Hypercentric Imaging in Coherent Optical Metrology (HyperCOMet)

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During production, technical parts are subject to unwanted geometrical distortions and surface defects such as fractures, scratches and dents. Therefore, shape inspection is a mandatory perquisite for industrial quality assurance. If high precision is required, interferometric methods are well established, because they work contactless and provide a measurement uncertainty down to a fraction of the illumination’s wavelength.Advancements in micro cold forming and micro systems engineering allow for the production of increasingly complex shaped micro parts, which leads to new challenges in optical metrology. Thus, in addition to the high precision, future interferometric measurement systems have to meet these requirements by offering the ability to measure highly complex shaped objects in a reasonable time frame. Currently available systems based on standard imaging systems cannot accomplish this task, because they have a limited viewing angle, and measuring the full 3D form of a complex geometry often requires a number of individual measurements from different observation directions. This requires extensive hardware and involves time-consuming calibration and stitching procedures to reconstruct the object’s form.A solution to this problem could be hypercentric (or pericentric) imaging, which has recently been introduced to the field of machine vision. It enables circumferential visibility (simultaneous imaging of the front and the side surfaces) of convex objects. Currently the technique is mainly used in combination with image processing, e.g. to rapidly detect irregularities of an object's surface texture. Even though interferometric shape measurement techniques could largely benefit from this imaging modality, an application of hypercentric imaging to the field of coherent optical metrology has, to the best of our knowledge, not been reported.The aim of the proposed project is to introduce hypercentric imaging to the field of interferometric shape measurement. It is expected that this concept provides fast and precise form determination in industrial quality inspection of complex shaped objects for all cases in which the objects have convex shape. Exemplarily, the two coherent optical shape measurement methods of multi wavelengths phase shifting interferometry (MW-PSI) and frequency domain white light interferometry (FD-WLI) will be adapted. This requires understanding of the hypercentric imaging process on a wave optics level with a focus on speckle statistics and coherence properties of the light in the image plane. Furthermore, a specific geometry model has to be developed, that links the observed path differences to the actual shape of the object, while taking the reversed magnification of hypercentric lenses into account. The results of the project could lead to a new generation of compact, flexible and precise interferometric sensors offering circumferential shape data with a measurement uncertainty down to the sub micrometer range.