# Estimation of Chlorophyll Fluorescence at Different Scales: A Review
## 6 Conclusions and Perspectives
### Research over the Atmospheric Effects on the Fluorescence Retrieval  
Since atmospheric effects are not considered, the SIF can be successfully used to monitor the vegetation at the leaf or canopy scale. SIF retrieved from the airborne/space-borne images still faces challenges. Thus, the retrieval of SIF in these situations needs to correct atmospheric effects [35; 86]. SIF detection from satellite data is a hot research topic and is regarded as a critical mission of the FLEX project. In recent years, SIF detection research has also achieved much progress, and SIF has been successful retrieved from satellite data, such as SCIMACHY, GOSAT, GOME-2, OCO-2, TROPOMI, and TanSat. Using the features of the interest window, these methods neglect the atmospheric effects or compute these effects via statistical methods. Although atmospheric effects, such as aerosol scattering and surface pressure, in fluorescence retrieval methods are frequently considered, other factors still require further analysis [86], including rotation Raman scattering (RRS) and stray light. These atmospheric effects result in less significant filling than the fluorescence in the oxygen absorption bands or Fraunhofer lines, but they still induce fluorescence retrieval errors.  
Given the weak fluorescence signal, atmospheric effects should be considered fully and deeply. In recent years, more people focused on research regarding the atmospheric effects on SIF estimation. [PERSON] et al. believed that the transmittance of an air column, the path radiance, and the adjacency effect are three main factors that affect the oxygen absorption band depth; they used MODTRAN 4 to compute the atmospheric and environmental parameters and then corrected the measured airborne radiance to obtain ground-level oxygen absorption bands relating to the SIF [135]. [PERSON] et al. analyzed how atmospheric effects impact SIF retrieval on proximal sensing (at tower scale) by using simulated data with MODTRAN and provided a rigorous oxygen compensation method by introducing the oxygen transmittance function into the FLD or SFM to improve SIF estimation [136]. [PERSON] et al. also estimated the upward and downward atmospheric transmittances using MODTRAN to obtain the downwelling irradiance and upwelling radiance at the canopy, and then retrieved the SIF by the 3-band FLD method [137]. [PERSON] et al. [88] and [PERSON] et al. [90] used the radiative transfer model inversion method to estimate the SIF. In the report for mission selection (an earth explorer to observe vegetation fluorescence), it stated that it was essential to process the atmospheric correction to mitigate error propagation in retrieved SIF, and it is believed that the atmospheric correction process mainly considered the presence of aerosols and the total atmospheric columnar water vapor (CWV) in oxygen absorption bands, which was carried out by the retrieval of aerosol and water vapor to derive the apparent reflectance, and then the full fluorescence spectrum through the SFM method was retrieved [138; 139]. In future research, more attention will be paid to the SIF retrieval based on rigorous atmospheric correction.