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Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor

Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor. K. Lenhard, M. Damm, P. Gege. Motivation. Large errors observed with ROSIS Target dependence of stray light! Vicarious calibration can not work.

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Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor

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  1. Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor K. Lenhard, M. Damm, P. Gege Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

  2. Motivation • Large errors observed with ROSIS • Target dependence of stray light! Vicarious calibration can not work. • Correction algorithm used for regular spectrographs adapted to our needs • Comparison: SeaWiFS experiences stray light ~ 10% Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

  3. Correction algorithm1 • Basic idea: measure Line Spread Function (LSF) • LSF quantifies how much signal neighbouring detector elements detect if one sensor element is illuminated • ROSIS is a pushbroom sensor → 2D detector array: Geometric and spectral stray light • LSFs can be merged into a matrix C: Each column contains the LSF of one channel/pixel Smeas=C∙Sin • Solve for Sin: Sin=C-1 ∙Smeas 1: Zong et al., Applied Optics, Vol. 45, No. 6 (2006) Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

  4. Measurement of Spectral Stray Light • Direct measurement of LSF difficult. Radiometric resolution/SNR too low • Used optical band pass filters to cover the bandwidth of ROSIS →higher total irradiance Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

  5. Measurement of Spectral Stray Light • Distinguish signal from stray light via treshholds • Estimation of stray light by comparison of expected and actual signal for each filter • Second iteration with corrected estimates • Drawback: large distance between illuminated and corrected channels Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

  6. Exemplary Correction • Promising results: Below 430 nm, regular signal is not expected • Large correction in the blue spectrum, as expected • Will be implemented in processing chain Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

  7. Measurement of spectral stray light • Saturate channel to observe stray light in adjacing channels • Reduce signal to allow for normalisation • This provides the LSF closer to the originating pixel than the filter approach • Yet to be measured systematically Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

  8. Geometric Stray Light • Simulation of error with inverse calculation for actual LSF of ROSIS • „Adjacency effect“ – dark spot surrounded by bright ground • Worst case error ~ 10% ! → Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

  9. Additional remarks • Ideally, geometric and spectral stray light have to be corrected simultaneously • Handling of tensor with (channels)2x(pixels)2≈5x109 entries computationally challenging → image sharpening algorithms • Reprocessing of ROSIS data would lead to different radiances and therefore to different results. Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

  10. Conclusion • Stray light in hyperspectral sensors can lead to large systematic, target-dependent radiometric errors. • Methods presented can be used to quantify amount of stray light • Method shown can help correcting stray light induced errors Implementation of algorithmic correction of stray light in a pushbroom hyperspectral sensor > Karim Lenhard > 17.3.2009

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