## 4 Discussion
### SIF Retrievals with FIREFLY
#### 4.1.1 Indirect Retrievals from Solar and Oxygen Absorption Features
Furthermore, the remaining  
Figure 18: (**a**) Orthorectified false color RGB FIREFLY image (red: 670.2 nm, green: 780.2 nm, and blue: 700.9 nm) of an in-flight acquisition of agricultural land and woodland in Maryland, United States (greens: vegetated targets; purples/pinks: non-vegetated targets); (**b**) lidar point cloud, colored by intensity. Note the wider swath width of G-LiHT lidar instruments; and (**c**) lidar point cloud, colored by height above ground. portion of the leaf reflectance region that is expected to be free of the atmospheric influences is also expected to have a more complex reflectance shape (Figure 16). These sorts of confounding interactions between influences are one of the main motivations behind direct SIF retrieval techniques such as PCA and SVD that remove observably systematic variation without attributing it to a specific origin. However, these techniques cannot be applied to indirect retrievals, and therefore this region will be challenging to use in an operational product. Finally, the spectral region of FIREFLY acquisitions in which SIF is expected to have a polynomial shape (\(>\)774 nm) could experience some additional uncertainty as a result of the slope varying across relatively short spectral distances. Furthermore, this nuanced influence of the SIF shape in this narrow spectral region serves to highlight the wider challenge of using indirect SIF estimates to derive a shape for SIF, considering the valley and peaks bands for each absorption feature will be influenced non-uniformly according to the local SIF shape (Figure 3). As demonstrated in Figure 14, reducing the spectral distance between valley bands and peak bands can help mitigate violations of the assumption that energy in the bands will be scaled equally. However, this is balanced against the consideration that selecting pairs of bands that result in an increase in the depth of the absorption feature also mitigates non-uniform influences. In this study, a search window of \(\pm\)3 bands (\(\pm\)0.153 nm) around a valley was used to select the peak that would provide the greatest depth of absorption feature. While this approach was chosen to balance the benefits of absorption feature depth and spectral proximity, further study is required to find optimum thresholds for absorption feature band selection. For an indirect SIF retrieval product, several factors have been discussed that would influence the decision of whether or not to include a particular absorption feature in a particular SIF retrieval effort. Figure 20 shows that this study has provided all of the relevant information needed to pursue this potential retrieval approach with FIREFLY. Following the lines attached to the \"A\" and \"B\" annotations in Figure 20 demonstrates the multi-step decision-making process for individual absorption features. Firstly, an absorption feature has to be represented in every spatial pixel of FIREFLY to be included in a retrieval effort, as indicated by an unbroken line of color across the left plot. Secondly, the expected proportional depth of the absorption feature might be required to meet a particular standard. As part of this study, the depth of each feature has been determined separately for each FIREFLY pixel, and is displayed in the left plot. Thirdly, there could be requirements for the spectral distance between the valley and peak band for each absorption feature, again derived for each feature in each FIREFLY pixel, and displayed in the right plot. The right plot also reveals that the optimum peak band changes for some absorption features across the FIREFLY spatial pixels, which could be another factor in filtering absorption features. Fourthly, the absorption features could be included or excluded based on the spectral regions they reside in, shown to the far right of Figure 20. It can be seen that absorption feature \"A\" would be subject to influences from water vapor and oxygen absorption, and leaf reflectance shape, whereas feature \"B\" resides in a region free from additional influences.