## 2 System Description and Data Collection  
### System Design  
The SIFSpec system mainly consists of the central control unit, a spectrometer with high spectral resolution and SNR, the light collection unit, and the temperature control unit. A schematic diagram of the configuration of SIFSpec is shown in Figure 1. All units (except the optical fibers) of the system are housed in a box with dimensions of 56 cm \(\times\) 35 cm \(\times\) 29 cm. Figure 1: Schematic diagram of the configuration of the solar-induced chlorophyll fluorescence (SIF) measurement (SIFSpec) system. TTL and IPC indicate the electronic shutter and the industrial personal computer, respectively. The core of the central control unit, an industrial personal computer, is responsible for data logging, control, collection, processing, and storage. The computer is equipped with a random access memory (RAM) of 2 GB and a read-only memory (ROM) of 256 GB. This memory is sufficient for saving measurements made during a one-year period as the size of the daily data file is generally no more than 40 MB if the data-sampling time interval is set as 3 min. Several pieces of software are included in the system for collecting, calibrating, browsing, displaying, and transforming the data. The spectrometer used in the SIFSpec system is a QE65 Pro spectrometer (Ocean Optics, Dunedin, FL, USA). The spectral resolution is determined by both the dispersion and pixel resolution, that is, the characteristics of both the grating and slit that are selected. The QE65 Pro used in this system is customized with a high spectral resolution of around 0.34 nm using a longpass filter (\(>\)590 nm), and an H6 grating (GRATING_#H6, Ocean Optics, Inc., Dunedin, FL, USA) with a groove density of 1200 mm\({}^{-1}\) and a 25 um slit (INTSMA-025, Ocean Optics, Inc.). The spectral sampling interval is about 0.155 nm. The spectral range of 649-805 nm and peak signal-to-noise ratio (SNR) of over 1000 make it possible to retrieve the full-band-width SIF spectrum with two peaks at 685 and 740 nm. The QE65 Pro spectrometer is equipped with a back-thinned full frame transfer Charge-Coupled Device (FFT-CCD) detector with 1044 \(\times\) 64 pixels and a 16-bit A/D converter. The QE65 Pro has a 2D arrangement of pixels (1024 \(\times\) 58) and a peak quantum efficiency of around 90%. The readout noise can be minimized and the processing speed can be enhanced by binning or summing the detector's columns as a pixel prior to the readout process. Additionally, the thermal noise is also reduced by the onboard Thermal Electric (TE) cooler. Thus, a superior dynamic range of 65,000:1 for a single acquisition can be delivered thanks to the combination of the QE65 Pro's low noise detector and 16-bit A/D converter, and the sensitivity can be up to \(\sim\)0.065 counts/e-. The amount of stray light can be lower than 0.08% at 600 nm and is 0.4% at 435 nm. In order to protect the spectrometer, a black customized thermotank is designed to separate the spectrometer from other parts of the SIFSpec system. The light collection unit is composed of a bifurcated optical fiber (CPATCH, Ocean Optics, Inc.), which has a bundle core 1000 um in diameter and two electronic TTL shutters that are used to collect the up-welling and down-welling radiation. The bifurcated optical fiber is a Y-shaped subassembly with two parallel fibers that have the same diameter and that merge to one in the middle. One 5 m long optical fiber is fixed in the upward direction and another is fixed in the downward direction. In the upward direction, the downward irradiance is measured using a cosine-corrected foreoptic (CC3-UV-S, Ocean Optics, Inc.), which can capture hemispherical measurements with a large field of view (FOV) of 180\({}^{\circ}\). The conical foreoptic uses a bare fiber with a small FOV of about 25\({}^{\circ}\) and is used to capture the upward radiance when pointing at the cropland canopy.