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Key Laboratory of Digital Earth Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100094, China; [EMAIL_ADDRESS] (X.L. ); [EMAIL_ADDRESS] (J.H. ); duss@radiexchange, satellite remote sensing is needed. However, the data from EC observations and satellite remote sensing cannot be linked directly due to the mismatch of spatial scales [9; 10]. To deal with this problem, continuous tower-based spectral observations in coordination with EC measurements is an efficient solution and can serve as a bridge between the eddy covariance (EC) measurements and satellite-based remote sensing data [9; 10; 11; 12]. In the last decade, a growing number of automatic tower-based spectroscopy systems have been initiated with the aim of making long-term, unattended observations of vegetation using different instruments and configurations. [PERSON] et al. [11] provided a review of the current optical systems and methods used for continuous vegetation optical sampling across European EC sites. Several hyperspectral systems have been proposed for continuous sun-induced fluorescence (SIF) detection out of the automatic optical systems currently installed at the European EC sites. These include the Multiplexer Radiometer Irradiometer (MRI) [12], the HyperSpectral Irradiometer (HSI) [13], the TriFLEX [14], the UNIEDI System [15], the FluoCSpec [16], and the SIF-Sys [17]. For passive SIF observations, the spectral resolution requirements are generally sub-nanometer [18]. Therefore, most of these systems provide high spectral resolution (FWHM \(\leq\) 1 nm) and their spectral ranges cover the SIF emission region (650-850 nm) [19]. Within this framework, both SpecNet [9] and the European equivalent COST Action ES0903 (EUROSPEC, [[http://cost-es0903.fem-environment.eu/](http://cost-es0903.fem-environment.eu/)]([http://cost-es0903.fem-environment.eu/](http://cost-es0903.fem-environment.eu/))) aim to establish common ground-based instruments, define measurement protocols for tower-based spectral observations and to promote the development of reliable, scale-appropriate and cost-effective optical global flux tower measurements. However, the mismatch between the footprints of different spectral flux observations is a key problem [10; 13]. Since the goal of the tower-based observations is to provide spectral signals in coordination with the flux data, the footprints of the two systems should match. A number of factors contribute to the flux footprint variability, including wind direction and speed, measurement height, and vegetation structure [20; 21]. A series of empirical flux footprint models (e.g., [22; 23; 24; 25]) have been proposed to estimate the footprint of flux measurements made under different conditions. The footprint of a spectral measurement is determined by the instrument configuration and a configuration suitable for the specific purpose of the measurements should be selected. Tower-based spectral measurements involve the measurement of the down-welling (incoming) and up-welling (both reflected and emitted) radiation fluxes from the canopy. Three main instrument configurations have been applied to in situ optical observations to quantify incoming and reflected radiation: bi-conical, hemispherical-conical and bi-hemispherical [10; 26]. The bi-hemispherical and hemispherical-conical configurations are the most commonly used for tower-based and long-term spectral observation [11]. The setups of the bi-hemispherical and hemispherical-conical configurations are shown in Figure 1. Both configurations use cosine-corrected foreoptic pointed at the zenith to acquire the downwelling irradiance values. For the up-welling measurements, hemispherical-conical configurations use a conical fore-optic (bare fiber) at nadir or off-nadir with a small field of view (FOV) (such as 25\({}^{\circ}\)) whereas bi-hemispherical configurations use a cosine-corrected foreoptic view at nadir with a large FOV of 180\({}^{\circ}\).