## 3 Measuring F on the Leaf and Canopy Level  
### Leaf Level  
#### 3.1.1 Measurement Setups  
The full range of emitted leaf fluorescence (600-800 nm) was originally measured in the laboratory with visible (VIS) near-infrared (NIR) spectrometers and detectors (e.g., 1024-channel intensified diode array detectors) using single-wavelength light sources, such as lasers, to prevent interference with the F emission range [5; 31; 55]. However, single-wavelength excitation sources have the disadvantages that (1) they impact the spectral shape as a result of the wavelength-specific absorbance and emission [56] and therefore affect the apparent spectral fluorescence yield (ASF) emitted by the leaf; (2) the applied illumination often represents only a fraction of the natural illumination conditions, and the plant reacts differently depending on the light intensity; and (3) they do not allow the simultaneous retrieval of ancillary data from the reflectance signal (e.g., Chl content, Photochemical Reflectance Index (PRI)) in the PAR (W m\({}^{-2}\)) range. Therefore, the use of a natural(-like) illumination source (lamp or sun) that provides a realistic PAR range is commonly preferred in the case of remote sensing applications, whereby the (full-range, spectrally resolved) F signal is measured in physical radiance units by a spectroradiometer. Measuring under natural(-like) light conditions, however, requires a retrieval method to decouple F from L. Using any of the remote sensing methods would also be possible at the leaf level, but the setup could be difficult, susceptible to the uncertainties associated with those indirect methods and limited to certain wavelengths. A more direct and practical approach is the use of a band-pass filter (see setups by [57; 58; 59; 60]) to omit the incoming light that interferes with the F range (i.e., \(>\)650 nm) and only slightly overlaps with the PAR range. For practical convenience, the FluoWat leaf clip was developed [61], which uses a filter that is easily inserted and removed and enables the insertion of a fiber optic in both the top and bottom positions to measure the upward and downward radiance fluxes perpendicular to the leaf surface (Lup(\(\lambda\)), Ldw(\(\lambda\)), W m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\), Figure 2). The placement of a sharp-edge low-pass filter (e.g., a high-performance OD4 short-pass filter, TechSpec, Edmund Optics GmbH, Germany) cuts off wavelengths longer than 650 nm, and upward and downward leaf-emitted F fluxes (Fup(\(\lambda\)), Fdw(\(\lambda\)), W m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\)) can be directly measured in the full 650-850 nm range [62]. The instrument requirements for measuring F directly under a low-pass filter are less strict compared with those for F retrieval from the narrow O\({}_{2}\)-absorption features (e.g., full-width at half maximum (FWHM) \(<\) 1 nm, SSI \(<\) 1 nm, see Section 3.2, Supplementary Table S2). Spectrometers used in combination with a filter to omit light that interferes with F should nevertheless comprise a sufficiently high signal-to-noise ratio (SNR) because the radiative flux is relatively weak (approx. 2.5 mW m\({}^{-2}\) sr\({}^{-1}\) @740 nm) (Supplementary Table S2); an example of this type of spectrometer is the FieldSpec (Analytical Spectral Devices (ASD) Inc., Boulder, CO, USA) [63; 64; 6]. To better interpret the link between passively measured F and photosynthesis, many of these setups also combine an active pulse-amplified modulating (PAM) fluorometer [57; 58; 59], gas exchange system [65], and/or leaf thermocouple to derive in parallel the parameters that are directly linked to photochemical yield, CO\({}_{2}\) uptake, and sensible heat dissipation.