# Estimation of Chlorophyll Fluorescence at Different Scales: A Review
###### Abstract  
Measuring chlorophyll fluorescence is a direct and non-destructive way to monitor vegetation. In this paper, the fluorescence retrieval methods from multiple scales, ranging from near the ground to the use of space-borne sensors, are analyzed and summarized in detail. At the leaf-scale, the chlorophyll fluorescence is measured using active and passive technology. Active remote sensing technology uses a fluorimeter to measure the chlorophyll fluorescence, and passive remote sensing technology mainly depends on the sun-induced chlorophyll fluorescence filling in the Fraunhofer lines or oxygen absorptions bands. Based on these retrieval principles, many retrieval methods have been developed, including the radiance-based methods and the reflectance-based methods near the ground, as well as physically and statistically-based methods that make use of satellite data. The advantages and disadvantages of different approaches for sun-induced chlorophyll fluorescence retrieval are compared and the key issues of the current sun-induced chlorophyll fluorescence retrieval algorithms are discussed. Finally, conclusions and key problems are proposed for the future research.  
ch chlorophyll fluorescence; Fraunhofer lines; physically-based method; statistically-based method 1  
[PERSON]  
The SIF retrieval methods have been successfully applied in the above-mentioned satellite data. To deeply understand the function of chlorophyll fluorescence in vegetation, the fluorescence explorer project (FLEX) was developed. The FLEX mission will map vegetation fluorescence to quantify photosynthesis activity. The FLORIS sensor will be in orbit in tandem with one of the Copernicus Sentinel-3 satellites, which will be launched in 2022 [52].  
In recent years, the reviews of sun-induced fluorescence have been published. [PERSON] et al. introduced the method and application of chlorophyll fluorescence in field and laboratory situations [53]. [PERSON] et al. summarized the sun-induced chlorophyll fluorescence (SIF) retrieval methods including the radiance-based methods and reflectance-based methods and its application at different scales [5]. [PERSON] et al. summed up the active and passive chlorophyll fluorescence measurements at canopy and leaf scales under different nitrogen treatments [54]. [PERSON] et al. introduced the state of the art of SIF measurement systems and the statistical methods for retrieval SIF on the canopy [55]. [PERSON] et al. published a review that introduced the origins of SIF, its relation to photosynthesis, and SIF retrieval at the canopy and global scale comprehensively [56]. [PERSON] et al. concluded the passive SIF measurement setups, protocols, and its application at the leaf to canopy level [57]. [PERSON] et al. wrote a review about the introduction and assessment of SIF retrieval methods for proximal sensing [57]. [PERSON] et al. introduced SIF and the relationship between SIF and photosynthesis from the view of light reactions [58]. These reviews introduced the generation, retrieval, and applications of SIF from different views. Different from these reviews, this paper introduces SIF from the near-ground to global scale and summarizes the existing SIF retrieval methods for quick understanding. In this article, chlorophyll fluorescence estimation methods are collated and summarized. First, chlorophyll fluorescence measurement near the ground is introduced. Next, SIF retrieval algorithms from satellite data are introduced in detail. Finally, we summarize the existing problems and conclusions in future research.