## 1 Introduction  
Spectrometer measurements of chlorophyll fluorescence (CF) capture photosynthetic activity and plant function information, have been shown related to gross primary production (GPP) and carbon uptake, and can capture signals of plant stress, e.g., [1, 2, 3]. CF is a physiological process undergone by plants to dissipate excess energy from photosynthesis to protect tissues by emitting radiation mostly in the red and far-red region of the spectrum (approximately 650 to 850 nm) and, as a by-product of photosynthesis, provides a mechanistic link to plant function [4, 5]. Thus, CF offers opportunities to assess vegetation status at leaf, plant, canopy, ecosystem, and global levels across spatial and temporal scales, e.g., [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]. Two predominant categories of CF measurement techniques are active and passive techniques. Widely used active techniques employ lasers, light emitting diode (LED) illumination, or lamps to excite chlorophyll and instruments, such as cameras or spectrometers, to record CF promptly or delayed and include techniques, such as pulse-amplitude-modulation (PAM) [4, 7, 12, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35]. By activating photosynthesis with a light of a known,narrow wavelength, the PAM technique allows measurements of several aspects of photosynthetic activity in ambient light conditions in great detail. Such active techniques, however, are restricted mainly to leaf level and have intricate protocols to follow for experiment design and are not easily scaled up. The other category is measuring CF in ambient conditions, mostly in sunlight. In contrast to PAM, CF measured with the so-called passive techniques can be made at leaf to ecosystem scales using spectrometers mounted on the laboratory bench, towers, drones, or on satellites. In addition to allowing data collection at multiple scales, passive CF measurements have also been shown to scale effectively to larger levels, such as globally, and are more generalizable because the light activating photosynthesis is usually sunlight, unlike the active technique [23; 36; 37; 38; 39; 40; 41]. CF can be challenging to measure because the CF signal is small compared to the amount of sunlight, or potentially alternative light, the plant requires to drive photosynthesis. For this reason, most methods to examine CF have remained complex, time intensive, require extensive knowledge of the instrument and illumination source specifications, uncertainties, protocols, and results are difficult to interpret. For example, current passive CF measurement methods include retrievals under sunlight requiring one of several algorithms to exploit the oxygen absorption bands that coincide with the CF emission spectrum centered at approximately 687 and 761 nm. The retrieved CF values are used for estimates or as model inputs to estimate the CF spectra, e.g., [22; 42; 43; 44; 45; 46]. Another option is to use an actinic light source having a \"window\" coincident with the CF spectrum can allow CF to be captured with a spectroradiometer. In many cases, a filter is placed within a chamber, such as within the source chamber for a leaf clip, to block incoming light in the CF wavelength range and then record the plant CF spectra in that \"window\". This filtering and measurement process can be tedious, require expensive components, and, in the case of using the leaf clip, restrict measurements to leaf scale similar to the PAM method [47; 48; 49; 50]. Therefore, despite advantages over active techniques, passive measurements of CF have remained complex. With the goal of identifying a potentially more straightforward CF-based technique to detect plant stress, an initial investigation was carried out to address the following: (1) what level of sensitivity to plant stress can be measured using a spectrometer and blue-red light emitting diode grow light, and (2) are CF spectra or CF metrics derived from these spectra indicative of stress earlier than visual inspection or traditional vegetation indices?