## 1 Introduction  
Imaging and non-imaging spectroscopy enables the detailed assessment of biochemical, structural, and even functional attributes of vegetation in natural environments at multiple scales [1; 2]. It has now been performed for more than 30 years [3; 4]. During this period, the technologies for spectroradiometric measurements have greatly advanced. Today, many high spectral resolution spectroradiometers (often used synonymously with spectrometer, although the latter is radiometrically calibrated) are available, and they are operated from different measuring platforms: (1) ground-based at the leaf level [5; 6; 7]; (2) ground-based at the canopy level, either carried by people or mounted on towers and other platforms [8; 9; 10; 11]; (3) airborne-based at the canopy level, flown at low-altitudes on, for example, UAVs [12; 13; 14; 15; 16; 17; 18]; (4) at high altitudes by aircraft [19; 20; 21]; and (5) spaceborne-based [22; 23; 24; 25]. This diversity provides the opportunity to monitor the dynamics of vegetation on different spatial and temporal scales. With advancements in sensor technology, these instruments can also measure fine spectral features. An example is sun-induced chlorophyll fluorescence (F), which is retrieved by exploiting telluric oxygen absorption bands (i.e., at 685 nm and 760 nm) [19] and single Fraunhofer lines [26; 27]. F emitted by chlorophyll-a molecules is one of three main de-excitation mechanisms for the energy captured by the light-harvesting pigments of plants. Under natural conditions, F represents only a few percent of the reflected radiance. Thus, detecting F is particularly challenging. Chlorophyll fluorescence has been widely used as a probe for photosynthesis at the level of chloroplasts, leaves, and plants in the laboratory and field studies (for reviews cf. [28; 29]). In the vast majority of these studies, an active light source was used to induce fluorescence, limiting the application of these active methods to setups in which close contact with the leaf is possible. However, there have been a few attempts to use these active approaches with a distance to the canopy of several meters [30; 31; 32; 33]. The ability to detect F with passive instruments largely extended the distance range and opened new possibilities to monitor functional vegetation dynamics at the ecosystem, landscape, or even global scale. Measurement setups and protocols for ground-based optical measurements have been already homogenized, e.g., for European flux sites [34], within the international SpecNet consortium [35], and for phenology monitoring [36; 37]. F measurement protocols, however, require particular consideration to prevent the misinterpretation of retrieved F signals due to its sensitive behavior in response to environmental factors and challenges related to the retrieval of this small signal. Furthermore, the very dynamic nature of F compared with other common vegetation traits derived from spectral data (e.g., chlorophyll content, leaf area index) makes the measurement setup and post-processing even more challenging, and it requires the careful consideration of environmental, physiological, structural, instrumental, and experimental aspects during radiometric measurements and F retrievals (Figure 1). Additionally, measurements of F at different levels (e.g., leaf and canopy) and scales (single leaf to landscape) require different protocols and attempts to link such cross-scale measurements. This also includes considerations given to necessary meta- and ancillary data. Figure 1: Environmental, physiological, physical, instrumental, and setup considerations at the leaf and canopy level for the acquisition, quality check, post-processing, retrieval, and interpretation of the sum-induced fluorescence (F) signal (FS; photosystem). Meeting these challenges requires not only reliable measurement setups, sophisticated measurement protocols, and robust F retrieval methods but also properly calibrated and characterized instruments.