## 1 Introduction  
Chlorophyll (Chl) fluorescence (ChlF) measurement is a powerful non-destructive technique used to assess the photosynthetic performance of plants [1; 2; 3; 4; 5; 6]. Various studies were conducted on the active measurement of ChlF signals using single point measurements [7; 8; 9; 10; 11; 12] or ChlF imaging [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. The ChlF principle is based on how light energy, which is absorbed by photosynthetic pigments such as chlorophylls and carotenoids is distributed [10]. Light energy received by Chl \(a\) has three main alternative fates; being used in photochemistry, being lost as non-radiative heat dissipation, or being emitted as fluorescence. By measuring ChlF, the efficiency of photochemistry and heat dissipation can be assessed [24]. ChlF offers a direct approach for actual plant photosynthetic activity measurement compared to reflectance measurements of vegetation captured by passive remote sensing sensors. In addition, plant stresses can be detected before visible signs of plant tissue deterioration or significant reduction in Chl content occurs [25; 26; 27]. The red and near-infrared (NIR) emissions by Chl coupled with quenching mechanisms have been used to estimate photosynthetic levels for decades. The process of \"quenching\", occurs through photochemical and non-photochemical mechanisms. The active and passive remote sensing of Chl was also frequently applied to detect plant stress on both leaf and canopy scales [28; 29; 30; 31; 32; 33; 34; 16; 35]. By the development of active techniques based on the pulse amplitude-modulated (PAM) method, effective measurement of Chl outside the laboratory became possible [12]. The PAM technique with saturating light pulses allows the estimation of several Chl parameters such as dark-adapted and light-adapted peak fluorescence (Fm and Fm', respectively) and steady-state Chl (Fs) to estimate the quantum yield of photosystem II (\(\Phi_{\text{PSII}}\)), the non-photochemical quenching (NPQ), and the photosystem II-based electron transport rate (ETR). \(\Phi_{\text{PSII}}\), NPQ, and ETR are three widely used Chl parameters, which are employed to measure photochemistry and the overall photosynthetic capacity of plants [9; 24]. However, the PAM fluorometry method is limited to short-distance applications for individual leaves and small plants attributed to the need for an artificial saturation light pulse and accurate pulse-synchronized and modulated fluorimetric techniques. A method based on the laser induced Chl fluorescence transients (LIFT) was introduced as a tool to expand the application of active Chl measurement methods to long-distance, canopy-scale studies [36; 37]. In this approach, low-intensity pulses, instead of a saturating pulse, are used to measure the fluorescence transient, which is interpolated to a maximum fluorescence level using a fluorescence model. Using LIFT, ChlF can be measured up to a distance of 50 m but there are still challenges for longer distance remote sensing applications using this method [36; 38; 39]. To address ChlF measurement shortcomings on larger scales, passive remote sensing of steady-state solar-induced ChlF (SIF) has recently attracted more attention [27]. The advantage of the SIF method is that the technique does not require accurate pulse-synchronized and modulated fluorimetric techniques as needed in the PAM and LIFT methods. The Fraunhofer line depth (FLD) method [40] is the most widely used SIF technique to measure ChlF emission under solar light. Using the FLD method, absolute variations in steady-state ChlF can be estimated but ChlF parameters cannot be retrieved and consequently ETR cannot be estimated. At the ground level, two broad oxygen absorption bands of terrestrial atmosphere at 760 nm (O\({}_{2}\)A) and 686 nm (O\({}_{2}\)B) are usually used to estimate the steady-state SIF. These wavelengths can be used for SIF measurement on the ground and for low flying aircraft. For satellite observation, other wavelengths in the near-infrared region are recommended [41].