## 1 Introduction  
The FIREFLY (fluorescence imaging of red and far-red light yield) instrument is a high performance spectrometer for estimating solar induced fluorescence (SIF). FIREFLY was operated in tandem with NASA Goddard's lidar, hyperspectral and thermal (G-LiHT) airborne imager [1] for flight operations in 2017 and 2018. SIF is the re-emittance, at longer wavelengths, of solar energy absorbed by plants. SIF occurs, alongside the production of heat, when absorption of light exceeds the utilization of light by photosynthesis [2]. As such, SIF is related to photosynthesis, and therefore gross primary productivity (GPP), mediated by the light use efficiency of the plant [3, 4]. Therefore, measuring SIF can provide quantitative insight into GPP [2, 5], species composition [3], phenology [6], and the physiological status of plants, including stress [3, 6, 7], as well as being useful for crop monitoring [8]. The history, theory, methodologies, applications, and current challenges facing the retrieval of SIF via remote sensing were the subject of a recent comprehensive review [9]. Since the ecological attributes and processes that can be inferred from SIF measurements are of interest at the global scale, and across spatial and temporal gradients, efforts have been made to retrieve SIF from remote sensing resources, particularly from spectroscopy [2, 3, 10]. Fluorescence retrievals have been reported from satellite-based instruments such as SCIAMACHY [11]; GOME-2 [12]; TANSO-FTS [10]; OCO-2 [7; 13]; and TROPOMI [14], resulting in global fluorescence products [5; 11; 12; 15]. SIF retrievals have also already been reported from airborne instruments including HyPlant [16] and the compact fluorescence imaging spectrometer (CFIS) [17]. From the perspective of spectroscopy, fluorescence is both a relatively weak signal [3; 18; 19] and can comprise only a small portion of the energy retrieved by spectroscopy in the relevant wavelengths (Figure 1). SIF has been reported as approximately 1% of total absorbed solar energy [6; 7; 20], 2% in some regions [4; 21], and 1-5% in the near infrared region [9; 22]. The recent review of [PERSON] et al. [9], depicted SIF with a peak of approximately 2.5 mW \(\mathrm{m}^{-2}\) sr \(\mathrm{}^{-1}\) nm \(\mathrm{}^{-1}\), and this representation is used as a benchmark to provide context in this study, and is shown in Figure 1.  
Considering that SIF comprises such a relatively small amount of energy, it is important for the attributes and performance of SIF-retrieving instruments to be particularly robustly characterized. In pursuit of this standard, many studies have investigated the influence of sensor characteristics on the ability to retrieve SIF measurements. The full-width at half maximum (FWHM) [19], band spacing (spectral sampling resolution) [19], stray light [7; 23], dark current [19], as well as instrument-specific sources of variation, such as non-linearity in detector responsivity [7], have all been shown to impact SIF estimations by masking the signal, or confounding the retrieval. Additionally, atmospheric conditions [24], aerosols, scattering and reabsorption [25], and bi-directional effects [26] have been shown to be important, particularly for more distant SIF retrievals from airborne and satellite platforms. Figure 2 summarizes the potential influences on SIF retrievals that need to be considered. The methods to retrieve SIF from spectrometer observations have diversified in accordance with the requirements of particular instruments, acquisition conditions, and data product specifications [27]. Some retrieval methods, such as Fraunhofer line discrimination (FLD) and improved Fraunhofer line discrimination (iFLD), use a modelled or retrieved proportional radiance relationship between spectral bands to infer the additive presence of SIF [18].