Efficient stray light characterisation: a white light interferometry-based method

Stray light is light that reaches the detector that is coming from sources other than the intended observed scene and is one of the main limiting factors for the performance of Earth Observation Space Optical Instruments. It often sets a working limit on the dynamic range of the system limiting the signal-to-noise ratio and minimizing the contrast between a bright and dark scene. Therefore, there is a need for detailed knowledge of the stray light behaviour of the instrument in order to correct the stray light effects in space instruments as much as possible. Characterising stray light to a high level of detail can be a lengthy task involving detailed scans requiring tuneable lasers or other sources of monochromatic light. This work presents a new method for Space Optical Instrument stray light characterisation based on White Light Interferometry. The characterisation method consists of a broadband light source that passes through a White Light Interferometer to generate spectral fringes in the exiting beam. This light is coupled to an illuminator to the instrument under test. Using the interferometer, Optical Path Difference step-scans are made for each of the spatial points in the instrument field of view. The corresponding interferograms of each illuminated detector pixel contain the full spectral information of direct- and/or stray light reaching that pixel. The dynamic range in the spectral information is increased by multiple exposure levels that are combined to result in high dynamic range spectra. Combining interferometer and spatial scans, the full spectral and spatial sensitivity of each pixel can be determined. This is demonstrated in this paper. The dynamic range in the resulting stray light kernels is shown to exceed seven orders of magnitude, limited only by the optical flux on the detector. To verify the method a dedicated high-contrast scene was used. The same scene is synthetically constructed out of the stray light characterisation data determined with the interferometric method and is compared with the measured verification scene. Differences in stray light between the measured and synthetic verification scene varied over the instrument detector on the order of 0.1% or less close to the bright part of the scene. The study shows that the white light interferometer based stray light characterisation is a promising alternative to the traditional methods involving monochromatic sources. The stability of the white light interferometer can be controlled to such level that accurate measurements are possible.