## 3 Measuring F on the Leaf and Canopy Level
### Canopy Level
#### 3.2.1 Measurement Setups from Proximal to the Airborne Scale  
A variety of spectroradiometers with different fore-optics and control systems are used to retrieve F. While some of the spectroradiometers already mentioned in the review in [40] are still used to measure F, overall, the trend has been toward spectroradiometers with higher spectral resolution. Today, the most used are spectroradiometers with different configurations from Ocean Optics, Inc. (Dunedin, FL, USA) [69]: HR2000+ [8; 11; 46; 47; 70], HR4000 [10; 71; 72; 73; 74], QE Pro [75; 76; 74; 77], and USB4000 [14; 77] (Supplementary Table S2). Ground-based setups that monitor the vegetation fluorescence emission employ several types of instruments that range from \(10^{-1}\) to \(10^{1}\) m above the canopy (Figure 4). Such permanently installed and automated systems can be used to follow the diurnal and seasonal changes in F. One example is the _FloX_ (JB Hyperspectral Devices UG, Dusseldorf, Germany), which is specifically designed to passively measure F under natural light conditions and is equipped with the QE Pro spectroradiometer. The instrument is based on prototypes, such as the Multiplexer Radiometer Irradiometer (MRI) [10], SFLUOR box, and SIF-System [78], developed by a collaboration between the Research Center Julich and the Remote Sensing of Environmental Dynamics Laboratory of the University Milano Bicocca. At the time of writing this manuscript, about 20 FloX instruments have been deployed worldwide. A similar system is the FluoSpex 2. It also consists of two spectroradiometers--a QE Pro and an HR2000+. A network of such systems called FluoNet has been deployed for various ecosystems, including grassland, forests, and agricultural fields [79]. Another spectroradiometer system (Piccolo) was developed by a group at the University of Edinburgh [17]. This system can also incorporate up to two Ocean Optics spectroradiometers that cover different spectral regions and has a custom-designed double-bifurcated fiber optic assembly. This approach enables the almost simultaneous measurement of L and E by each spectroradiometer. In addition, the Piccolo fiber optic assembly incorporates fore-optics with integral shutters so that there is no light loss at in-line shutter interfaces. There are currently 10 Piccolo systems in use either at fixed installations or mounted on tractors or rotary-wing UAVs. Often, the systems are mounted several meters above the canopy on or close to flux towers for purposes such as the evaluation of the relationship between F and parameters such as gross primary productivity (GPP) [8; 75; 76; 77]. Specialized structures have been developed to measure F from greater heights. One example is the TriFLEX system [11]. It consists of a 21-meter-high crane equipped for F measurements. The crane is installed in the middle of fields dedicated to agricultural research. Owing  
Figure 4: Different ground-based F measurement setups in Grosseto, Italy (top left); Juelich, Germany (top, right); Majadas, Spain (bottom, left); and Washington, DC, USA (bottom, right). to a 24-meter jib and a 100-meter railway, F measurements can be performed for nadir-viewing over a plot of \(100\times 200\) m and a plot of \(60\times 150\) m with field crops. Often, the spectroradiometers of fixed installation systems are shielded from the environment to stabilize their temperature [11; 72]. This is necessary to keep the noise levels, the radiometric response, and especially the spectral features of the spectroradiometer constant [38] (in review, this issue). Additionally, the measurement of the downwelling E is a critical component. Ideally, it should be measured close to the target and exposed to a similar E field and not be shaded or illuminated by other targets. Depending on the setup, this is not always straightforward, especially when cosine diffusers requiring a fixation structure are used.