## 5 Summary and Discussion  
Within the last decade, research on F has intensified. The announcement of FLEX as the next ESA Earth Explorer mission [157] has triggered the development of dedicated instrumentation and protocols. These advancements have improved the reliability of spectroradiometric measurements and eventually enabled the measurement of subtle F signals with well-calibrated and temperature-stabilized [38] this issue state-of-the-art spectroradiometers for operation at the leaf and canopy level. Still, highly reliable radiance measurements of F require not only further technological development, but also the sufficient precision of measurements, correct setups, and well-postulated measurement protocols. The current trend of instrumentation is moving toward a higher spectral resolution and a high signal-to-noise ratio (SNR). While spectroradiometers with an FWHM of a few nanometers may allow one to estimate F760, only actively cooled sensors (enabling high SNR at a very high spectral resolution (FWHM \(<\) 1 nm), e.g., the QE Pro spectroradiometer (OceanOptics, Inc., Dunedin, FL, USA) offer the ability to retrieve F in both atmospheric oxygen bands (i.e., F760 and F687). Point spectroradiometers with such a configuration are used for ground- and tower-based measurements as well as UAV setups. Imaging systems with a very high resolution and SNR are barely available. This poses particular challenges when investigating the spatial F dynamics. Although some measurement setups are yet able investigate different plant functional types, e.g., broadleaf forest and grassland pasture at the Majadas research site (Spain), most of the current setups monitor only one vegetation type. In addition to UAV systems, installations similar to field phenotyping platforms (e.g., [158; 159]) could overcome this issue by their ability to repeatedly measure multiple points in the field in an automated fashion. Canopy F emissions are modulated by several external environmental factors, such as incoming PAR, air temperature, and wind, in combination with plant-specific traits, e.g., canopy chlorophyll content and architecture and the actual status of physiological processes [72]. Hence, more studies investigating the spatial distribution of F for multi-species canopies and research aiming to link temporal continuous measurements with spatial continuous measurements are needed. Such studies must consider all issues related to the comparability of imaging and non-imaging data and different reflectance factors (hemispherical-conical vs. hemispherical-directional), such as those outlined in [13; 80]. Standardization of measurement protocols has been mostly advanced by standardizing F-measuring systems, resulting in systems such as the FluoWat, FloX, and Piccolo. Also, measurement setups have become more robust. The current continuous ground-based measurements enable investigations of F temporal dynamics under real environmental conditions. This is important for not only getting a deeper insight into plant physiological processes but also understanding temporally discrete F observations, such as those from airborne platforms and current and future spaceborne missions. Temporally continuous measurement setups should be extended with auxiliary information, which is necessary to interpret the F signal correctly. It is important to mention that current continuous measurements usually cover only a small area (most of the time, only one discrete point in space). Larger areas could be characterized by discrete overflights of airborne systems, but carrying out an airborne campaign requires more efforts and planning compared with fixed-ground and tower-based installations. Moreover, one needs to think about the extrapolation of airborne observations through time. Regarding flexibility, leaf clip F measurements on single leaves are easy to carry out since the logistics and organizational efforts are low. Yet, they require a lot of human interactions and provide only a limited number of measurements at a given time.