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The measurement of the bi-directional scatter distribution function (BSDF) is a well established process on reflective surfaces and largely also on transmissive optical elements. Such measurements are crucial and frequently used as input to assess the straylight performance of optical space instruments. The straylight performance of grating based spectrometers, such as used in Sentinel-5 or FLEX, is to a large extent driven by the grating itself. Thus, a BSDF characterization of gratings for such spectrometers is necessary early in the spectrometer development. However, performing such measurements on gratings turn out to be challenging for a number of reasons that will be presented and addressed in this paper. In this paper we address some of the challenges experienced when measuring BSDF at different wavelengths from UV up to SWIR for several type of gratings. Scatterometers are usually designed to measure the BSDF of a single flat optical surface. The particular form or construction of such gratings, for example transmissive gratings with at least two optical interfaces or gratings with optical power, requires a reconfiguration of the classical measurement set-up to minimise the errors in the BSDF characterization. Additionally, there is a difficulty of measuring the near-specular scatter of those components due to the inherent optical aberrations and potentially their curvature. The dispersive property of gratings imposes the use of a very stable and spectrally pure light source for the measurement. We suggest some strategies and configurations to mitigate the above-mentioned difficulties. Some BSDF measurements on curved and immersed gratings are presented in this paper for illustration.
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