## 4 Discussion
### SIF Retrievals with FIREFLY
#### 4.1.2 Direct Retrievals
An influence of this nature would again be a significant problem for SIF retrievals, as signals from neighboring ground targets would be mixed in each pixel. There is no evidence to suggest that spatial stray light is an influence in FIREFLY, with sharp boundaries between cover types visible in many FIREFLY acquisitions (Figures 18,19 and 23). However, there is no substitution for a thorough characterization of the PSF, and this will be a focus of future characterization efforts. Spectral shift and squeeze describe the redistribution of the recorded wavelength of energy as a function of the temperature and pressure conditions experienced by the instrument during acquisitions. Any inconsistency in the wavelength of recorded energy would be confounding for SIF retrievals. In an effort to investigate spectral shift and squeeze, the approximate locations of peaks of ten large absorption features inside the region of influence by the oxygen A feature were established with inflection analysis (Section 2.3.1). The FIREFLY band in which the local peak for each feature occurred was then retrieved in samples from every in-flight acquisition used in this study, on the assumption that these acquisitions incorporated a range of temperature and pressure conditions. Figure 24 shows that the peak radiance for each absorption feature did move between FIREFLY bands, but that it rarely moved by more than one band from the modal peak band for each feature, suggesting random noise could be a factor. However, there were some examples found (shown in Figure 24) where the peaks moved in a consistent spectral direction, which could be evidence of a minor spectral shift. Further investigation is recommended, and will be facilitated by the tool demonstrated in Figure 24. Additionally, a more rigorous test for spectral shift and squeeze could involve mounting a panel of known reflective profile over the instrument port during in-flight acquisitions, accompanied by measurements of temperature and pressure conditions. Figure 23: (**a**) False color RGB FIREFLY image (red: 670.2 nm, green: 780.2 nm, blue: 700.9 nm) from an in-flight acquisition in Puerto Rico, USA (greens: vegetated targets; purples/pinks: non-vegetated targets); (**b**) high resolution aerial photograph from G-LiHT RGB camera, with zoom of mixed cover (light blue box), and vegetation (red box) shown. Spectral smile is the term used when the detected wavelength of energy shifts as a function of the spatial pixel in which it occurs, due to the inconsistent distribution of energy by the detector grating. This effect would have a serious impact on SIF retrievals, but there is no evidence that FIREFLY has a spectral smile. A spectral smile could be identified by a shift in the cumulative distribution function of energy in cross-track targets, but after correction for FWHM effects, there were no cross-track effects observable in either the controlled acquisitions of the panels (Figure 10), or the cross-track homogenous targets found in in-flight acquisitions (for example, the cross-track road in the left panels of Figure 19.  
In-flight acquisitions were also used to look for a keystone effect, whereby energy intended for one set of spatial pixels would be distributed onto another, varying as a function of wavelength. Functionally, a keystone influence mixes portions of spectra from different spatial pixels, which would be extremely confounding to SIF retrievals, especially if the underlying SIF emission function is to be derived from multiple estimates across the spectral range of the instrument, or by spectral fitting methods. There is no evidence to suggest that FIREFLY has a keystone effect, but it is hard to be certain from in-flight acquisitions, as there would have to be sharp boundaries between heterogeneous targets aligned with the spatial sampling boundaries of the instrument.