The accurate simulation of straylight is essential for the verification of the contrast requirements in optical instruments.
In a spectrometer, the scattering from reflective gratings is hardly understood and difficult to characterize while
contributing significantly to the overall system straylight and reduction of the spectrometer contrast. In this article we present an experimental setup for, and measurement results from, the characterization of the bidirectional scattering distribution function (BSDF) of a grating in the scope of the FLORIS project of the ESA FLEX Mission. The grating is an Engineering Model and will be subject to further optimization. Measurement of the BSDF showed approximately a Harvey-Shack profile parallel to the grating grooves, consistent with a dominant contribution from roughness scatter and minor distinctive features. Moreover, we observed distinct straylight peaks out of the diffraction plane, which are called “satellites”. The main challenges in the measurement of grating BSDFs arise from the near angle limit, the determination of the instrument signature and the selection of the appropriate sampling (2D or 3D). Theoretical analysis has been performed to investigate the influence of, and limitations introduced by, the measurement setup combined with the convex curvature of the grating. The next step is to introduce these measured BSDFs into straylight simulation. We have done that by fitting appropriate functions to the measured BSDF and defining them in the optical analysis software ASAP as a user-defined BSDF.