Spatially and angularly resolved scatterometry equipment for a quantitative characterization of scratches, digs, and contamination

Qualifying light scattering on optical surfaces is challenging: for high-end instruments with demanding requirements like gravitational wave detectors and satellite instruments, the impact of localized defects can be critical for the instrument performances. These defects, typically micrometric, emerge during manufacturing or integration, with a sparse distribution (less than one defect per 100 micrometer diameter disk). However, these defects can significantly contribute to scattering, necessitating precise quantification. To address this challenge, the Light Scattering Group (CONCEPT) of the Institut Fresnel has developed the SPatially and Angulary Resolved Scatterometry Equipment (SPARSE). This innovative instrument integrates the principles of a scatterometer with an imaging system, allowing for spatially resolved Bidirectional Reflectance Distribution Function (BRDF). SPARSE can resolve up to around 400 000 microsurfaces of 26 μm x 26 μm on a one-inch diameter component. The instrument can measure scattering levels (<i>BRDF</i> cos(&theta;)) as low as 10⁻⁷ <i>sr</i>⁻¹, and its data processing is designed to discern and quantify the influence of localized defects, contamination, scratches, and roughness within the scattering budget. Here, we provide a brief description of the experimental setup and its metrological qualification. Additionally, we showcase some examples of measurements taken on representative samples, highlighting the versatility of SPARSE in advancing our understanding of light scattering for both gravitational wave detectors optical surfaces and satellite instruments.