Carré Phase Shifting Algorithm for Wavelength Scanning Interferometry

Abstract

Wavelength scanning interferometry is an interferometric technique for measuring surface topography without the well-known 2π phase ambiguity limitation. The measurement accuracy and resolution of this technique depends, among other factors, on the algorithm used to evaluate its sinusoidal interference pattern. The widely used fast Fourier transform analysis experiences problems such as waviness error across the measured surface due to spectral leakage. This paper introduces a new fringe analysis method based on the Carré phase shifting algorithm combined with a least squares fitting approach. Numerical simulation was carried out to assess the performance of the Carré algorithm in comparison to fast Fourier transform analysis, and the same was validated by presenting four experimental case study examples (a surface flat, a ceramic ball bearing, a flexible thin film, and a discontinuous step height sample). The analysis results show that the proposed Carré algorithm with least squares fitting can significantly eliminate the waviness error, especially when measuring steep surfaces.

Single-shot dispersive interferometry for inline surface inspection

“…To overcome this issue, white light interferometry (WLI) using broadband illumination has been used to determine the absolute distance between the testing surface and the reference surface [4]. Additionally, depending on the scanning methods used when performing the measurement, WLI can be divided into three categories, namely vertical scanning interferometer (VSI) [5], wavelength scanning interferometry (WSI) [6][7] and dispersive interferometry [8][9]. Dispersive interferometry has been of considerable interest due to the advantage of single-shot measurement over the VSI and WSI and could be potentially applied to in-line surface inspection.…”