Editing Adaptive optics (section)

=== Measurement of ocular aberrations ===
[[Optical aberration|Ocular aberrations]] are generally measured using a [[wavefront sensor]], and the most commonly used type of wavefront sensor is the [[Shack–Hartmann wavefront sensor|Shack–Hartmann]]. Ocular aberrations are caused by spatial phase nonuniformities in the wavefront exiting the eye. In a Shack-Hartmann wavefront sensor, these are measured by placing a two-dimensional array of small lenses (lenslets) in a pupil plane conjugate to the eye's pupil, and a CCD chip at the back focal plane of the lenslets. The lenslets cause spots to be focused onto the CCD chip, and the positions of these spots are calculated using a centroiding algorithm. The positions of these spots are compared with the positions of reference spots, and the displacements between the two are used to determine the local curvature of the wavefront allowing one to numerically reconstruct the wavefront information—an estimate of the phase nonuniformities causing [[Optical aberration|aberration]].