Synthetic aperture interferometry: High-resolution optical measurement over an exceptionally large field of view
Lead Research Organisation:
Loughborough University
Department Name: Wolfson Sch of Mech, Elec & Manufac Eng
Abstract
This proposal is concerned with the development of synthetic aperture interferometry (SAI) for high resolution measurement of large scale, precision engineered parts (e.g. aerospace components and photovoltaic panels). SAI is a new way to link several small interferometers together to make one large interferometer with dramatically increased resolutution and/or field of view. In principle this is similar to the way that arrays of radio telescopes can be linked to provide the equivalent of a much larger telescope. For the case of surface metrology however, it is also important to consider the object illumination as this has an equally important effect on resolution.
In this work we propose to build a flexible system using an array of 15x15 5MP cameras and illumination at 3 separate laser wavelengths diverging from up to 225 fibre-optic sources. This is represents around 1TB of data and is at the limits of the processing capability of a high specification desktop computer. In this proposal, however, we will consider a hierachical approach to data analysis that will initially interogate a limited number of cameras and illumination conditions to indicate regions that may be out of tolerance. Subsequently, high resolution analysis will be used to measure the highlighted regions.
We will work closely with instrument manufacturers and end users to ensure that the technology is embedded within the UKs expanding high value manufacturing industry.
In this work we propose to build a flexible system using an array of 15x15 5MP cameras and illumination at 3 separate laser wavelengths diverging from up to 225 fibre-optic sources. This is represents around 1TB of data and is at the limits of the processing capability of a high specification desktop computer. In this proposal, however, we will consider a hierachical approach to data analysis that will initially interogate a limited number of cameras and illumination conditions to indicate regions that may be out of tolerance. Subsequently, high resolution analysis will be used to measure the highlighted regions.
We will work closely with instrument manufacturers and end users to ensure that the technology is embedded within the UKs expanding high value manufacturing industry.
Planned Impact
The idea that is central to the outline proposal will benefit our collaborators and the wider community as follows:
i) Manufacturing Industry.
Quality control is increasingly important in high-value manufacturing. Machine vision systems are integral to many
inspection processes (e.g. food packaging) but cannot work to the strict tolerances of large-scale, precision
engineering. At present quality control is limited to a few important contacting measurements and as such proof of
compliance is similarly limited. In the future, industries that can prove compliance are likely to succeed. We have received
cash support from several industries and are presently in discussion with others.
ii) Instrument Manufacturers.
The UK has a strong history of innovation in precision metrology and is a world leader in contacting measurements. The
idea in this proposal will bring precision metrology onto the production line and into the machine vision market that is
currently estimated to be worth over $3bn. We will work with instrument manufacturers to make this a reality.
iii) The Academic Community.
Our idea is generic since, in essence, optical metrology can only "illuminate and observe response". The flexible, multiple
illumination, multiple coherent observation instrument provides a platform that can emulate most optical instruments. It can
therefore be configured to optimise speed and resolution compromises. We are currently considering how to share this
capability with other academics through a suitable user engagement strategy.
i) Manufacturing Industry.
Quality control is increasingly important in high-value manufacturing. Machine vision systems are integral to many
inspection processes (e.g. food packaging) but cannot work to the strict tolerances of large-scale, precision
engineering. At present quality control is limited to a few important contacting measurements and as such proof of
compliance is similarly limited. In the future, industries that can prove compliance are likely to succeed. We have received
cash support from several industries and are presently in discussion with others.
ii) Instrument Manufacturers.
The UK has a strong history of innovation in precision metrology and is a world leader in contacting measurements. The
idea in this proposal will bring precision metrology onto the production line and into the machine vision market that is
currently estimated to be worth over $3bn. We will work with instrument manufacturers to make this a reality.
iii) The Academic Community.
Our idea is generic since, in essence, optical metrology can only "illuminate and observe response". The flexible, multiple
illumination, multiple coherent observation instrument provides a platform that can emulate most optical instruments. It can
therefore be configured to optimise speed and resolution compromises. We are currently considering how to share this
capability with other academics through a suitable user engagement strategy.
Organisations
- Loughborough University (Lead Research Organisation)
- UNIVERSITY OF NOTTINGHAM (Collaboration)
- University of Huddersfield (Collaboration)
- Scitek Consultants Ltd (Project Partner)
- National Physical Laboratory NPL (Project Partner)
- The Manufacturing Technology Centre Ltd (Project Partner)
- Epigem Ltd (Project Partner)
- Taylor Hobson Ltd (Project Partner)
Publications

Coupland J.
(2015)
The influence of tilt on surface roughness measurement using the focus variation microscope
in Proceedings of the 15th International Conference of the European Society for Precision Engineering and Nanotechnology, EUSPEN 2015


Garcia-Armenta J
(2020)
Coherent imager module with a large field of view for synthetic aperture interferometry applications.
in Optics express

Garcia-Armenta J
(2024)
Transmission mode synthetic aperture interferometry using a compact coherent imager and switched fiber-optic illumination
in Optics and Lasers in Engineering

Middleton R.
(2016)
Design of a Compact Digital Holographic Camera with Large Numerical Aperture

Nikolaev N
(2016)
Focus variation microscope: linear theory and surface tilt sensitivity.
in Applied optics

Park IS
(2018)
Characterization of the reference wave in a compact digital holographic camera.
in Applied optics

Su R
(2021)
Scattering and three-dimensional imaging in surface topography measuring interference microscopy.
in Journal of the Optical Society of America. A, Optics, image science, and vision
Description | This project concerns the design and construction of a Synthetic Aperture Interferometer (SAI) - an instrument to measure the surface profile of a large area (100x100 mm^2) with the precision of a high magnification interference microscope. Work has progressed well in the following areas: i) Design and construction of miniature (12.5x12.5x12.5 mm^3) digital holographic cameras. These cameras are capable of measuring both the phase and amplitude of the light scattered by an object in order to effect interferometric measurement of surfaces. We have characterised the inexpensive Sony CMOS Sensor within Raspberry Pi NOIR (8mpixel v2) cameras and have two developed designs of coherent camera for use in the near IR using this device. An "all-glass" design (where the CMOS sensor, pinhole fibre optic reference beam are cemented to a 5mm cube beamsplitter (to form a monolithic block) and a simpler "air-spaced" design utilising a special side emission fibre optic. In both cases a key enabling technology has been the development of fibre-optic/pinhole sources with high numerical aperture (NA). ii) The concept of aperture synthesis has been demonstrated by a) combining the output of an array of coherent cameras when an object is illuminated using a single source and b) combining the output of a single coherent camera when an object is illuminated sequentially by an array of sources. In each case the resolution of the single camera is increased dramatically in accord with the diffraction limit of the resulting synthetic aperture. iii) An important aspect of this work is the design of a calibration protocol which allows the position of the cameras to be found to interferometric precision (approx. 100nm) during every phase of the measurement process (i.e. on-the-fly). Two methods have been investigated to achieve this. In our preliminary experiments we devised a "guide-star" method that phase locks the holographic images such that a diffraction limited image of a nearby known object (e.g. point source) is reconstructed. This is similar in concept to correction of atmospheric aberration in astronomy. With this approach we were able to calculate the diffraction limited image of a fibre source to a resolution of 1.6um with a coherent camera with a native resolution of 100um. The second and more generally applicable method exploits a novel, chrome-on-glass calibration plate that we have designed, developed in collaboration the JD PhotoData (Hitchen, UK) and characterised in collaboration with the National Physical Laboratory, Teddington. This work has shown that manufacture/characterisation of a suitable calibration plate is at the limit of current capability (RMS deviation is approximately 1um). However, a key finding of the project is that it is possible to calculate the camera/source position to the required accuracy using a method that is similar in concept to the guide star but uses the a-priori knowledge of the calibration plate as a start point. iv) The integration of 225 digital holographic cameras into a SAI has been modelled and proven numerically. With lateral resolution of 0.5 um a single SAI measurement of and area of 100mmx100mm contains approximately 4.10^10 data points (about 320 Gb). Data processing is therefore a significant challenge. Our chosen solution exploits a parallel computing solution based on single board computers - Raspberry Pi. a small, 24 board, Raspberry Pi cluster has been demonstrated. In this way each camera collects 675 video frames. A key finding of this work is that it is possible to image a small region defined by the native resolution of the individual cameras (approx. 100umx100um) by locally processing such that only 4 numbers are extracted from each image. We call this the "spotlight" method. Finally, using the knowledge and tools we have developed under this research we have been able to demonstrate for the first time, interferometric imaging over an exceptionally large field of view. |
Exploitation Route | The project is a collaboration with: i) An instrument manufacturer - Taylor-Hobson ii) End users Epigem Ltd., Scitek Consultants Ltd. and others represented by the Manufacturing Technology Centre iii) National Physical Laboratory In order to realise the calibration plate we have also worked in close collaboration with JD PhotoData (Hitchen) We have interest in this technology from others e.g. Prof Xiangqian (Jane) Jiang, Huddersfield University (EPSRC Metrology Hub) and Prof Richard Leach, Nottingham University. |
Sectors | Aerospace Defence and Marine Agriculture Food and Drink Digital/Communication/Information Technologies (including Software) Electronics Energy Healthcare Manufacturing including Industrial Biotechology Pharmaceuticals and Medical Biotechnology Transport |
Description | Since finishing this project we have been increasingly engaged by Zygo Inc. who are now part of the Ametek Group that is the parent of our project partners Taylor-Hobson Ltd. Some of the ideas that follow on from the SAI concept concern the calibration on existing coherence scanning interferometers and the methods are being beta-tested at Zygo (Connecticut) |
First Year Of Impact | 2020 |
Sector | Manufacturing, including Industrial Biotechology |
Impact Types | Economic |
Description | Revisiting optical scattering with machine learning (SPARKLE) |
Amount | £196,575 (GBP) |
Funding ID | EP/R028842/1 |
Organisation | Engineering and Physical Sciences Research Council (EPSRC) |
Sector | Public |
Country | United Kingdom |
Start | 05/2018 |
End | 05/2021 |
Description | Huddersfield |
Organisation | University of Huddersfield |
Country | United Kingdom |
Sector | Academic/University |
PI Contribution | Since starting the SAI project we have become partners in the EPSRC Metrology Hub led by Huddersfield (Prof Jiang). We have also been asked to participate in a Marie Curie RISE award (Huddersfield led with work based on SAI in collaboration with Xi'an Jiaotong University, Fudan University, Shanghai Jiao Tong University and Huazhong University of Science and Technology ).Our contribution here is the analysis of optical metrology using the linear theory we have developed and is central to SAI. |
Collaborator Contribution | The main contribution is exchange of knowledge of end user requirements. |
Impact | None |
Start Year | 2017 |
Description | Nottingham |
Organisation | University of Nottingham |
Country | United Kingdom |
Sector | Academic/University |
PI Contribution | We have worked extensively with the Metrology Group at Nottingham (Prof Leach). This collaboration rests on the linear theory of surface measurement that we have developed and is central to SAI and also some computational tools for rigorous calculation of surface scattering. |
Collaborator Contribution | Experimental results |
Impact | We have several multi-institutional papers (see publications list |
Start Year | 2015 |
Description | Euspen Micro/Nano Manufacturing Workshop Tutorial |
Form Of Engagement Activity | Participation in an activity, workshop or similar |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Industry/Business |
Results and Impact | This was a 2 hour workshop on optical metrology hosted by NPL Teddington |
Year(s) Of Engagement Activity | 2015 |
Description | Invited talk at MetMap (Sheffield AMRC 2019) |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | Regional |
Primary Audience | Industry/Business |
Results and Impact | This was a mixed group of industrialists/academics |
Year(s) Of Engagement Activity | 2019 |
URL | https://www.nottingham.ac.uk/conference/fac-eng/metmap-2019 |