Open-Source Python Toolbox Turns Drone Images into Plant Fluorescence Maps

Plants are constantly talking to us through light. When chlorophyll absorbs sunlight to power photosynthesis, a small fraction of that absorbed energy is re-emitted as a faint glow in the far-red part of the spectrum, a signal known as solar-induced fluorescence, or SIF. Because this glow rises and falls with the actual physiological state of vegetation, it has become one of the most sought-after measurements in modern plant science, offering a direct window into photosynthetic activity and stress that no ordinary camera can provide. Now, a team of researchers has released an open-source software package designed to make this elusive signal accessible at an extraordinary level of detail, turning ordinary drone flights into centimeter-scale maps of plant health.

The software, called SIFMap, was developed by J. Buffat, A. Elibol, H. Scharr, S. Choza-Farías, S. Salattna and J. Bendig and described in the journal SoftwareX. It is a Python toolbox purpose-built to process data from a novel high-resolution imaging sensor known as SIFcam, which is carried aboard uncrewed aerial vehicles. The package automates the entire journey from raw image pairs captured in flight to finished, stitched maps of fluorescence at 760 nanometers, a wavelength where the fluorescence signal can be cleanly separated from reflected sunlight. The code is freely available under the GPL 3.0 license, with documentation, example datasets and two complete flight datasets published alongside the paper.

The trick behind measuring SIF from the air lies in a clever piece of physics called Fraunhofer Line Discrimination, or FLD. Deep in the solar spectrum there are narrow dark bands, Fraunhofer lines, where absorption in the sun’s own atmosphere dims incoming sunlight. Within one of these dark lines, the light reaching a plant is weak, so any fluorescence the plant emits stands out clearly against it. SIFcam exploits this by carrying two cameras fitted with ultra-narrowband interference filters centered at 757.9 and 760.7 nanometers, straddling the oxygen absorption feature in the far red. By comparing the two channels, the software can compute fluorescence using the most basic formulation of the FLD method, combining the reflectance in both bands with an estimated irradiance to isolate the faint fluorescent flux.

What makes SIFcam unusual is that it behaves like a multispectral camera array, capturing two synchronized views of the same scene in a single snapshot. Flying at a typical altitude of 25 to 30 meters over crops such as wheat, the system produces images with spatial resolution on the order of centimeters, fine enough to distinguish patterns within and between individual plants. But that snapshot design creates a serious computational challenge: because the fluorescence calculation requires combining individual pixels from both channels, the two images must be aligned with subpixel accuracy. Vibration, wind and differences in integration time between the channels mean that even a rigidly mounted camera pair produces slightly shifting geometry from one exposure to the next.

SIFMap solves this with a homography-based alignment strategy. When a camera looks down at a scene that is effectively flat, as is the case for a closed crop canopy viewed from more than 25 meters up, the relationship between two views can be described by a single planar homography matrix, a mathematical transformation borrowed from the geometry of multiple views. The toolbox estimates this homography independently for every image pair using a Random Sample Consensus, or RANSAC, procedure applied to Scale Invariant Feature Transform, or SIFT, keypoints, the workhorse of modern image matching. This per-pair estimation absorbs the distortions introduced by flight dynamics and ensures the pixel-level precision the fluorescence retrieval demands.

Once individual images are internally aligned, the software must figure out how hundreds of overlapping frames fit together into one map. A typical SIFcam flight consists of 300 to 1000 image pairs with 80 percent forward overlap and 70 percent sidelap. SIFMap tackles this in two stages. First, a registration module identifies which images actually overlap by comparing feature descriptors across all image pairs in an all-against-all nearest-neighbour search, then verifies the candidates with projective matching and RANSAC, producing a graph of connected images from which isolated or corrupted frames are automatically removed. Second, a global alignment module places every image into a common coordinate frame. It picks a reference image using a minimum spanning tree of the matching graph, accumulates pairwise motions along the shortest paths, and then refines all transformations simultaneously by minimizing a nonlinear symmetric transfer error with least-squares optimization, complete with analytically computed Jacobian matrices, sparsity analysis and iterative outlier removal.

Transparency is the heart of the project. Commercial photogrammetry packages such as Agisoft Metashape and Pix4Dmapper can process SIFcam data, but their core algorithms are proprietary, making it difficult for scientists to understand how parameter choices affect the radiometry of the final product. The open-source alternative WebODM offers more control but relies on a full Structure-from-Motion pipeline that is computationally heavy, and it does not recognize SIFcam as a multispectral array, processing only one spectral band at a time. In benchmark tests on a 32-core machine, SIFMap stitched a 342-pair dataset in roughly 487 seconds of mapping time and a 931-pair dataset in about 912 seconds, while WebODM needed more than two hours for either dataset. SIFMap matched commercial software on speed while using less memory and, crucially, exposing every step to scrutiny, including pixel-level statistics that reveal how many images contribute to each map pixel.

The architecture is deliberately modular. The toolbox is organized into three main modules, SIFcam, match and align, supported by a data module that defines the underlying structures. Each stage of the pipeline, from radiometric preprocessing through band alignment, registration, global alignment and visualization, is a distinct unit that can be inspected, adjusted or replaced. Preprocessing converts raw digital numbers into calibrated at-sensor radiance using dark-field and flat-field corrections, then into reflectance products at both wavelengths by interpolating against in-flight measurements of Lambertian reflectance panels. The visualization stage offers several aggregation strategies for overlapping images, including a distance-weighted scheme that smooths out lighting changes caused by shifting sun-observer geometry. Because the design is generalized, the same registration and alignment machinery can be applied to other image types, and the developers note that maps could even be generated from ordinary RGB imagery.

For users, the package ships with a small 24-pair test dataset and two complete flights, together with configuration files that parameterize every stage. The most influential settings concern the registration and alignment steps: a RANSAC residual threshold that governs how strict the feature matching is, a minimum correspondence count that decides when two images count as connected, and outlier-removal parameters that control how aggressively noisy feature matches are pruned between successive optimization rounds. The default configuration has proven reliable across comparable SIFcam datasets, but the documentation walks users through the cases where dataset-dependent tuning is warranted. Support is provided by the developers at Forschungszentrum Jülich, where the work was carried out with funding from the German Federal Ministry of Education and Research.

The implications reach well beyond one sensor. Remote sensing of chlorophyll fluorescence has matured over five decades from ground instruments to satellite products that track ecosystem photosynthesis across the globe, yet a persistent gap has remained at the field scale, where researchers need resolution fine enough to see how stress spreads through a crop canopy. Existing airborne and ground-based SIF systems face trade-offs in resolution, cost and deployment flexibility, and low-cost UAV sensor setups have been held back by the lack of adapted processing software. By delivering a fast, transparent, end-to-end mapping pipeline, SIFMap lowers that barrier, letting plant scientists, breeders and agronomists turn routine drone flights into quantitative maps of photosynthetic performance. The current version is restricted to flights above 25 meters over relatively flat, crop-like canopies, because the homography approach assumes planar scenes, but the team is already outlining solutions for closer, more structurally complex targets in future releases, along with additional retrieval and mosaicking methods.

Subject of Research: Open-source software for mapping solar-induced chlorophyll fluorescence from UAV-borne snapshot imaging

Article Title: SIFMap: A python toolbox for creating solar-induced fluorescence maps from UAV-borne snapshot imaging data

Article References: SIFMap: A python toolbox for creating solar-induced fluorescence maps from UAV-borne snapshot imaging data. (n.d.). https://doi.org/10.1016/j.softx.2026.103065

Image Credits: AI Generated

DOI: 10.1016/j.softx.2026.103065

Keywords: solar-induced fluorescence, SIFMap, UAV remote sensing, chlorophyll fluorescence, SIFcam, Fraunhofer Line Discrimination, image mosaicking, Python, plant phenotyping, photosynthesis, open-source software, precision agriculture