Spectroradiometric calibration and temperature sensitivity analysis of a fluorescence box

The fluorescence box (FloX) is a hyperspectral instrument designed for unattended observation of sun-induced chlorophyll fluorescence (SIF) and reflectance. FloX instruments are being used as part of a validation network for the upcoming Fluorescence Explorer imaging satellite. Hence, these devices are expected to deliver continuous observations with high accuracy. We perform the spectroradiometric calibration and temperature sensitivity analysis of a FloX unit, which comprises two spectrometers. We present the results of a thermal cycling test of the FloX using a refrigerated incubator, spanning temperatures from 40°C to 0°C in steps of 5°C, during which the instrument's optical fibers used an access port to carry out external measurements of the reflected radiance from a Spectralon panel illuminated by a tungsten lamp. The mean radiance values within the SIF spectral range exhibited changes of over 3% for the high-resolution sensor and over 4% for the broadband sensor, relative to the nominal radiance values at 20°C. We examine the contributions to the combined standard uncertainty to show how these radiometric shifts are a systematic effect of changing environmental temperature.