## 4 Discussion
### Comparison of Hemi-Con and Bi-Hemi Configurations  
When the upwelling sensor was equipped with a diffuser (CC-3), the IT of this sensor may be doubled due to the light attenuation of the diffuse material of the cosine corrector. Even so, the ITs of SI and SII were still much lower than SIV (as reference) (Figures 7 and 9). Therefore, the most affected by the bi-hemi configuration are vegetation reflectances and SIF.  
Reflectances measured with the hemi-con and bi-hemi configurations were indistinguishable when the target was soil ground (Figure 8). However, the differences of reflectance are notable when the target was the plot of _E. aureus_ as the hemispherical reflectance was higher in the visible region and lower in the near infrared region than the conical reflectance (Figure 11). This is similar with the feature that the soil reflectance is also higher in the visible region and lower in the near infrared region than the vegetation reflectance, indicating that soil ground signals may be captured by hemispherical sensor when measuring the small _E. aureus_ plot (only \(2.5\times 2.5\) m\({}^{2}\)). As the soil ground is homogeneous and continuous while the vegetation plot is heterogeneous and discontinuous at this small scale, different viewing geometries and footprints of hemispherical and conical sensors resulted in the differences of vegetation reflectance even though more than 90% of the conical signal is from the vegetation canopy [(43)], but also had a rare influence on ground reflectance (Figure 16). This result impelled us to test what the footprint of CC-3 may truly be.  
The photon-receiving part of CC-3 is built 1-2 mm below the rim, meaning that the actual FOV is definitely less than 180\({}^{\circ}\). [PERSON] et al. [(43)] reported that 90% of the total radiation captured by CC-3 is from an FOV of 144\({}^{\circ}\), of which the radius is approximately triple the sensor's height from the canopy. By moving the vegetation target away from the center of FOV, we found that far-red SIF signal became negative before the radius of FOV reached triple of the sensor height (Figure 12). Meanwhile, the far-red SIF of the plot of _E. aureus_ measured with bi-hemi configuration were much lower than those measured with hemi-con one (Figure 15) but were consistent when measuring a plot of grassland (Figure 14). This is probably due to multiple scattering of heterogeneous canopy in the far-red region causing a mismatch between estimated and real footprints. Furthermore, the actual viewing geometry may be different with the assumed one when hemispherical and conical sensors were set at different heights (Figure 15A). Therefore, considering the shape of canopy reflectance of _E. aureus_ (Figure 11), far-red SIF of this kind of heterogeneous canopy is interfered with by the soil ground signal. However, the case is different for red SIF, which is highly reabsorbed by the canopy but rarely multiple scattered thus measured red SIF is mainly from top of the canopy. As long as the assumed footprints of upwelling  
Figure 16: Reflectance of grassland measured by SII of bi-hemi configuration and SIV of hemi-con configuration.  
hemispherical and conical sensors were the same (Figure 15C), red SIF values were the same even the viewing geometry were different. While all upwelling sensors were set at the same height, red SIF from conical sensor was larger than others as the conical sensor has a smaller footprint but larger probability of seeing more sunlit leaves in its FOV when the solar zenith angle is small (around noon) (Figure 15D).