## 4 Discussion  
### Comparison of Different SIF Systems  
Several types of ground-based systems have been employed for continuously monitor vegetation SIF during the past few years. Up to now, the most acceptable construction of this kind of system (e.g., FLoX, FluoCSpec2, Photospec and FAME) integrates spectrometers from Ocean Optics Inc.: FLAME/HR2000+, QEPro, with splitters fiber and inline TTL for sequentially measuring and switching between solar irradiance and vegetation radiance [18; 23; 27]. Our new built SIFprism was derived from a prism system developed by [PERSON] and [PERSON] (Department of Global Ecology, Carnegie Institution for Science), and the part of splitter fiber and inline TTL was replaced with a rotary prism box. For the first time, we tested four different constructed systems (SI-SIV) of different configurations to analyze the feasibility of monitoring SIF acquired from different sites using different devices across the world. SI and SIII were built by our team based on existing constructions, and SIV (FLoX) was a commercial product we bought, all of which could be treated as existing well-accepted SIF observation systems. SII was newly developed and built, which was specifically compared with the other three systems. The IT of four systems measuring soil ground (Figure 7) and vegetation (Figures 9 and A4) were compared. The spectrometer with a larger spectral range (650-800 nm) or slit (25 um) has a larger FWHM but higher light sensitivity. Therefore, the ITs of SI and SII were much shorter than those of the other two. Prism can reduce more than 50% the light loss through splitter fiber and inline TTL as IT of SIFprsim (SII) was about half of SI whether the upwelling sensors were equipped CC-3 or not. A shorter IT could help to improve the SNR (Figure A2) and enable the almost simultaneous measurements of downwelling and upwelling radiation to minimize the temporal mismatch between them as much as possible [35]. This will also be helpful for high-quality data acquisition under fast changing weather conditions and in the precision of SIF retrieval [34]. When four systems were setup to measure grey reference under fast changeable (in seconds) radiation, irradiances from four systems were consistent while radiances and reflectance were different (Figure 6). As these four systems are well radiometric calibrated, there are two possible reasons causing this result. The sampling temporal interval was set as one minute for all the four systems, thus the irradiances from the four systems were almost synchronously. However, due to different ITs, the sequential acquisition of radiances from them was non-synchronous. A shorter IT of SII makes it more relatively simultaneously measure irradiance and radiance comparing to other systems. Another potential reason is that even the four upwelling sensors were bound together and pointed to the grey reference, and they may measure different spots of the grey reference of different reflected radiance, which is hard to avoid. Spectral shifting is an obstacle to guaranteeing the precision of SIF measurement [29; 35]. The prism system is able to avoid spectral shifting as all light from two channels penetrating through unique optical path into spectrometer without light mixing via the splitter fiber. This issue for the system with splitter fiber is inevitable and hard to be resolved. One possible way is keeping straight and slack in the joint place of the splitter fiber, which may need many tests to get a 'right' shape of reflectance. An alternative option is to perform wavelength calibration respectively for each optical path and to recalculate the wavelength for each band of the two channels [14; 32]. Solar irradiance and canopy radiance measured precisely at the same wavelengths and with the same accuracy is an ideal case for SIF observation, which is unachievable with exist technology for now [34]. Nevertheless, red and far-red SIF from the four systems of hemi-con configuration varied parallel over the day even under variable weather conditions (Figure 13).