Synthetic Aperture Interferometry for High-Resolution Optical Measurement Over a Large Field of View (SAI)
High-resolution optical inspection is an important technique used for surface defect detection in high-value engineering, however, conventional imaging is restricted to a small field-of-view. A new approach is required to allow fast inspection of large surfaces with sub-micron precision.
As high-value manufacturing reshapes the UK economy there is an increasing need for inspection and measurement systems that can verify large, expensive components with strict tolerances. In these sectors 100% inspection might require detection and measurement of sub-micron sized defects on surfaces measuring several hundred millimetres or more. No instrumentation currently offers the dynamic range and throughput required for these tasks.
The overall aim of this project is to demonstrate a novel optical instrumentat for high-resolution inspection and precision measurement of large engineering components. A prototype instrument based on an array of fibre-optic sources and phase-locked coherent imaging systems to synthesise a large numerical aperture has been developed. Efficient numerical algorithms to process the captured information have been studied; first to identify regions of interest within the field of view and subsequently to measure imperfections with high-resolution.
Examples of end-users for the new technology include the reel-to-reel production of thin film photovoltaic (PVs) structures, the verification of aerospace parts such as turbine blades, and the measurement of aspheric or freeform optical surfaces.
The project team has worked closely with instrument manufacturers (e.g. Taylor-Hobson Ltd) and end-users (e.g. Epigem Ltd). We have also collaborated directly with suppliers of key enabling technologies (e.g. JD Photo Data Ltd to optimise the production of chrome on glass calibration artefacts with feature sizes smaller than 1.6μm).
A new instrument has been designed based on an array of 225 coherent imagers and 225 fibre-optic illumination sources at 3 wavelengths that can measure a surface area of 100×100mm2 with a precision of better than 1μm (see schematic diagram).
The object is illuminated by each fibre optic source sequentially, with data collection completed in less than 23 seconds. The recorded coherent images can then be combined using a distributed computing platform to give an interferogram that is equivalent to that recorded by a coherence scanning interferometer (CSI). Phase locking of the coherent imagers is achieved using a custom calibration artefact that allows position of cameras and sources to be determined with interferometric accuracy.
Key outcomes include:
- A compact (12.5mm) low-cost digital holographic camera with large numerical aperture (NA=0.5). The compact design is achieved by exploiting highly divergent reference beams derived from novel in-house micro-machined fibre-optics.
- A bespoke chrome on glass calibration artefact based on a 2D design with fiducial markers in the form of Fresnel Zone Plates suitable for determining the positions of the fibre sources and coherent imagers to an accuracy better than λ/4. The smallest line feature in this design has a 1.6µm pitch which at the limit of the capability of the etching process.
- Distributed computing to combine data from coherent imagers and demonstrate aperture synthesis. A single surface height measurement over a complete surface area of 100x100mm2 generates more than 80GB of raw data. Efficient analysis is achieved using novel reconstruction algorithms implemented on a distributed processing cluster.
Professor Jeremy Coupland, Professor of Applied Optics and Associate Dean (Research)
“I am convinced that the coherent addition of information collected by inexpensive sensor arrays will displace expensive lenses and large sensors in optical systems and will form the basis of future metrological instruments.”