## 4 Discussion  
### Uncertainties in the SIF Retrieval and \(APAR_{chl}\) Measurements  
The SIF retrieval accuracy is dependent on the spectral characteristics of the sensor, such as the SR (spectral resolution) and SNR (signal-to-noise ratio). In this paper, the spectral data was acquired by a QE Pro spectrometer with an SR of 0.3 nm and a peak SNR \(>\)1000. According to the work by [PERSON] et al. [16], using the 3 FLD method, the SIF retrieval error should be less than 15%. Therefore, because of the high SR and SNR of the QE Pro spectrometer, it is reasonable to believe that the accuracy of the retrieved SIF is good and, hence, that the results describing the ability of the SIF\({}_{canopy}\) to estimate the APAR\({}_{chl}\) described here are reliable. The significant correlation between SIF\({}_{canopy}\) and APAR\({}_{chl}\) indicates that the SIF may be a better proxy for APAR\({}_{chl}\) estimation.  
The studies by [PERSON] et al. [44] and [PERSON] et al. [45] showed that, due to variations in the direct and diffuse radiation and also the bi-directional reflectance distribution function (BRDF) characteristics of the canopy, the illumination conditions also have an influence on the retrieval of SIF. The different absorption depths at the oxygen absorption bands that absorb direct and diffuse radiation can lead to an overestimate or underestimate of the SIF at specific observation geometries. In this study, all the measurements of SIF\({}_{canopy}\) were carried out at nadir conditions. According to the simulations, the error in the SIF retrieval caused by the nonlinear mixing of direct and diffuse radiation is less than 5% for nadir observations under clear-sky conditions and a solar zenith angle larger than 10\({}^{\lx@math@degree}\). It is, therefore, reasonable to neglect the influence of this nonlinear mixing of direct and diffuse radiation.  
The method proposed by [PERSON] et al. [43] was adopted to measure the fPAR\({}_{green}\). According to the validation carried out by [PERSON] et al. [43], which was based on a comparison of fPAR measurements made by a digital camera and those made by a SunScan instrument (Delta-T, Inc., Cambridge, UK) in the 400-700 nm spectral range with a resolution of 0.30 \(\upmu\)mol\({}^{-1}\) m\({}^{-2}\) s\({}^{-1}\), there was a significant correlation between the two types of measurement--the mean absolute error was 0.031 and the mean relative error 6.037%. Therefore, the accuracy of fPAR measurements can be considered reliable. In the present study, the ratio of fPAR\({}_{chl}\) to fPAR\({}_{green}\) was calculated using a simulated dataset with different values of Cab and LAI and a mean value of the ratio of fPAR\({}_{chl}\) to fPAR\({}_{green}\) depending on different Cab content level was used. For example, if Cab was 5 and 10 \(\upmu\)g cm\({}^{-2}\), the ratio used the simulated value. The range of this ratio was 0.775-0.81 for Cab greater than 20 \(\upmu\)g cm\({}^{-2}\) and so the uncertainty in the calculation of fPAR\({}_{chl}\) and APAR\({}_{chl}\) should have been smaller than 3%.  
Figure 13: The relationship between APAR\({}_{chl}\) and SIF\({}_{canopy}\) at the O\({}_{2}\)-A (**a**) and O\({}_{2}\)-B (**b**) bands for different plant types based on the experimental data.