## 1 Introduction  
Hyperspectral airborne imaging for applications in plant science has a long history and started with the AVIRIS sensor in 1987, which was built by the Jet Propulsion Laboratory of NASA (2016). To date, several comparable sensors have also been developed and used extensively, such as HyMap (HyVista Corp., Australia), APEX (University of Zurich, Switzerland, and Flemish Institute for Technological Research (VITO), Belgium), CASI (ITRES Research Ltd., Canada), Aisa (Specim Ltd., Finland) and HySpex (Norsk Elektro Optikk, Norway). These sensors more or less cover the spectral range from 400-2500 nm with a full width at half maximum (FWHM) of a few nanometers for each single band. Image data acquired by the above-mentioned sensors are very useful because they enable cost-effective and non-destructive assessment of structural (e.g., leaf area index (LAI) and biomass) and biochemical (e.g., pigment and water content) vegetation properties (e.g., (2016; 2017; 2018)). In addition to reflecting light, plants also emit a small amount of light in the red and near-infrared spectral domain from 670-780 nm. This signal is known as sun-induced chlorophyll fluorescence (SIF). Since SIF is emitted from the core of the photosynthetic machinery, it provides information directly linked to photosynthetic efficiency (2017; 2018). SIF, therefore, adds complementary information to reflectance-based optical spectroscopy. The complexity of the signal, however, still causes several open points, including the scale dependency of SIF, full understanding of its information content and the way forward to mechanistically exploit SIF for advanced predictions of ecosystem photosynthesis and related gas-exchange processes. In recent decades, several devices have been developed to measure SIF on leaves at ground (e.g., FluoWat (2018)) and canopy level (e.g., FLoXBoX (2018)). In addition, data from different atmospheric chemistry satellites, such as GOSAT (2018; 2018; 2018), OCO-2 (2018), GOME-2 (2018) and TROPOMI (2018; 2019), have been used to derive SIF at different spatial and temporal scales. These studies have already demonstrated the potential of satellites to measure SIF from space and the importance of this information to better understand the photosynthetic functioning of vegetation. To explore this potential further, the European Space Agency (ESA) selected the FLuorescence Explorer (FLEX) in 2015, the first satellite solely designed to measure SIF on a global scale, to become ESA's Earth Explorer 8. FLEX will be operated in a tandem constellation with Sentinel-3 and will deliver landscape- and ecosystem-scale data with a spatial resolution of 300 m (2018). Besides ground- and space-borne platforms, unmanned aerial vehicles (UAVs) and classical aircraft also have the potential to carry sensors measuring SIF at an intermediate spatial scale to close the gap between proximal and satellite measurements. Due to technical limitations, to date, only a few studies have demonstrated the retrieval of SIF from UAV platforms using point spectrometers and small imaging devices (e.g., (2018; 2018; 2019)). In contrast, airborne-based imaging spectroscopy already provides critical infrastructure to bridge ground- and satellite-based knowledge and technology to detect SIF on canopy scale. In this context, the AirFLEX line scanner was the first airborne instrument used to measure SIF in different campaigns between 2005 and 2008 (2018). The potential of other imaging spectrometers, such as ROSIS (2018), CASI-1500 (2018), HYPER (2018) and APEX (2018), was also investigated to measure SIF from airborne platforms. The spectral resolutions of these instruments (2.2-7.0 nm), however, were sub-optimal for SIF retrieval, and only facilitated relative proxies of fluorescence (2018).