## 3 Results  
### Measurements of Grey Reference and Soil Background  
Irradiance, radiance, and reflectance are the first things should be checked to investigate the quality of data collected from observation systems. Figure 6 shows the irradiance, radiance and reflectance measured by different systems of hemi-con configuration while the downwelling bare fiber pointing toward grey reference around noon under slightly cloudy (left volume) and changeable (right volume) conditions. The irradiance measured by these four systems showed marginal difference, while radiance from different systems varied dramatically (Appendix A.3 and Figure A3 as a complement showing the consistency of the radiance acquired from the four systems under clear and stable illumination condition). As the four systems all using sequential observation are measuring the incident irradiance first and then the reflected radiance, under changeable weather conditions, irradiance and radiance are measured asynchronously and probably under different radiation conditions, causing variations of measured reflectance. The radiation conditions were more stable at 12:04 than that at 11:06. The standard reflectance of the grey reference is 0.5 at far-red region, and reflectance measured by SI, SII, and SIII were close to this value at 12:04, but reflectance measured by all four systems were less than 0.5 at 11:06. Generally, reflectance measured by SII and SII were closer to 0.5 than the other two systems at different time, indicating the stability of SII and SIII used in fast changing weather situations.  
All downward pointing sensors of the four systems with the hemi-con configuration were conducted to measure bare soil ground for entire days. Integration time (IT) could be a useful factor to evaluate the efficiency of these systems. Figure 7 displays the diurnal variations of ITs of different systems measuring (A and B) irradiance and (C and D) radiance. ITs of SI and SII were much lower than the other two systems. IT of SIV was always the longest and reaching the offset maximum IT (Figure 7C) when measuring soil ground. IT of SII was around half of that of SI, showing high observation efficiency, which was also the reason for SII having high SNR.  
Figure 6: Irradiance, radiance and reflectance measured by different systems while the downwelling bare fiber pointing toward grey reference around noon under slightly cloudy (**A**,**C**,**E**) and changeable (**B**,**D**,**F**) conditions.  
The main difference between hemi-con and bi-hemi configurations is the different viewing geometry as that the hemi-con measurements can be taken by observing the canopy at nadir or off nadir, while both nadir and off-nadir contribute to the bi-hemi measurements but in different weights [28]. Reflectance of soil ground measured from SI, SII and SIII of hemi-con (Figure 8A) and bi-hemi (Figure 8B) configurations of are shown in Figure 8. Reflectances measured by different systems had marginal difference by the bare fiber for hemi-con configuration, and those from the cosine corrector for bi-hemi configuration were also similar. Even though measurements of the downward sensor with bare fiber and cosine corrector have different viewing geometries, the soil was generally homogeneous, such that the reflectances measured by the bare fiber and cosine corrector were almost not affected by different viewing geometries.