SSI Metrology is driving higher manufacturing standards.

project13 Metrology is a critical piece of the optics fabrication process, and fabrication of high quality, advanced optics is often seriously hampered by the lack of accurate and affordable metrology. Even today, optics manufacturers and their customers struggle to cope with serious limitations in conventional interferometry. As the adage goes, “If you can’t measure it, you can’t make it.” And yet, manufacturers today often go with a “close enough” or “best effort” approach to quality control, making assumptions and relying on spot checks as an indication of final surface quality. Since many customers lack the necessary tools to verify the specification, this practice has become widely accepted in the industry.

 “If you can’t measure it, you can’t make it.”
Comprehensive metrology improves the efficiency, flexibility, capability and reliability of any optics manufacturer. Phase measuring interferometry has proven to be a precise, accurate, and flexible metrology technology.  Yet the majority of metrology systems today can only measure a subset of the optics produced (60-70%). The remaining 30-40% are either too large, or too steep to be viewed over the full aperture accurately and affordably.

 

Stitching makes the difference
A measurement of the entire surface (or full-aperture) is clearly the answer. However, full aperture metrology is often available only at great cost, or with significant lead-time. Subaperture Stitching Interferometry (SSI) technology makes it possible to perform absolute metrology on nearly every optic, without expensive investments, keeping manufacturing costs lower. Using stitching technology helps reduce the dependence on test plates and radius benches, it helps manufacturers reduce over-all cycle times and costs.

The key to the QED's revolutionary metrology capabilities is subaperture stitching. The stitching process essentially reconstructs a full-aperture map of a surface from a collection of smaller (sub-aperture) maps, each covering only a portion of the whole surface. While this would be fairly straightforward for flat (plano) optics, it becomes much more complicated for non-flat surfaces, especially for aspheric shapes.

Advanced software and hardware developments enable, with very high accuracy, an automated, subaperture metrology solution that was previously unrealizable. QED’s stitching algorithms use an advanced optimization method to minimize the discrepancies between overlapping regions of subapertures caused by several sources of systematic and random measurement error. Through its integrated calibration capability, QED's metrology systems can deliver better accuracy than a standard full-aperture test.

 QED then combines interferometry, patented stitching algorithms and software to automate the entire package. Integration of the measurement design, motion control, phase data acquisition and data analysis allows complex stitched measurements to be made with a simple push of a button.