## 3 Measuring F on the Leaf and Canopy Level
### Canopy Level
#### 3.2.2 Measurement Protocols from Proximal to the Airborne Scale
Also, in most cases, a bare fiber without fore-optics is used to measure the radiance of the canopy if the measurements are carried out rather close to the canopy. However, setups that measure farther away from the canopy often use lenses that limit the FOV [11] to constrain the extent of the area sampled. Some setups also use oblique-viewing geometries [8] and rotate to capture multiple targets [103; 104; 105]. Different viewing geometries have been shown to have a great impact on reflectance measurements for a given sun incidence angle [106; 107; 108; 13]. The impact on F760 measurements were initially investigated in [82]. The authors found that F760 varied as a function of the sun's incidence angle and viewing incidence angle during the midday measurement. In general, F760 tended to increase with higher viewing angles, especially when the sun incidence angle was between 35\({}^{\circ}\) and 100\({}^{\circ}\) with respect to the leaf normal. This needs to be accounted for if measurements with different viewing geometries are to be compared. The subtlety of the F signal requires a robust measurement protocol that, on one hand, reduces the temporal offset of the reference and the target measurement and, on the other hand, optimizes the SNR of the instrument for the current illumination condition. The latter includes the optimization of the integration time, as well as an estimation of the dark current at that integration time and the current sensor temperature. Most studies on F have reported that reference measurements are carried out quite close to canopy measurements. Most of the time, they are carried out sequentially (for cases in which just one spectroradiometer is used) [8; 71]. For dual spectroradiometer setups, it is even possible to carry out both measurements simultaneously [11]. To use the full dynamic range of the spectroradiometer, the measurement integration time should be adapted to the current illumination. This is done either manually or by means of an auto-integrating function that iteratively changes the integration time until the sensor signal is within a range of predefined digital numbers that do not exceed the spectroradiometers saturation value. This can be done by optimizing either the integration time to the target or the target and reference channel. After the integration time is set, the actual measurements can start. In a typical measurement sequence, the canopy measurement is \"sandwiched\" between reference measurements [10; 109]. Additionally, as an alternative to modeling the dark noise [110; 111], it can also be quantified for the specified integration time(s) within the cycle by closing the shutters. The further the distance between the object and the sensor, the more complex the atmospheric correction becomes (cf., Section 4.1.). Additionally, estimation of the irradiance is more complex at the airborne scale. While a few studies have used radiometric ground reference panels for an empirical correction, others have employed physical-based corrections [16; 93; 112]--for instance, those based on atmospheric transfer models, such as MODTRAN [113] or the SMARTS [114]--to estimate incoming radiance and other essential atmospheric transfer functions. For more specific applications, such as fluorescence retrievals, semi-empirical corrections of only selective wavelength regions are possible [115]. Some current UAV systems able the recording of the downwelling irradiance directly on the UAV (cf., AirSIF system in Figure 5), while other correction schemes are designed, although residual atmospheric disturbances typically remain [14]. The following section reviews the current knowledge on the need for atmospheric influence.