## 3 Measuring F on the Leaf and Canopy Level
### Canopy Level
#### 3.2.2 Measurement Protocols from Proximal to the Airborne Scale  
Under natural outdoor conditions, the illumination changes constantly because the position of the sun changes throughout the diurnal and seasonal cycles and the composition of the atmosphere is highly dynamic. While cumulus clouds are rather visible changes, the unaided eye and human cognitive process might not perceive thin high-altitude cirrus clouds or changes in the aerosol or water content of the atmosphere [96]. Even changes in the air pressure influence the permeability of the atmosphere, and this influences the illumination at the top of the canopy, as well as affecting signal loss and scatter in the target-optics optical path. These changes are wavelength-dependent; they influence both the intensity of the irradiance and the irradiance field and affect the retrieval of biochemical plant parameters from spectral data and fluorescence [89]. Thus, for high-precision radiance measurements, such as the estimation of F, the measurement of the radiance emitted and reflected by the canopy and the irradiance received by it need to be captured nearly simultaneously to estimate a target's spectral response to the illumination conditions present during the time of the acquisition [97]. The irradiance incident upon the target area can be approximated by measuring a diffuse (Lambertian) reflectance reference panel or by using a vertically pointing receptor that displays a cosine response. When only one spectroradiometer is used, this can be facilitated by alternating measurements of the reference and the target with the same fore-optic arrangement (single field-of-view (FOV) setup) or using a splitter and shutter system that switches between two fibers: one looking toward the target, and the other one looking toward a reference panel or a toward the zenith with an attached cosine receptor (dual FOV setup) [74; 98]. The single FOV setup can be used manually or in an automated fashion with, for example, a rotation arm [99] or a moving panel [11; 100]. Examples of the dual FOV setups are described in [8; 73; 76]. The setups can also be realized with multiple spectroradiometers, where one is typically pointed toward a reference target or  
Figure 5: Two different F sensors installed on different platforms: HyPlant installed in an aircraft (**left**) and AirSIF system [95] with QE pro of the Terraluma group installed on a multirotor UAV (**right**). an attached cosine receptor is pointed to the zenith, and the other one is pointed toward the vegetation target (dual spectroradiometer setup) [11; 46; 47]. There are also some special cases that turn their FOV toward the canopy and the zenith [99]. The dual spectroradiometer setup has the advantage that radiance and irradiance can be captured at nearly the same time. Unfortunately, two spectroradiometers never have the exact same spectral and radiometric characteristics. Thus, accurate cross-calibration is required and is very challenging; it is not always possible for very high spectral resolution spectroradiometers and likely to create biases in the data [14; 98; 101; 102]. The impact of spectral shifts increases with the spectral resolution of the sensor. [PERSON] et al. [38] showed that when F retrieval relies on measured downwelling irradiances (not simulated with atmospheric radiative transfer models (RTMs)), any spectral shifts will induce errors that will only be canceled out if they affect the measurement of both (up- and downwelling) fluxes equally. In practice, this can only be achieved if they are recorded by the same sensor. Thus, the dual spectroradiometer approach is not recommended for F retrieval since it is incredibly sensitive to even sub-nanometric spectral shifts between the measured up- and downwelling radiation fluxes. The different setups also differ by the viewing geometry that is used to measure surface radiance. Most setups adopt a nadir surface-viewing orientation.