Abstract
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.
Highlights
Lateral optical distortion is present in most optical imaging systems
The method demonstrated in this paper provides a new methodology for lateral distortion correction in 3D optical imaging systems
In which the distortion correction has to rely on some precision-manufactured and calibrated standard artefact with structured patterns, the new approach makes use of an arbitrary surface that can be found, e.g. a coin surface, such that the spatial sampling of the distortion is no longer limited by the structures in the artefact
Summary
Lateral optical distortion is present in most optical imaging systems. Distortion, unlike optical aberrations such as spherical aberration, coma and astigmatism, is not responsible for a lack of sharpness of the image; rather it is related to the form of the image, and the degree of the distortion is dependent on the position in the image plane [1]. Optical aberrations are usually suppressed for optimisation of the optical resolution in a commercial coherence scanning interferometry (CSI) system, but a significant amount of distortion can be present [2,3]. In areas such as computer vision, distortion in a camera system may cause errors in pattern analysis and recognition, dimensional and displacement measurement, threedimensional (3D) image reconstruction, etc [4]. In CSI [5,6], distortion may cause field-dependent systematic errors in the measurement of surface topography. Distortion should be corrected in order to achieve a non-distorted image field and a uniform lateral sampling distance of the image projected on to the camera, such that the measurement accuracy can be improved
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