The Fluorescence Explorer (FLEX) mission has been selected as ESA's 8th Earth Explorer mission. The primary objectives of the mission are to provide global estimates of vegetation fluorescence, actual photosynthetic activity, and vegetation stress. FLEX will fly in tandem formation with Sentinel-3 providing ancillary data for atmospheric characterization and correction, vegetation related spectral indices, and land surface temperature. The purpose of this manuscript is to present its scientific payload, FLORIS, which is a push-broom hyperspectral imager, flying on a medium size platform. FLORIS will measure the vegetation fluorescence in the spectral range between 500 nm and 780 nm at medium spatial resolution (300 m) and over a swath of 150 km. It accommodates an imaging spectrometer with a very high spectral resolution (0.3 nm), to measure the fluorescence spectrum within two oxygen absorption bands (O 2A and O 2B ), and a second spectrometer with lower spectral resolution to derive additional atmospheric and vegetation parameters. A compact opto-mechanical solution is the current instrument baseline. A polarization scrambler is placed in front of a common dioptric telescope serving both spectrometers to minimize the polarization sensitivity. The telescope images the ground scene onto a double slit assembly. The radiation is spectrally dispersed onto the focal planes of the grating spectrometers. Special attention has been given to the mitigation of stray-light which is a key factor to reach good accuracy of the fluorescence measurement. The absolute radiometric calibration is achieved by observing a dedicated Sun illuminated Lambertian diffuser, while the spectral calibration in flight is performed by means of vicarious techniques. The thermal stabilization is achieved by using two passive radiators looking directly to the cold space, counterbalanced by heaters in a closed loop system. The focal planes are based on custom developed CCDs. The opto-mechanical design is robust, stable vs. temperature and easy to align. The optical quality is very good as recently demonstrated by the latest tests of an elegant breadboard. The scientific data products comprise the Top Of Atmosphere (TOA) radiance measurements as well as fluorescence estimates and higher-level products related to the health status of the vegetation addressing a wide range of applications from agriculture to forestry and climate.

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The FLuorescence EXplorer (FLEX) European Space Agency (ESA) mission with its payload, the FLuORescence Imaging Spectrometer (FLORIS), aims at performing quantitative measurements of the solar induced chlorophyll fluorescence from space, with the purpose of improving the monitoring of the health of Earth vegetation. The retrieval of a faint signal, such as the one from chlorophyll fluorescence, requires very low Stray-Light (SL) levels. The SL reduction in FLORIS implied constraints impacting the design, by means of using low roughness optical components, the integration, through a strict contamination control, but also the data processing with the need of a very accurate correction strategy making use of numerical models well correlated with experimental data, to be acquired during the On Ground Calibration (OGC) activities. In order to assess and validate the correction strategy, an accurate SL characterization has been anticipated during the FLORIS Optical Model Refurbished (OMR) campaign. Different methods, such as out-of-field and out-of-band measurements, have been investigated in order to avoid the detector blooming affecting measurements with high input signals. By combining the results from the different approaches, it has been possible to achieve up to 9-10 decades of explored dynamic range. The model, correlated with measurements, has finally proved to be capable to correct SL with a reduction factor >10, down to a level less than 40% of the required fluorescence error (10% of a fluorescence level of about 2 mW/m²/sr/nm).

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