Form deviations caused by lateral displacement errors in annular subaperture stitching interferometry

Optical form measurement employing a tiltable line scanning, low coherence interferometer for annular subaperture stitching interferometry

“…This contribution demonstrates the application of a tiltable low coherence interferometer with a line-scan camera sensor and a plane reference wavefront for ASSI [4,[8][9][10][11]20]. The employed Michelson interferometer features an actuated reference mirror and high precision scanning axis combining optical path length modulation for phase-shifting interferometry and unambiguous distance measuring in the local coordinate system by depth scanning white light interferometry.…”

“…Subaperture stitching interferometry (SSI) is a commonly used approach to combine axial and lateral resolution of an interferometer with a variable field of view [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. SSI is a non-null test method, which does not require a fringe free interferogram generated by matching the reference wavefront to the specimen's topography.…”

“…Employing a tilted plane wave as reference removes the tilt information from each subaperture, while using a spherical reference wave additionally removes the local radius information of the subaperture [3]. If absolute form measurement is required, the lost information can be restored, applying precise knowledge of the reference wavefront form and its translational errors between subapertures, additional information from overlapping regions of subsequently measured subapertures or global optimization algorithms [9][10][11]. However, the restoration of the absolute specimen topography in global coordinates is challenging and usually the restored parameters of global tilt and radius are retrieved with a much higher uncertainty than the locally measured topography features of higher order.…”

“…The employed Michelson interferometer features an actuated reference mirror and high precision scanning axis combining optical path length modulation for phase-shifting interferometry and unambiguous distance measuring in the local coordinate system by depth scanning white light interferometry. The application of a plane reference wavefront reduces the translational errors between subapertures to piston and tilt and enables the application of a modified stitching algorithm based on [1,10,11,21], which combines a Cartesian polynomial fitting for the translational errors with a Zernike polynomial fitting for the global wavefront reconstruction in a global model fitting approach. The results of the global model fitting are compared to a cumulative stitching approach, retrieving the lost tilt information of the subapertures from the overlapping regions of adjacent rings.…”

“…For each interval r k a section of the locally unwrapped topography w(r k , θ) is depicted (blue-, red-, yellow line), the interferometer should always be aligned in the balanced path length distance Lm = L ref . [11] is proposed to model the influence of radial displacement errors on the modified topography stitching and to receive an estimate of the expanded measurement uncertainty. The virtual experiment is adapted to emulate the donut shaped locally unwrapped subapertures the interferometric topography measurement yields.…”