One limitation of the conventional Shack–Hartmann sensor is that the spots of each microlens have to remain in their respective subapertures. We present an algorithm that assigns the spots to their reference points unequivocally even if they are situated far outside their subaperture. For this assignment a spline function is extrapolated in successive steps of the iterative algorithm. The proposed method works in a single-shot technique and does not need any aid from mechanical devices. The reconstruction of a simulated steep aspherical wave front (∼100λ/mm slope) is described as well as experimental results of the measurement of a spherical wave front with a huge peak-to-valley value (∼400λ). The performance of the method is compared with the unwrapping method, which has been published before.
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