Precision optics to make plant health visible from space

On September 15, 2026, the European Space Agency’s (ESA) Earth observation mission FLEX is set to launch, carrying advanced optical components developed by researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) in Jena, Germany. These components include a silicon-based double-slit assembly and two high-precision mirrors designed for the satellite’s Fluorescence Imaging Spectrometer, or FLORIS. The instrument is engineered to measure the faint fluorescence emitted by plants when exposed to sunlight, offering unprecedented insight into their health and environmental conditions. FLORIS operates using two distinct light channels. One channel captures high-resolution data on closely spaced wavelengths, while the other spans a broader spectrum. This dual-channel approach necessitates an exceptionally precise dual-slit assembly, which has been meticulously crafted by the Fraunhofer team. The slits, each measuring 85 micrometers in width over a length of 44.15 millimeters, are manufactured with a tolerance of just ±1 micrometer. Such tight tolerances ensure that the amount of light reaching the detector remains consistent, allowing accurate interpretation of the data collected. The fabrication of the slits involves a specialized lithographic process applied to silicon wafers. The wafers undergo masking, patterning, and controlled wet etching to create the intricate structures. To enhance optical performance, the slits are coated with a black layer that minimizes unwanted reflections. The assembly process further demands extreme precision, with the silicon elements needing to be aligned within a mechanical holder to within 5 micrometers. Maintaining high parallelism between the slits and ensuring flatness of less than 10 micrometers is critical for reliable operation under the harsh conditions of space travel. To support the functionality of the spectrometer, Fraunhofer IOF also produced two mirrors that guide incoming light toward the detectors. These mirrors feature surfaces with a roughness of less than 0.3 nanometers root mean square, roughly equivalent to the spacing between individual atoms. This level of smoothness is essential for minimizing signal distortion and maximizing the clarity of the measurements. The FLEX mission, known as the “Fluorescence Explorer,” aims to generate comprehensive global maps of plant fluorescence. This phenomenon occurs during photosynthesis and serves as a key indicator of plant function. By analyzing these emissions, scientists hope to gain deeper understanding of how plants respond to environmental stressors such as drought, heat, and nutrient deficiencies. The data gathered could significantly enhance efforts to monitor agricultural productivity, forest health, and ecosystem dynamics. The development of the optical components represents a major technological achievement. The double-slit assembly, in particular, showcases the ability to combine high spectral resolution with broad spectral coverage, a capability previously unattainable in spaceborne instruments. Dr. Falk Kemper, project manager for the FLEX project at Fraunhofer IOF, emphasized the importance of precision in achieving reliable results. “The fluorescence signals emitted by plants are very weak,” he stated. “For FLORIS to analyze these signals reliably, the optical components must be manufactured and assembled with exceptional precision.” As FLEX prepares for its launch, the success of the mission hinges on the flawless integration of these cutting-edge technologies. The data it returns will mark a pivotal step in remote sensing capabilities, offering a new dimension to our understanding of Earth’s vegetative systems.