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The local inclination of the wavefront across the Shack-Hartmann microlens array causes a distortion of the spot array. Each spot is deviated from the local optical axis of its microlens depending on the ray direction. Thus the spot position is a measure for the wavefront inclination and the ensemble of all measurements points can be used to reconstruct the wavefront by numerical methods.

In phase-shifting interferometry, several camera images are collected while inducing a phase-shift between "fringe" images. With several intensity values for each pixel one determines the relative phase. Subsequent phase unwrapping yields the wavefront.

In many instances, the two methods are complimentary. For a given measurement task, considerations such dynamic range, lateral resolution, measurement speed and choice of wavelength/light source(s) will often lead to the more efficient solution.