## 3 Measuring F on the Leaf and Canopy Level
### Canopy Level
#### 3.2.1 Measurement Setups from Proximal to the Airborne Scale
When the downwelling E is measured using a calibrated highly reflective reference (usually a Spectralon@panel (Labsphere, Inc., North Sutton, NH, USA)), the challenge is to place it appropriately (fixed level and in a similar E field as the target). Any reference surface (WR panel, cosine diffuser) should be kept free from dirt due to birds, insects, or dust and periodically checked for degradation [80]. Furthermore, the fiber optics should be tightly fixed since any movement will alter the radiometric throughput, leading to inappropriate calibration coefficients being applied. Several of the mentioned systems are used for calibration and validation efforts in the framework of the ESA FLEX mission preparation. However, imaging spectroradiometers are also used. The SPECIM PS V10E (Specim, Spectral Imaging Ltd., Oulu, Finland) [81] is a pushbroom system with 1392 spatial pixels. The sensor was mounted on a long scanning bar, which was used to move the system and generate 2D hyperspectral images of the canopy. A custom-made retrieval with careful preprocessing facilitated the retrieval and spatial mapping of F from this ground-based imagery [9; 82]. Recently, the high-performance ground-based imager HyScreen (Specim, Spectral Imaging Ltd, Oulu, Finland) has been developed particularly to derive F.  
Canopy measurements are also carried out using airborne platforms. The first airborne instrument dedicated to measuring F was the AIRFLEX multichannel radiometer [83]. Less than 10 years after that, the HyPlant imaging spectroradiometer system ([19], Figure 5, left) was developed to serve as an airborne demonstrator for the European Space Agency's Earth Explorer mission Fluorescence Explorer (FLEX) [84]. The sensor package operates in the pushbroom mode and consists of two optical imaging modules: (1) the DUAL-image, a line-imaging VIS-NIR-SWIR spectroradiometer consisting of two sensors integrated in a single housing using common fore-optics to provide contiguous spectral information from 370 to 2500 nm, with a 3 nm spectral resolution in the VNIR spectral range and 10 nm spectral resolution in the SWIR spectral range; (2) the fluorescence imager (FLUO), a special module that acquires data at a very high spectral resolution (0.25 nm) in the spectral regions of the two atmospheric oxygen absorption bands (670-780 nm) and is dedicated to measuring F [19; 73; 85; 86; 87; 88]. Other airborne sensors not specifically designed to retrieve F have also been used. [PERSON] et al. [89] investigated the impact of varying irradiance on vegetation indices and F derived from spectroscopy data captured with the Airborne Prism Experiment (APEX) [20] and compared APEX-based F retrievals with EC-derived GPP. APEX is an airborne dispersive pushbroom imaging spectroradiometer covering the 375-2500 nm spectral region in up to 532 narrow contiguous spectral bands. The Chlorophyll Fluorescence Imaging Spectroradiometer (CFIS) [90; 91] has also been used to retrieve F from aircraft. F measurements have also been carried out from UAV platforms. These were predominantly carried out with fixed-wing platforms at ground altitudes of above 500 m and with instruments characterized by spectral resolutions of 5 nm and larger that were not specifically designed for F retrievals [16; 92; 93; 94]. Very recently, multirotor UAV sensing systems were developed with a very high spectral resolution [14]. Another system that can also be carried on UAV systems is the Piccolo doppia [17]. Similar to the AirSIF system [95], illustrated in Figure 5 (right), it is based on the QE Pro spectroradiometer, and two fiber optics are used to measure the downwelling E and the L in a sequence. With the ongoing miniaturization of spectral sensing systems and the advances in robotics [12], we expect more UAV sensing systems to emerge in the future. Taking all systems together, the different observational scales could be interlinked and connected with the satellite observational scale [24; 26; 27; 90].