High-performance X-ray mirrors are fundamental to the operation of synchrotrons like Diamond Light Source, where their surface accuracy directly influences beam quality, focus and experimental performance. As beamline optics become increasingly sophisticated, with larger apertures, steeper curvatures and freeform geometries, new metrology approaches are needed to characterise these challenging optical surfaces.
A team of researchers at Diamond’s Optics and Metrology group have developed a laser Speckle-based Curvature Optical Metrology (SCOM) instrument that directly measures two-dimensional surface curvature. Instead of measuring the mirror’s height directly, SCOM measures it curvature using laser speckle patterns- tiny random patterns created when laser light is scattered on a surface. By tracking how these patterns move, the instrument can build up a detailed picture of the mirror’s shape.
Measuring challenging optical surfaces
Interferometric techniques remain the gold standard for measuring flat and gently curved optics. However, strongly curved mirrors present significant challenges, as steep slopes can lead to fringe crowding and unstable measurements. SCOM addresses this by using laser speckle patterns as wavefront markers. A digital image correlation algorithm tracks subtle speckle displacements to determine surface curvature with high sensitivity. The instrument has demonstrated reliable measurements for mirrors with radii of curvature ranging from 10 metres down to just 100 millimetres – surfaces that are particularly challenging for conventional interferometers.
Dr Hongchang Wang, Principal Optics Scientist and leading and corresponding author of the study, said: “Strongly curved and freeform mirrors are becoming increasingly important for advanced beamline designs, yet they remain difficult to measure with traditional interferometry. SCOM provides a practical and robust solution that expands our metrology capabilities.”
Read more on the Diamond website
Image: Dr Hongchang Wang is aligning the optics for the measurement with SCOM system