Light source for stray light characterisation of EnMAP spectrometers

EnMAP is a German hyperspectral satellite mission. The Remote Sensing Technology Institute of the German Aerospace Center is responsible for the stray light characterisation of the satellite’s spectrometers. This paper describes the light source developed for that purpose and how it will be employed to characterise the in-band, in-field stray light. This type of stray light can be characterised by measuring the point spread function (PSF) at multiple locations on the detector. Because of the cumulative effect of stray light, it is necessary to characterisethe PSF in the near- and far-field. The PSF can span a range of more than 6 orders of magnitude from the peak to the wings at the edges of the detector, exceeding the intrinsic dynamic range of the instrument. Therefore a high dynamic range (HDR) PSF measurement is required: A spectrally and angularly narrow spot of light is guided onto the instrument then measurement frames of multiple signal levels are acquired. The signal level is controlled by changing the instrument’s integration time or changing the neutral density filter of the light source. The overexposed pixels in the higher signal level frames are excluded, then the multiple image frames are combined into one HDR frame which can then be used in the stray light correction algorithm. This HDR ’bracketing’ technique is demonstrated here with DLR’s HySpex VNIR-1600 airborne imaging spectrometer