Editing Adaptive optics (section)

== In retinal imaging ==
[[File:Adaptive optics system full.svg|thumb|Illustration of a (simplified) adaptive optics system. The light first hits a tip–tilt (TT) mirror and then a deformable mirror (DM) which corrects the wavefront. Part of the light is tapped off by a beamsplitter (BS) to the wavefront sensor and the control hardware which sends updated signals to the DM and TT mirrors.]]
[[Aberrations of the eye|Ocular aberrations]] are [[distortion]]s in the wavefront passing through the pupil of the [[Human eye|eye]]. These [[optical aberration]]s diminish the quality of the image formed on the retina, sometimes necessitating the wearing of spectacles or [[contact lens]]es. In the case of retinal imaging, light passing out of the eye carries similar wavefront distortions, leading to an inability to resolve the microscopic structure (cells and capillaries) of the retina. Spectacles and contact lenses correct "low-order aberrations", such as [[defocus]] and [[astigmatism]], which tend to be stable in humans for long periods of time (months or years). While correction of these is sufficient for normal visual functioning, it is generally insufficient to achieve microscopic resolution. Additionally, "high-order aberrations", such as coma, [[spherical aberration]], and trefoil, must also be corrected in order to achieve microscopic resolution. High-order aberrations, unlike low-order, are not stable over time, and may change over time scales of 0.1s to 0.01s. The correction of these aberrations requires continuous, high-frequency measurement and compensation.

=== 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]].

=== Correction of ocular aberrations ===
Once the local phase errors in the wavefront are known, they can be corrected by placing a phase modulator such as a deformable mirror at yet another plane in the system conjugate to the eye's pupil. The phase errors can be used to reconstruct the wavefront, which can then be used to control the deformable mirror. Alternatively, the local phase errors can be used directly to calculate the deformable mirror instructions.

=== Open loop vs. closed loop operation ===
If the wavefront error is measured before it has been corrected by the wavefront corrector, then operation is said to be "open loop".

If the wavefront error is measured after it has been corrected by the wavefront corrector, then operation is said to be "closed loop". In the latter case then the wavefront errors measured will be small, and errors in the measurement and correction are more likely to be removed. Closed loop correction is the norm.

=== Applications ===
Adaptive optics was first applied to flood-illumination retinal imaging to produce images of single cones in the living human eye. It has also been used in conjunction with [[scanning laser ophthalmoscopy]] to produce (also in living human eyes) the first images of retinal microvasculature and associated blood flow and retinal pigment epithelium cells in addition to single cones. Combined with [[optical coherence tomography]], adaptive optics has allowed the first [[three-dimensional]] images of living cone [[Photoreceptor cell|photoreceptors]] to be collected.<ref>{{cite journal |doi=10.1364/OE.14.004380|pmid=19096730|pmc=2605071|title=High-speed volumetric imaging of cone photoreceptors with adaptive optics spectral-domain optical coherence tomography |journal=Optics Express |volume=14|issue=10|pages=4380–94|year=2006|last1=Zhang|first1=Yan|last2=Cense|first2=Barry|last3=Rha|first3=Jungtae|last4=Jonnal|first4=Ravi S.|last5=Gao|first5=Weihua|last6=Zawadzki|first6=Robert J.|last7=Werner|first7=John S.|last8=Jones|first8=Steve|last9=Olivier|first9=Scot|last10=Miller|first10=Donald T.|bibcode=2006OExpr..14.4380Z}}</ref>