrally and temporally proving the biophysical origin of the signal. We propose
rigorous tests for instruments intended to measure SIF and show ways to further improve the
presented methods.