## 4 Discussion  
The relationship between SIF and GPP is still not very clear, and there are a lot of uncertainties in both the mechanism and the observations [6]. In this paper, we focused on the match between the footprints of SIF and flux observations. For the tower-based observation of up-welling irradiance of vegetation, there are mainly two different configurations: conical observation and hemispherical observation [8; 26]. We compared the footprints of the spectral observations of the two configurations, and modeled the ERTPL of hemispherical observations for atmospheric correction at the oxygen absorption band. In recent years, more and more automatic tower-based spectral observation systems were established to obtain long-term observations for vegetation in coordination with flux measurements for linking remotely sensed data to ecosystem characteristics [12; 13; 14; 15; 16; 17]. For those observations, one of the scientific challenges is to determine the most suitable FOV of spectral observations to match with the flux footprint. [PERSON] et al. [11] reviewed the configuration of 55 optical systems at 42 flux tower sites in 2011, and found that 17 out of the 55 systems used hemispherical observations of up-welling irradiance, and the others used the conical observations with FOVs from 5\({}^{\circ}\) to 60\({}^{\circ}\). [PERSON] et al. [10] claimed that the hemispherical measurements had great advantage of enabling the sampling of a wider area. According to the results of this paper, 90% of the total radiation comes from an FOV of width 72\({}^{\circ}\), which in turn covered 75.68% of the source area of the flux measurements. So the hemispherical measurements have an obvious advantage to match with the flux footprint. For conical measurements, some alternative ways for better matching with the flux footprint were also proposed. For example, [PERSON] et al. [40; 41] used spectral observations collected over a circular area centered at the flux tower with a rotating system; [PERSON] et al. [42] introduced a tram system to make spatially representative observations within the flux footprint. However, there are also a lot of disadvantages for the hemispherical spectral measurements. Firstly, the tower body and its shadow will be in the field of view. Moreover, the influence of them will vary with the illumination geometry [10], which will cause some uncertainties in the observation. For example, if the diameter of the projection of the tower body on the ground is 5 m, and the height of sensor is 20 m, the tower body will cover a FOV of about 14.25\({}^{\circ}\) (about 8% of the total FOV of the hemispherical observation). Secondly, the surface of vegetation canopy is not isotropic. Both the reflectance and SIF are directional [33]. For hemispherical observations, radiance from all directions with different weight will be collected. So the directional characteristics of reflectance or SIF should be carefully considered for different cases of application. Thirdly, as the transmittance of cosine-corrected foreoptic is limited (about 25-30% at the far-red band for the Ocean Optics CC-3 cosine corrector), the signal-to-noise ratio of the hemispherical spectral observations will decrease to some extent, which, clearly, could reduce the accuracy of the SIF retrieval. Another important issue to be considered for the hemispherical observation is the more complex path of radiation transfer. For the conical observation, the FOV is usually very small, and the RTPL can be estimated as the height of the sensor. But for the hemispherical observation, the FOV is 180\({}^{\circ}\), and the ERTPL would be much longer than the sensor's height. For the tower-based observations for reflectance, wavelengths at atmospheric windows are usually used, so the influence of atmospheric radiation transfer is usually neglected. However, for SIF observation, the atmospheric absorption bands are needed, and the influence of atmosphere is significant [27]. So the ERTPL for tower-based hemispherical observations needs to be modeled.