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Ekberg, Peter
Publications (3 of 3) Show all publications
Ekberg, P. & Mattsson, L. (2018). Traceable X,Y self-calibration at single nm level of an optical microscope used for coherence scanning interferometry. Measurement science and technology, 29(3), Article ID 035005.
Open this publication in new window or tab >>Traceable X,Y self-calibration at single nm level of an optical microscope used for coherence scanning interferometry
2018 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 29, no 3, article id 035005Article in journal (Refereed) Published
Abstract [en]

Coherence scanning interferometry used in optical profilers are typically good for Z-calibration at nm-levels, but the X,Y accuracy is often left without further notice than typical resolution limits of the optics, i.e. of the order of similar to 1 mu m. For the calibration of metrology tools we rely on traceable artefacts, e.g. gauge blocks for traditional coordinate measurement machines, and lithographically mask made artefacts for microscope calibrations. In situations where the repeatability and accuracy of the measurement tool is much better than the uncertainty of the traceable artefact, we are bound to specify the uncertainty based on the calibration artefact rather than on the measurement tool. This is a big drawback as the specified uncertainty of a calibrated measurement may shrink the available manufacturing tolerance. To improve the uncertainty in X, Y we can use self-calibration. Then, we do not need to know anything more than that the artefact contains a pattern with some nominal grid. This also gives the opportunity to manufacture the artefact in-house, rather than buying a calibrated and expensive artefact. The self-calibration approach we present here is based on an iteration algorithm, rather than the traditional mathematical inversion, and it leads to much more relaxed constrains on the input measurements. In this paper we show how the X, Y errors, primarily optical distortions, within the field of view (FOV) of an optical coherence scanning interferometry microscope, can be reduced with a large factor. By self-calibration we achieve an X, Y consistency in the 175 x 175 mu m(2) FOV of similar to 2.3 nm (1 sigma) using the 50x objective. Besides the calibrated coordinate X, Y system of the microscope we also receive, as a bonus, the absolute positions of the pattern in the artefact with a combined uncertainty of 6 nm (1s) by relying on a traceable 1D linear measurement of a twin artefact at NIST.

Place, publisher, year, edition, pages
optical microscope, 2D accuracy, self-calibration, traceable, high precision
National Category
Engineering and Technology
urn:nbn:se:kth:diva-223779 (URN)10.1088/1361-6501/aaa39d (DOI)000425138000003 ()2-s2.0-85042553462 (Scopus ID)
EU, FP7, Seventh Framework Programme, 309672

QC 20180307

Available from: 2018-03-07 Created: 2018-03-07 Last updated: 2018-03-07Bibliographically approved
Ekberg, P., Daemi, B. & Mattsson, L. (2017). 3D precision measurements of meter sized surfaces using low cost illumination and camera techniques. Measurement science and technology, 28(4), Article ID 045403.
Open this publication in new window or tab >>3D precision measurements of meter sized surfaces using low cost illumination and camera techniques
2017 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 28, no 4, article id 045403Article in journal (Refereed) Published
Abstract [en]

Using dedicated stereo camera systems and structured light is a well-known method for measuring the 3D shape of large surfaces. However the problem is not trivial when high accuracy, in the range of few tens of microns, is needed. Many error sources need to be handled carefully in order to obtain high quality results. In this study, we present a measurement method based on low-cost camera and illumination solutions combined with high-precision image analysis and a new approach in camera calibration and 3D reconstruction. The setup consists of two ordinary digital cameras and a Gobo projector as a structured light source. A matrix of dots is projected onto the target area. The two cameras capture the images of the projected pattern on the object. The images are processed by advanced subpixel resolution algorithms prior to the application of the 3D reconstruction technique. The strength of the method lays in a different approach for calibration, 3D reconstruction, and high-precision image analysis algorithms. Using a 10 mm pitch pattern of the light dots, the method is capable of reconstructing the 3D shape of surfaces. The precision (1 sigma repeatability) in the measurements is < 10 mu m over a volume of 60 x 50 x 10 cm(3) at a hardware cost of similar to 2% of available advanced measurement techniques. The expanded uncertainty (95% confidence level) is estimated to be 83 mu m, with the largest uncertainty contribution coming from the absolute length of the metal ruler used as reference.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2017
3D reconstruction, large area measurement, camera calibration, structured light, image processing, image metrology
National Category
Computer Vision and Robotics (Autonomous Systems)
urn:nbn:se:kth:diva-205442 (URN)10.1088/1361-6501/aa5ae6 (DOI)000395884500001 ()2-s2.0-85014505994 (Scopus ID)

QC 20170522

Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2018-01-13Bibliographically approved
Ekberg, P., Su, R. & Leach, R. (2017). High-precision lateral distortion measurement and correction in coherence scanning interferometry using an arbitrary surface. Optics Express, 25(16), 18703-18712
Open this publication in new window or tab >>High-precision lateral distortion measurement and correction in coherence scanning interferometry using an arbitrary surface
2017 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 16, p. 18703-18712Article in journal (Refereed) Published
Abstract [en]

Lateral optical distortion is present in most optical imaging systems. In coherence scanning interferometry, distortion may cause field-dependent systematic errors in the measurement of surface topography. These errors become critical when high-precision surfaces, e.g. precision optics, are measured. Current calibration and correction methods for distortion require some form of calibration artefact that has a smooth local surface and a grid of high-precision manufactured features. Moreover, to ensure high accuracy and precision of the absolute and relative locations of the features of these artefacts, requires their positions to be determined using a traceable measuring instrument, e.g. a metrological atomic force microscope. Thus, the manufacturing and calibration processes for calibration artefacts are often expensive and complex. In this paper, we demonstrate for the first time the calibration and correction of optical distortion in a coherence scanning interferometer system by using an arbitrary surface that contains some deviations from flat and has some features (possibly just contamination), such that feature detection is possible. By using image processing and a self-calibration technique, a precision of a few nanometres is achieved for the distortion correction. An inexpensive metal surface, e.g. the surface of a coin, or a scratched and defected mirror, which can be easily found in a laboratory or workshop, may be used. The cost of the distortion correction with nanometre level precision is reduced to almost zero if the absolute scale is not required. Although an absolute scale is still needed to make the calibration traceable, the problem of obtaining the traceability is simplified as only a traceable measure of the distance between two arbitrary points is needed. Thus, the total cost of transferring the traceability may also be reduced significantly using the proposed method. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License.

Place, publisher, year, edition, pages
National Category
Atom and Molecular Physics and Optics
urn:nbn:se:kth:diva-214895 (URN)10.1364/OE.25.018703 (DOI)000409326900033 ()2-s2.0-85027888823 (Scopus ID)

QC 20171023

Available from: 2017-10-23 Created: 2017-10-23 Last updated: 2017-11-29Bibliographically approved

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