New speckle-based metrology system enhances measurement of advanced x-ray mirrors

High-performance X-ray mirrors are fundamental to the operation of modern synchrotron facilities such as Diamond Light Source. Their surface accuracy directly affects beam quality, focus, and overall experimental performance. As beamline optics become increasingly complex—incorporating large apertures, steep curvatures, and freeform geometries—conventional interferometric metrology techniques face growing limitations.

 

To address these challenges, researchers have developed a laser Speckle-based Curvature Optical Metrology (SCOM) instrument capable of directly measuring two-dimensional surface curvature. The system offers a compact, flexible, and highly adaptable alternative to traditional height-based interferometric methods.

 

Measuring What Was Previously Difficult to Measure

Interferometric techniques remain the gold standard for flat and gently curved optics. However, for strongly curved surfaces, steep slopes lead to fringe crowding and measurement instability. SCOM overcomes this issue by using laser speckle patterns as wavefront markers. A digital image correlation algorithm tracks subtle speckle displacements to retrieve curvature information with high sensitivity.

 

The system has demonstrated reliable measurements for mirrors with radii of curvature ranging from 10 metres down to just 100 millimetres—conditions that are highly challenging for conventional interferometers.

 

Dr Hongchang Wang, Principal Optics Scientist and leading and corresponding author of the study, explains the advantages of the SCOM technique: 

“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.”

 

Supporting Fabrication and In-Situ Monitoring

Beyond standalone measurements, the compact design of SCOM enables integration directly onto fabrication platforms and metrology gantries. This makes it particularly suitable for on-machine metrology and real-time process monitoring.

 

The system has already been successfully applied to:

• Deterministic figuring of X-ray mirrors
• In-situ stress monitoring during multilayer deposition
• Characterisation of deformable mirrors
• Metrology of complex freeform optical components

 

Importantly, the instrument produces full two-dimensional curvature maps. From these, slope and height profiles can be derived, providing rich surface information. With optimized parameters, spatial resolution better than 0.2 mm can be achieved—allowing characterization within the critical mid-spatial-frequency range that influences X-ray beam performance.

 

Dr Kawal Sawhney, Optics and Metrology group leader at Diamond and a Co-author of the paper on this new metrology technique, adds: 

“Having access to full 2D curvature maps allows us to directly link mirror surface features to beam distortions. This helps us predict and optimise beam quality much more effectively.”

 

Complementing Existing Metrology Techniques

While high-precision interferometric systems deliver exceptional nanometre-scale height repeatability, they often require careful alignment, long acquisition times, and custom optical components for different mirror geometries. SCOM offers a complementary balance between precision and flexibility, with greater portability and adaptability for complex surfaces.

 

By extending metrology capability to mirrors that are currently difficult—or previously impossible—to measure reliably, SCOM opens new possibilities for advanced optical design and fabrication.

 

As synchrotron science continues to push toward higher brightness and greater precision, innovations in metrology such as SCOM will play a critical role in ensuring that next-generation X-ray optics meet the demanding standards required for world-class research.

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