Two-step carrier-wave stitching method for aspheric and freeform surface measurement with a standard spherical interferometer.

Measurement of aspheric and freeform surfaces remains challenging due to the various shapes of the surface under test (SUT), especially when the SUT has a large aperture and strong deviation from the spherical surface. This paper proposes a two-step carrier-wave stitching method to enlarge the measurement bandwidth of a digital Moiré interferometry. Then, measurements of aspheric and freeform surfaces with a standard spherical interferometer without a phase-shifting mechanism are demonstrated. Experimental results with a root-mean-square repeatability of better than 1/200λ present good consistency to UA3P contact measurement results. Further simulation results with different residual wavefronts confirm measurement accuracies of peak-to-valley value of 10-3 λ. The method is effective for large residual wavefronts and thus has potential for flexible measurement of aspheric and freeform surfaces.