07 Sep Measuring plant fluorescence from orbit
During photosynthesis, plants release a small portion of the absorbed solar energy as fluorescence. This weak light signal can provide information about photosynthetic activity and indicate whether plants are experiencing stress from factors such as drought or heat. Researchers now aim to measure the spectrum of this fluorescence from Earth orbit. The Fluorescence Imaging Spectrometer (Floris) on ESA’s Flex Earth observation mission is designed for this purpose.
For the spectrometer, researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF have developed and manufactured a silicon-based double-slit assembly and two high-precision mirrors. Floris operates with two spectral channels: one provides high-resolution information on closely spaced wavelengths, while the other covers a broader spectral range. The double slit enables both high spectral resolution and coverage of a broader spectral range.
High precision in manufacturing and assembly
Fraunhofer IOF manufactured the double slit from silicon. Image: Fraunhofer IOF
The two slits in the assembly manufactured at Fraunhofer IOF are each 44.15 mm long and 85 µm wide. The permissible deviation in slit width is ±1 µm. According to the developers, larger deviations would cause too much or too little light to reach the detector, affecting the evaluation of the measurement data.
Fraunhofer IOF developed a lithographic process chain for manufacturing the silicon wafers. The wafers were masked, patterned and wet-etched using a time-controlled process. The slits were then given a black coating to minimize unwanted reflections.
For precise assembly, the silicon element had to be positioned parallel to the apertures in the mount with an accuracy of better than 5 µm. The slit flatness had to remain below 10 µm. A combination of form-fit mounting, clamping and adhesive bonding is designed to ensure that the assembly withstands the vibrations, accelerations and temperature fluctuations encountered during launch.
The fragile slits must be installed highly parallel to each other while remaining robust enough to withstand the extreme environmental conditions encountered in space. Image: Fraunhofer IOF
Fraunhofer IOF also manufactured two mirrors integrated into the slit assembly. They direct the incoming light onto the spectrometer detectors. According to the researchers, the mirror surfaces had to meet a roughness requirement of 0.3 nm rms.
Plant fluorescence as a measurement parameter
Flex stands for Fluorescence Explorer. The ESA mission is designed to produce global maps of plant fluorescence. Because this fluorescence is linked to photosynthesis, it can provide information about plant activity and indicate stress caused by factors such as drought or heat. The data will be used, among other purposes, to monitor ecosystems, agricultural land and forests and to support climate and vegetation research.
According to the project partners, the Flex mission is scheduled for launch on September 15, 2026. The double-slit assembly was developed and manufactured at Fraunhofer IOF under contract to OHB System. Spectrometer integration and further mission preparations were led by Leonardo and Thales Alenia Space.
Source: www.iof.fraunhofer.de