## 1 Introduction  
Remote sensing methods have long been used for monitoring vegetation, usually using reflectance-based vegetation indices (VIs) and other vegetation parameters derived from spectral measurements [1]. However, these VIs may fail to detect short-term changes in plant function as most of them do not have a direct connection with the photosynthetic functioning of the plants [2]. Recently, the remote sensing of solar-induced chlorophyll fluorescence (SIF) has become a promising tool to monitor a plant's physiological condition thanks to developments in observational technology during the last decade. Satellite SIF products have been used to estimate regional and global distributions of ecosystem gross primary production (GPP) [3; 4; 5; 6; 7; 8; 9; 10; 11]. In support of promoting the application of space-borne SIF in indicating functional parameters and processes, airborne and ground-based image or non-image canopy spectroscopy observations are conducted to investigate the spatiotemporal character of SIF at different observational scales [12; 13; 14; 15; 16; 17; 18; 19]. Compared to airborne platforms, either manned or unmanned aircraft, ground-based platforms are easier to operate and can be adapted to various vegetation types. An imaging spectrometer named SPECIM (Spectral Imaging Ltd., Oulu, Finland) has been setup for SIF observation [15]. However, the operation and maintenance of imaging equipment for long-term observations costs too much; therefore, many studies have deployed non-imaging devices. During the past few years, many kinds of instruments were employed for continuous vegetation canopy spectra observation and were usually set up at flux tower sites [20]. Based on prototypes (e.g., TriFLEX, MRI, SFLUOR box, SIF-system) [12; 21; 22], many improved systems (e.g., FluoSpec, SIFSpec, PhotoSpec) [13; 18; 23; 24] were developed in addition to existing commercial products such as FLoX (JB Hyperspectral Devices UG, Dusseldorf, Germany) and Piccolo Doppio (University of Edinburgh, Edinburgh, UK) [14]. Thus far, the widely accepted spectrometer for SIF measurement is QEPro (Ocean Optics Inc., Dunedin, FL, USA), which has a typical signal to noise ratio (SNR) of 1000 and can be customized with red and far-red regions (mostly 730-780 nm or 650-800 nm) granting the full width half maximum less than 0.3 nm. A spectrometer with full spectral range from 300-1000 nm, such as FLAME, USB2000, HR2000, or HR4000 (all from Ocean Optics Inc.), is often used to measure canopy reflectance, which can be used to calculate typical VIs (e.g., normalized difference vegetation index (NDVI), enhanced vegetation index (EVI)). To collect high-precision data, spectrometers are usually embedded in temperature-controlled box, keeping away from humidity and fluctuation of temperature, which may cause reduces of SNR. Such automatic and long-term mounted systems are able to collect temporally continuous data and to monitor diurnal and seasonal dynamics of SIF, especially to analyze SIF with concurrently measured bioclimatic parameters and carbon flux [18; 25; 26; 27; 28]. Under natural conditions, the critical process in the measurement of SIF is how to precisely and synchronously measure solar and vegetation radiation as a spectrometer has only one optical path. The ideal case is using two sensors (i.e., two spectrometers for now) to capture the solar spectrum and vegetation spectrum at the same time. However, in practice, the two sensors need highly accurate cross-calibration in order to avoid any spectral shifts and biases created in the data, which is almost impossible for such high spectral resolution spectrometers used for SIF retrieval [29; 30; 31; 32; 33]. Therefore, a construction using one spectrometer and with extra protocols enabling a single optical channel to measure both solar and vegetation radiation is recommended.