This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: When the ESA Earth observation mission FLEX launches into space on September 15, 2026, as scheduled, it will carry high-precision optical components from Jena, Germany. Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF have developed and manufactured a silicon-based double-slit assembly as well as two high-precision mirrors for the spectrometer on board the satellite.
The spectrometer is designed to detect the fluorescence of plants excited by sunlight from Earth's orbit. The data is expected to provide new insights into the photosynthetic activity, health, and stress levels of vegetation. The FLEX mission will address the question: How much light do plants emit, and what can this light emission tell us about the health of the plants?
At the heart of the satellite will be the "Fluorescence Imaging Spectrometer," or FLORIS for short. Unlike many other spectrometers, FLORIS does not operate with a single light channel but with two: One channel provides particularly high-resolution information on closely adjacent wavelengths, while the second covers a broader range of the light spectrum. This requires an extremely precise dual-slit assembly.
"The fluorescence signals emitted by plants are very weak. For FLORIS to analyze these signals reliably, the optical components must be manufactured and assembled with exceptional precision," says Dr. Falk Kemper, project manager for the FLEX project at Fraunhofer IOF.
"The double slit enables a combination of high spectral resolution and broad spectral coverage. Its fabrication pushed the limits of what is technically feasible." Each of the two slits in the assembly developed at Fraunhofer IOF is exactly 85 micrometers wide over a length of 44.15 millimeters—with a permissible deviation of only plus or minus 1 micrometer. Deviations beyond this would result in too much or too little light hitting the detector, thereby impairing the evaluation of the measurement data.
For the manufacturing process, Fraunhofer researchers developed a specialized lithographic process chain for silicon wafer structuring. The wafers were masked, patterned and wet-etched in a time-controlled manner. The slits were then coated with a black layer to achieve the required optical properties and minimize unwanted reflections.
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