Adaptive optics (AO) refers to a technique to compensate for distortions caused by optical aberrations in the media between the camera and the object being imaged. It was originally developed for use in astronomical telescopes to compensate for optical distortions induced by the inhomogeneous earth atmosphere. It has since evolved to become a powerful clinical tool in ophthalmology.
In an ophthalmic AO system, a wavefront sensor, such as the Hartmann-Shack wavefront sensor, measures the distorted wavefront emerging from the eye, made irregular by aberrations of the cornea and crystalline lens.
Note that even for a normal eye, as the pupil enlarges, optical aberrations in the peripheral areas of the anterior segment come into play and may result in considerable distortions to the retinal image. For example, under low lighting conditions, the pupil dilates to approximately 5–7 mm in diameter. Higher-order aberrations become significant and lead to broadening of the point spread function (PSF, the image of a point source of light; see Chapter 2).
After measuring the distortion of the retina-reflected light, the AO system “undistorts” the beam via reflection by a deformable mirror. This is a very special mirror with a flexible surface and multiple electric actuators that can rapidly deform the mirror surface to modify the impinging aberrated wavefront accordingly, thereby effectively removing the distortions, as represented in Video 8-1.
Correction of aberrations via reflection of a deformable mirror.
Animation developed by Kristina Irsch, PhD.
Access all Section 3 videos at www.aao.org/bcscvideo_section03.
AO thus enables imaging of the human retina with unprecedented resolution, such as revealing individual photoreceptors or the walls of blood vessels. Note that AO by itself does not provide an image; rather an AO subsystem is incorporated into an existing imaging device, as indicated in Figure 8-25. AO subsystems have thus far been successfully integrated into three ophthalmic imaging devices: fundus cameras, scanning laser ophthalmoscopes, and the OCT device.
Figure 8-25 Basic principle of adaptive optics. The distorted emerging wavefront from the eye, made irregular by aberrations of the cornea and lens, gives a blurred retinal image at the start. The irregularities in the pattern of multiple images of the object produced by a micro-lenslet array allows us to determine what exactly are the distortions produced by the interfering ocular media, and then with a deformable mirror, we can modify the beam accordingly, so as to remove the distortions, which results in a high resolution retinal image. Note that adaptive optics by itself does not provide an image; rather an AO subsystem is incorporated into an existing imaging device, as indicated in red.
(Courtesy of Christopher Dainty, National University of Ireland, Galway.)
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.