Prof. Mark Wenig Profile

Prof. Mark Wenig

at Ludwig-Maximilians-Univ. München

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Publications (2)

Proceedings Article | 11 June 2021 Open Access

Experimental validation of a 2D-slit homogenizer for space based imaging spectrometers

Timon Hummel, Claude Coatantiec, Xavier Gnata, Tobias Lamour, Rémi Rivière, Christian Meister, Jasper Krauser, Dennis Weise, Mark Wenig

Proceedings Volume 11852, 118520L (2021) https://doi.org/10.1117/12.2599167

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The analysis of recent Earth observation spectrometer missions revealed the impact of spatially heterogeneous Earth radiance scenes on the spectral accuracy of the instruments. One of the most critical observations is the distortion of the instrument spectral response function (ISRF) induced by radiometric contrast in the Earth radiance scene. In order to meet the high precision and accuracy of quantifying the spatial distribution of the atmospheric composition, stringent requirements on the ISRF knowledge are defined such as shape stability, centroid position of the spectral channel centre and the Full Width at Half Maximum (FWHM). In the framework of the CO2M A/B1 study, Airbus investigated a new slit concept called 2D-slit homogenizer (2DSH) for the mitigation of spatially non-uniform scenes. This is done by replacing the classical spectrometer slit with non-circular core multimode fibres, which scramble the light in along-track (ALT) and across-track (ACT) direction and hence average the contrast of the Earth scene in both dimensions. The final 2DSH will be made of several adjoined fibres, assembled in a bundle. A single fibre core dimension defines the spectral extent of the slit (ALT) and the minimum achievable spatial sample (ACT). Consequently, the full swath width covered by the instrument is given by the total size of the fibre bundle in ACT. Here, we present an experimental validation of the 2DSH in terms of scrambling efficiency and radiometry. In order to probe the fibre characteristics for non-uniform scenes, we designed and constructed a setup which allows us to track and tune multiple high contrast scene cases as a fibre input facet illumination. The scrambling efficiency performance of the fibre is assessed by measuring the near- and far-field intensity distribution of light transmitted through the fibre for different scene cases. With the acquired data, we propagate the fibre output through the subsequent spectrograph by simulation and thereby translate the measured near- and far-field distributions in ISRF errors. In particular we quantify the ISRF shape, centroid and FWHM for different Earth scenes. Furthermore, we analyze the impact of focal ratio degradation in terms of radiometric losses and compare the results in the NIR and SWIR wavelength for different stress cases on the fibre.

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Proceedings Article | 25 September 2018 Paper

A novel measurement approach to quantify diffuser induced Spectral Features

Florian Richter, Jasper Krauser, Corneli Keim, Dennis Weise, Mark Wenig

Proceedings Volume 10785, 107851Y (2018) https://doi.org/10.1117/12.2501943

KEYWORDS: Diffusers, Sensors, Spectrometers, Speckle pattern, Calibration, Telescopes, Light scattering, Speckle, Signal to noise ratio, Radiometry

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Spectrometers for Earth Observation require inflight radiometric calibration, for which the sun can be used as a known reference. For wide field instruments, a diffuser is placed in front of the spectrometer, scattering incoming sun light into the entrance slit and ensuring a homogenous illumination. As drawback, the diffuser induces a specific radiometric error caused by interference, which is called Spectral Features. The scattering of the incident light at the diffuser induces a random path difference yielding a specific interference pattern at the entrance slit, known as speckle pattern. These speckles are propagated through the disperser to the detector plane and further integrated by the detector pixels. The resulting feature can yield a significant signal error contribution, whose spectral variation is referred to as spectral features. The magnitude of this error is evaluated in terms of the Spectral Features Amplitude (SFA), the ratio of the signal standard deviation with its mean value over a specific wavelength range. There have been several ways implemented to measure the SFA of a spectrometer, e.g. end-to-end measurements with representative instruments. Typically the measurement accuracy is not sufficient to isolate the SFA from other radiometric errors. As a consequence, the instrument layout can hardly be optimized to suppress Spectral Features. We propose a novel characterization technique for Spectral Features based on the direct acquisition of monochromatic speckle patterns at the entrance slit. This allows the observation of Spectral Features below the level of the spectrometer spectral and spatial resolution. The Spectral Features are derived from various observed speckle patterns by properly mimicking the real spectrometer in the data analysis. With this measurement technique we are able to gain insight into the mechanism behind speckle induced Spectral Features. This insight will be used to develop a parameterized model facilitating the design of future space based spectrometers.

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