B-spline approximation of a wavefront measured by Shack-Hartmann sensor

Conventional Shack-Hartmann sensor uses Zernike polynomials in order to approximate the wavefront of the light. Zernike approximation is well-known, well-established and widely used technique. And in most cases the quality of approximation is good enough, especially if the measured light beam has circular aperture. But when the light beam is rectangular or ringshaped (for example, if one need to measure the surface flatness of the detail that is ring-shaped), the approximation using Zernike polynomials fails. In this work we implemented the approach of the approximation of the wavefront using Bspline polynomials. We present the results of approximation of a complex simulated wavefront (Franke surface) and an experimentally measured wavefront of the ring-shaped detail using B-Spline polynomials.