## Materials and Methods  
### Instrumental Constructions  
In this study, we chose four observation systems of DFOV protocol for test. Both hemi-con and bi-hemi configurations were also tested. Figure 1A shows the setup of the observation systems. To measure both solar radiation and canopy radiation for SIF retrieval, in most of the SIF observation systems in use, TTL (Ocean Optics Inc., Dunedin, FL, USA), an optical shutter, and splitter fiber are often used to separate the single optical path of the spectrometer into two channels, and to switch between them. Figure 1B shows the schematic diagram of this construction. Three of the four systems used in this study are built from this kind of construction. We have also developed a new SIF observation system replacing TTL with a device contained a prism for achieving the function of TTL (SIFprism), of which construction is schematically shown in Figure 1C. The main distinction between the two kinds of systems is the different optical path switching methods, which are either TTL with a splitter fiber [18; 27] or a prism with a single fiber. The prism is embedded in a box along with an electric motor and is connected to spectrometer by the single fiber optic. The motor controls the directions (right and left) of the prism to receive the downwelling or upwelling radiation, which is then reflected to the output port connected with the spectrometer via the single fiber optic. SIFprism is controlled by a MATLAB GUI program called NJUSpec Controller (Figure A1). NJUSpec Controller runs all the tasks of SIFprism, including regulating the temperature of TCB, managing the position of the prism, closing or opening the shutter of the light path, setting the integration time of each spectrum, displaying the running status of the SIFprism system. The collecting processes of spectra through the NJUSpec Controller are as follows:  
Once the system is started, the NJUSpec Controller enables the external TEC of the TCB to ensure the spectrometer is working in a constant 25 \({}^{\circ}\)C ambient environment (Figure A2). The internal temperature of the spectrometer is controlled by an internal TEC to ensure that the temperature of charge-coupled device (CCD) is constantly \(-10\)\({}^{\circ}\)C (Figure 11). Moreover, temperature and relative humidity sensors are put into the TCB to monitor the environmental dynamic (Figure 11). The temperature and the relative humidity inside and outside of TCB are displayed on the interface (the bottom left in Figure 11) of NJUSpec Controller. NJUSpec Controller controls the motor to change directions (right and left) of the prism switching between the radiation received from the downwelling and upwelling fiber optics. As data quality is mostly decided by IT, a simple optimal algorithm based on the work of [PERSON] et al. (2017) is used to calculate the optimized IT for each spectral measurement to adapt changeable weather conditions. Sequentially, measurements of incident solar radiation, corresponding dark current (DC), reflected canopy radiation and corresponding DC are carried out around every 45 s.  
The spectra of radiance and irradiance, and their ITs are displayed on the interface of NJUSpec controller (upper-left of Figure 11). The reflectance and both red and far-red SIF of the target are retrieved and also displayed on the interface (upper-right of Figure 11, bottom right of Figure 11), respectively. All these data are saved in a Matlab file (.mat) every 30 min. In this study, four deliberate observation systems we chose with different constructions (Table 1) to compare SIF retrieved from them. Spectrometers used in all systems were the same model (QEpro) but with different spectral regions and sizes of slit, which were all customized to measure radiance in the fluorescence emission wavelength range.